Fire Weather Climatology

Set-up

This practice case study represents an example of a rapid upper-ridge breakdown event over the western U.S. on 10 May 2005.

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following describe the situation during the morning of 10 May 2005? (Choose all that apply then click Done.)

1. The mid-level thermal ridge, dry slot and wind gradient are superimposed over portions of Arizona, New Mexico and west Texas during the morning of 10 May.

2. Poor to moderate humidity recoveries can be expected over most of Nevada and Utah

3. Overnight warm temperatures and subsequent poor to moderate humidity recoveries can be expected over portions of the Southwest.

Answer

Answers 1 and 3 are true, but the other is not. The focus area for this event was across portions of the Southwest, including parts of west Texas.

As the closed low translated over the Great Basin and Central Rockies, a mid-level wind gradient strengthened over portions of the Southwest. A mid-level dry slot and thermal ridge provided the primary focus for warm overnight temperatures and poor to moderate humidity recoveries as indicated by the 12 hour RAWS summaries below. This pre-conditioned the surface fuel bed across portions of Arizona and New Mexico and set the stage for later that day.

RAWS Summaries

In the following RAWS data, Tn is the minimum temperature and Rx is the maximum humidity recorded over the preceding 12 hour overnight period. Note also the wide range of elevations at which these measurements were taken. Comparing elevations with observed weather will help you develop an appreciation for the diversity of effects often observed in areas of complex terrain.

Arizona RAWS 12hr(overnight)Summary Ending:  Tue May 10 2005 14:26Z
Use with caution.  Data may be incomplete or contain errors.  Times are GMT. 
State                        Newest  Oldest   Total        MaxRH          Peak
ID Name                 Elev Time    Time     Obs   Tx  Tn  Rx  Rn  Pcpn  Gust
==============================================================================
AZ ALPINE               8031 10/1406 10/0206  13    56  38  52  27  00.00  16
AZ AZSCA_PORT1          5600 10/1405 10/0205  13    67  42  55  18  00.00  12
AZ BLACK HILLS          3300 10/1331 10/0131  13    82  57  35  12  00.00  20
AZ BLACK ROCK           7080 10/1401 10/0201  13    50  33  77  47  00.00  35
AZ BRIGHT ANGEL         8134 10/1411 10/0211  13    49  38  72  39  00.00  41
AZ BUCKSKIN LO          7400 10/1422 10/0222  13    55  44  63  30  00.00  24
AZ BUCKSKIN MTN         6400 10/1410 10/0210  13    58  43  69  29  00.00  24
AZ CARR                 5400 10/1419 10/0219  13    66  49  38  17  00.00  11
AZ CARRIZO              6832 10/1419 10/0219  13    58  47  56  24  00.00  27
AZ CHEDISKI PEAK        7323 10/1422 10/0222  13    54  41  69  29  00.00  24
AZ CHUCK BOX            7475 10/1422 09/1722   9    64  42  59  18  00.00  22
AZ CIBECUE RIDGE        6725 10/1420 10/0220  13    59  47  48  19  00.00  21
AZ CIBOLA               250  10/1348 10/0148  13    80  58  54  31  00.00  25
AZ COLUMBINE            9521 10/1402 10/0202  13    47  39  49  30  00.00  20
AZ CORONADO #1 PORTABL  5483 10/1324 10/0124  13    76  42  39  13  00.00  13
AZ COTTONWOOD RIDGE     6860 10/1400 10/0200  13    60  45  50  22  00.00  25
AZ CROWN KING           5900 10/1354 10/0154  13    60  46  56  26  01.90  19
AZ DEER SPRINGS         7211 10/1420 10/0220  13    55  43  59  23  00.00  23
AZ DRY LAKE             7428 10/1336 10/0136  13    61  45  54  20  00.00  18
AZ EMPIRE               4650 10/1402 10/0202  13    74  45  53  11  00.00  16
AZ FLAGSTAFF            7000 10/1406 10/0206  13    54  44  70  37  00.00  26
AZ FORT APACHE RX#1     5400 10/1414 10/0214  13    57  42  58  26  00.00  15
AZ FOUR SPRINGS         6560 10/1340 10/0140  13    61  48  45  25  00.00  39
AZ FRAZIER WELLS        6770 10/1415 10/0215  13    53  37  81  31  00.00  40
AZ GILA RIVER           1093 10/1422 10/0222  13    83  65  43  14  00.00  13
AZ GLOBE                3560 10/1406 10/0206  13    72  54  48  18  00.00  15
AZ GOODWIN MESA         4200 10/1403 10/0203  13    68  44  82  26  00.00   0
AZ GRASSHOPPER          6390 10/1420 10/0220  13    57  45  54  23  00.00  20
AZ GREENBASE            6923 10/1416 10/0216  13    56  41  74  35  00.00  28
AZ GREER                8200 10/1408 10/0208  13    53  32  49  23  00.00  26
AZ GUNSIGHT             5280 10/1403 10/0203  13    64  50  43  24  00.00  38
AZ GUTHRIE              6340 10/1404 10/0204  13    67  53  29  13  00.00  23
AZ HAVASU               475  10/1404 10/0204  13    84  45  62  19  00.00  20
AZ HEADQUARTERS         5400 10/1412 10/0212  13    72  47  33  12  00.00  16
AZ HEBER                6635 10/1405 10/0205  13    58  46  54  24  00.00  28
AZ HILLTOP              5720 10/1405 10/0205  13    63  34  77  23  00.00  15
AZ HOPI                 5602 10/1413 10/0213  13    63  44  62  25  00.00  32
AZ HOPKINS              7120 10/1419 10/0219  13    67  46  44  15  00.00  19
AZ HORSE CAMP CANYON    4040 10/1405 10/0205  13    76  57  33  12  00.00  26
AZ HOUSEROCK            5400 10/1405 10/0205  13    63  53  41  25  00.00  43
AZ HUMBUG CREEK         5250 10/1406 10/0206  13    63  50  54  29  00.00  30
AZ HURRICANE            5445 10/1406 10/0206  13    59  44  70  31  00.00  15
AZ IRON SPRINGS         5000 10/1406 10/0206  13    61  42  77  31  00.00  22
AZ JUMP-OFF RIDGE       7090 10/1422 10/0222  13    56  45  49  21  00.00  31
AZ JUNIPER RIDGE        6920 10/1420 10/0220  13    59  44  48  19  00.00  27
AZ LAKESIDE             7000 10/1400 10/0200  13    62  47  51  26  00.00  22
AZ LIMESTONE CANYON     6800 10/1336 10/0136  13    59  42  46  18  00.00  32
AZ MICRO 6-FM           5837 10/1421 10/0221  13    67  50  37  12  00.00   9
AZ MORMON LAKE          7400 10/1403 10/0203  13    54  43  70  36  00.00  20
AZ MOSS BASIN           5920 10/1408 10/0208  13    58  37  71  38  00.00   0
AZ MOUNT LOGAN          7200 10/1408 10/0208  13    50  37  72  37  00.00  20
AZ MOUNTAIN LION        5483 10/1414 10/0214  13    56  36  73  28  00.00   5
AZ MULESHOE RANCH       4560 10/1409 10/0209  13    75  52  37   9  00.00  16
AZ MUSIC MOUNTAIN       5420 10/1409 10/0209  13    59  43  58  30  00.00  33
AZ NIXON FLATS          6500 10/1402 10/0202  13    55  40  64  32  00.00  14
AZ NOON CREEK           4925 10/1337 10/0137  13    71  53  32  14  00.00  16
AZ O W SADDLE           7300 10/1419 10/0219  13    54  43  59  30  00.00  22
AZ OAK CREEK            4924 10/1403 10/0203  13    63  52  54  29  00.00  19
AZ OLAF KNOLLS          2900 10/1410 10/0210  13    72  53  42  23  00.00  34
AZ PARIA POINT          7235 10/1340 10/0140  13    58  45  49  28  00.00  36
AZ PAYSON               4975 10/1215 10/0015  13    71  52  52  18  00.00  22
AZ PINEY HILL           8102 10/1341 10/0141  13    57  40  67  27  00.00  17
AZ PLEASANT VALLEY      5050 10/1341 10/0141  13    68  45  62  25  00.00  20
AZ PROMONTORY           7800 10/1409 10/0209  13    50  39  74  35  00.00  25
AZ RED LAKE             6200 10/1359 10/0159  13    58  32  98  33  00.00  13
AZ RINCON               8240 10/1357 10/0157  13    55  42  53  23  00.00  18
AZ ROBINSON TANK        5560 10/1411 10/0211  13    61  47  57  25  00.00  26
AZ ROOSEVELT            2180 10/1411 10/0211  13    78  58  45  12  00.00  16
AZ RUCKER               5700 10/1408 10/0208  13    76  40  33   8  00.00  27
AZ SAGUARO              3100 10/1408 10/0208  13    81  59  40  12  00.00  18
AZ SAN CARLOS #1        2840 10/1414 10/0214  13    79  51  50  14  00.00  20
AZ SASABE               3500 10/1333 10/0133  13    82  50  60  15  00.00  16
AZ SCOUT CAMP           7554 10/1337 10/0137  13    61  43  46  18  00.00  18
AZ SELLS                2366 10/1414 10/0214  13    79  51  51  20  00.00  20
AZ SMITH PEAK           2500 10/1331 10/0131  13    79  58  51  17  00.00  23
AZ STANTON              3600 10/1412 10/0212  13    70  51  63  23  00.00  24
AZ STRAY HORSE          7020 10/1419 10/0219  13    56  40  58  26  00.00   8
AZ SUNSET POINT         2960 10/1412 10/0212  13    72  57  43  19  00.00  28
AZ TONTO PORTABLE #2    4905 10/1336 10/0136  13    65  40  64  20  00.00  43
AZ TRAIL CABIN          6279 10/1408 10/0208  13    67  46  41  20  00.00  14
AZ TRUXTON CANYON       5350 10/1414 10/0214  13    60  41  67  36  00.00  34
AZ TUSAYAN              6697 10/1408 10/0208  13    56  42  78  37  00.00  19
AZ TWEEDS POINT         5200 10/1413 10/0213  13    61  41  53  32  00.00  49
AZ TWIN WEST            5936 09/1316 09/0816   6    49  45  62  43  00.00  11
AZ TWIN WEST            5936 10/1416 10/0216  13    56  40  68  37  00.00  28
AZ UNION PASS           3520 10/1413 10/0213  13    70  48  40  22  00.00  33
AZ VERDE                3100 10/1405 10/0205  13    73  53  48  19  00.00  18
AZ WARM SPRINGS CANYON  8010 10/1323 10/0123  13    51  38  66  35  00.00  34
AZ YELLOW JOHN MOUNTAI  6160 10/1410 10/0210  13    56  41  71  35  00.00  36
NWS - Boise, Idaho

New Mexico RAWS 12hr(overnight)Summary Ending:  Tue May 10 2005 14:26Z
Use with caution.  Data may be incomplete or contain errors.  Times are GMT. 
State                        Newest  Oldest   Total        MaxRH          Peak
ID Name                 Elev Time    Time     Obs   Tx  Tn  Rx  Rn  Pcpn  Gust
==============================================================================
NM ALB PORTABLE #2      8140 10/1406 10/0206  13    58  31  62  22  00.00  11
NM ALBINO CANYON        7160 10/1422 10/0222  13    60  42  47  21  00.00  18
NM BARTLEY              8339 10/1404 10/0204  13    55  40  53  28  00.00  26
NM BATDRAW              4425 10/1345 10/0145  13    78  68  17   8  00.00  17
NM BEAR WALLOW          9953 10/1419 10/0219  13    46  39  36  23  00.00  19
NM BEAVERHEAD           6700 10/1421 10/0221  13    61  27  74  20  00.00  13
NM BLUEWATER CREEK      7624 10/1409 10/0209  13    62  25  96  22  00.00  19
NM BLUEWATER RIDGE      8289 10/1405 10/0205  13    57  41  49  26  00.00  10
NM BOSQUE               4500 10/1403 10/0203  13    75  33  78  12  00.00  19
NM BRUSHY MOUNTAIN      8789 10/1349 10/0149  13    60  41  41  19  00.00  21
NM CAPROCK              4210 10/1326 10/0126  13    77  61  56  12  00.00  22
NM CARLSBAD PORTABLE    3532 10/1340 10/0140  13    81  55  26   7  00.00  17
NM CHUPADERA            6520 10/1327 10/0127  13    72  50  23  10  00.00  21
NM CIMARRON             8744 10/1409 10/0209  13    54  36  67  28  00.00  14
NM COSMIC               9100 10/1415 10/0215  13    52  45  32  19  00.00  14
NM COYOTE               8800 10/1240 10/0040  13    62  35  50  15  00.00  17
NM CUBA                 6172 10/1329 10/0129  13    69  36  56  14  00.00  16
NM DATIL                8300 10/1411 10/0211  13    58  40  46  22  00.00  14
NM DEADMAN PEAK         8450 10/1407 10/0207  13    57  42  29  14  00.00   0
NM DRIPPING SPRINGS     6172 10/1414 10/0214  13    69  56  26  11  00.00  18
NM DUNKEN               5500 10/1327 10/0127  13    74  47  27  12  00.00  30
NM EIGHT MILE DRAW      3697 10/1418 10/0218  13    75  46  40  13  00.00  24
NM GILA CENTER RAWS     5600 10/1419 10/0219  13    70  32  62  15  00.00  15
NM GRANTS               8449 10/1416 10/0216  13    55  37  55  27  00.00  11
NM HACHITA VALLEY       4291 10/1419 10/0219  13    79  48  29   9  00.00  24
NM JARITA MESA          8803 10/1416 10/0216  13    53  40  47  30  00.00  14
NM JEMEZ                7999 10/1410 10/0210  13    53  40  57  32  00.00  14
NM LAGUNA               5773 10/1333 10/0033  13    76  45  47  15  00.00  20
NM LINCOLN PORTABLE     7102 10/1330 10/0130  13    63  55  21  16  00.00  15
NM MAGDALENA            8500 10/1249 10/0049  13    67  44  28  12  00.00  15
NM MALPIAS LAVA FLOW    7514 10/1421 10/0221  13    60  33  60  21  00.00  19
NM MAYHILL              6558 10/1401 10/0201  13    65  44  34  16  00.00  15
NM MESCAL               6227 10/1349 10/0149  13    69  40  49  17  00.00  19
NM MILLS CANYON         5856 10/1405 10/0205  13    69  46  53  19  00.00  25
NM MOUNTAINAIR          6500 10/1405 10/0205  13    65  42  41  19  00.00  16
NM OAK FLATS            7550 10/1250 10/0050  13    67  42  45  19  00.00  18
NM PADUCA               3510 10/1328 10/0128  13    80  51 100   9  00.00  18
NM PECOS                8600 10/1220 10/0020  13    67  47  29  14  00.00  19
NM PELONA MOUNTAIN      8080 10/1323 10/0123  13    64  42  27  10  00.00  25
NM QUEEN                5605 10/1413 10/0213  13    72  57  22  16  00.00  15
NM RAMAH                7038 10/1334 10/0134  13    65  31  76  21  00.00  20
NM ROSWELL #1 PORTABLE  3572 10/1344 10/0144  13    65  30  71  19  00.00  16
NM ROSWELL #2 PORTABLE  3572 10/1420 10/0220  13    73  51  33  16  00.00  16
NM SAN ANDRES           6138 10/1404 10/0204  13    69  59  20  10  00.00  15
NM SANDIA LAKES         5000 10/1342 10/0142  13    72  43  85  22  00.00  11
NM SIERRA DE LAS UVAS   5000 10/1330 10/0130  13    77  62  16   7  00.00  24
NM SLAUGHTER            8680 10/1419 10/0119  13    59  39  42  21  00.00  16
NM SMOKEY BEAR          6900 10/1400 10/0200  13    67  42  38  16  00.00  16
NM SOCORRO PORTABLE     7407 10/1343 10/0143  13    65  46  33  16  00.00  22
NM TAOS PORTABLE #2     9122 10/1348 10/0148  13    54  40  47  26  00.00  22
NM TOWER                6500 10/1421 10/0221  13    65  51  30  19  00.00  15
NM TRUCHAS              8340 10/1410 10/0210  13    56  31  84  29  00.00  11
NM WASHINGTON PASS      9370 10/1341 10/0141  13    51  36  78  33  00.00  24
NM zz STONE LAKE        7440 10/0000 09/1200  13    69  49  38  16  00.00   0
NM zz ZUNI              6320 10/0100 09/1300  13    74  41  59  13  00.00   0
NWS - Boise, Idaho

NWS/BLM

Evolution

How did this event unfold? Examine the data products below (check images to see the data products). Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

....an intense, and rapidly translating closed low aloft impacted portions of California and the Great Basin.

...gusty winds and drier air mixed down to the surface across portions of the Southwest and west Texas.

...a vigorous mid-level cold front progressed eastward across portions of Arizona and New Mexico.

...cooler surface conditions and higher humidity followed, reducing fire danger across portions of the Four Corners area.

...very strong, dry southwesterly mid-level flow became established over portions of the Southwest and Southern plains.

...moisture and significant lift remained well to the north and east of the main area of interest (New Mexico), extending the duration of dry southwesterly flow for this area.

Fire Weather

Take a second look at the loops of 700-500 hPa winds, 700 hPa temperatures, and satellite VIS with overlaid 700 hPa dewpoint depressions (checkimages to see the data loops). Try to decide which of the following statements accurately identify key weather elements and transitions in this case study.

Question

Which of the following statements is/are true?(Choose all that apply then click Done.)

1. Flow associated with the closed low aloft provided significant mid-level moisture over most of the Arizona/New Mexico region during this event.

2. Diurnal heating was synchronized well with strong mid-level wind flow, a thermal ridge, and a dry slot positioned across portions of the Southwest and Southern Plains.

3. This case illustrated a strong and rapid breakdown of the upper-level ridge providing the highest fire danger across portions of New Mexico.

Answer

Choices 2 and 3 are correct. Diurnal heating was synchronized well with the various breakdown signatures and the primary focus area, based on alignment of these features, was New Mexico. Distinct frontal characteristics (temperature change, wind shift, etc.) were apparent both at the surface and aloft, indicating a relatively deep cold front. (More explanation on the next page.)

Outcomes

Strong mid-level flow associated with the translating closed low affected portions of the Southwest and west Texas during the period of interest. As mixing progressed during the daylight period, this band of strong southwesterly winds, coincident with a robust mid level dry slot and thermal ridge, brought gusty winds, low humidity, and warm temperatures to the surface.

The southwesterly flow also prevented the dry line from progressing very far into eastern New Mexico during the afternoon period. For a time, it was pushed a significant distance into west Texas, creating high fire danger conditions across far west Texas. This type of east-west oscillation of the surface dry line across portions of the Southern Plains can be difficult to forecast during the primary fire season.

Cooler temperatures and higher humidity levels followed the vigorous cold front, which reduced fire danger levels across portions of the Four Corners area.

NWS offices in Albuquerque, El Paso, and Tucson all issued Red Flag Warnings for 10 May.

You have now completed this case study. Additional data, including the Red Flag Warning and fire danger class maps and 24-hour RAWS summaries are included for your interest below.

Red Flag Warnings issued by the National Weather Service 10 May 2005.

Observed Fire Danger Class for 10 May 2005. Fire danger ratings ranged from high to extreme across southern Arizona & New Mexico and west Texas.

24-Hour RAWS Summaries

Arizona RAWS 24hr Summary Ending:  Wed May 11 2005 14:03Z
Use with caution.  Data may be incomplete or contain errors.  Times are GMT. 

State                        Newest  Oldest   Total                       Peak
ID Name                 Elev Time    Time     Obs   Tx  Tn  Rx  Rn  Pcpn  Gust
==============================================================================
AZ ALPINE               8031 11/1306 10/1306  23    68  27  80  15  00.00  38
AZ AZSCA_PORT1          5600 11/1305 10/1305  23    76  35  65  14  00.00  26
AZ BLACK HILLS          3300 11/1331 10/1331  23    86  47  34  11  00.00  27
AZ BLACK ROCK           7080 11/1301 10/1301  23    42  30 100  56  00.08  27
AZ BRIGHT ANGEL         8134 11/1311 10/1311  23    46  28  72  29  00.00  39
AZ BUCKSKIN LO          7400 11/1322 10/1322  24    61  36  63  26  00.00  43
AZ BUCKSKIN MTN         6400 11/1310 10/1310  23    54  33  73  24  00.00  37
AZ CARR                 5400 11/1319 10/1319  24    76  42  44  15  00.00  25
AZ CARRIZO              6832 11/1319 10/1319  24    64  37  55  22  00.00  56
AZ CHEDISKI PEAK        7323 11/1322 10/1322  24    60  35  69  26  00.00  39
AZ CIBECUE RIDGE        6725 11/1320 10/1320  24    66  37  45  16  00.00  39
AZ CIBOLA               250  11/1348 10/1348  24    83  44  68   9  00.00  19
AZ COLUMBINE            9521 11/1302 10/1302  23    54  31  56  27  00.00  34
AZ CORONADO #1 PORTABL  5483 11/1324 10/1324  23    80  43  47  15  00.00  21
AZ COTTONWOOD RIDGE     6860 11/1400 10/1400  23    66  37  44  20  00.00  41
AZ CROWN KING           5900 11/1354 10/1354  17    60  36  52  12  01.90  28
AZ DEER SPRINGS         7211 11/1320 10/1320  24    61  36  58  21  00.00  43
AZ DRY LAKE             7428 11/1336 10/1336  23    68  35  48  18  00.00  32
AZ EMPIRE               4650 11/1302 10/1302  23    79  37  52  12  00.00  27
AZ FLAGSTAFF            7000 11/1306 10/1306  23    52  23  87  24  00.00  38
AZ FORT APACHE RX#1     5400 11/1314 10/1314  23    64  26  81  21  00.00  31
AZ FOUR SPRINGS         6560 11/1340 10/1340  24    55  33  82  19  00.00  54
AZ FRAZIER WELLS        6770 11/1315 10/1315  23    48  33  76  27  00.00  36
AZ GILA RIVER           1093 11/1322 10/1322  24    83  48  41  12  00.00  24
AZ GLOBE                3560 11/1306 10/1306  23    79  39  54  20  00.00  23
AZ GOODWIN MESA         4200 11/1303 10/1303  23    64  39  76  18  00.00   0
AZ GRASSHOPPER          6390 11/1320 10/1320  24    64  40  51  21  00.00  68
AZ GREENBASE            6923 11/1316 10/1316  24    52  20  86  19  00.00  35
AZ GREER                8200 11/1308 10/1308  23    62  23  61  14  00.00  37
AZ GUNSIGHT             5280 11/1303 10/1303  23    58  34  70  15  00.00  33
AZ GUTHRIE              6340 11/1304 10/1304  23    73  46  33  13  00.00  52
AZ HAVASU               475  11/1304 10/1304  23    84  38  71   9  00.00  14
AZ HEADQUARTERS         5400 11/1312 10/1312  23    79  39  45  11  00.00  44
AZ HEBER                6635 11/1305 10/1305  23    65  24  65  23  00.00  40
AZ HILLTOP              5720 11/1305 10/1305  21    70  24  89  23  00.00  24
AZ HOPI                 5602 11/1313 10/1313  22    66  26  77  17  00.00  40
AZ HOPKINS              7120 11/1319 10/1319  24    73  40  39  16  00.00  31
AZ HORSE CAMP CANYON    4040 11/1305 10/1305  23    79  49  32  14  00.00  40
AZ HOUSEROCK            5400 11/1305 10/1305  23    60  38  73  18  00.00  42
AZ HUMBUG CREEK         5250 11/1306 10/1306  23    59  43  53  15  00.00  38
AZ HURRICANE            5445 11/1306 10/1306  23    52  34  77  29  00.00  18
AZ IRON SPRINGS         5000 11/1306 10/1306  23    57  30  75  18  00.00  32
AZ JUMP-OFF RIDGE       7090 11/1322 10/1322  24    60  37  49  20  00.00  58
AZ JUNIPER RIDGE        6920 11/1320 10/1320  24    65  37  46  17  00.00  45
AZ LAKESIDE             7000 11/1400 10/1400  23    65  26  80  20  00.00  37
AZ LIMESTONE CANYON     6800 11/1336 10/1336  23    64  36  45  19  00.00  45
AZ MICRO 6-FM           5837 11/1321 10/1321  24    78  44  35  15  00.00  19
AZ MORMON LAKE          7400 11/1303 10/1303  23    53  26  79  24  00.00  40
AZ MOSS BASIN           5920 11/1308 10/1308  23    55  35  57  20  00.00   0
AZ MOUNT LOGAN          7200 11/1308 10/1308  23    43  30  70  40  00.00  21
AZ MOUNTAIN LION        5483 11/1314 10/1314  23    69  28  77  20  00.00  20
AZ MULESHOE RANCH       4560 11/1309 10/1309  23    80  44  37  10  00.00  40
AZ MUSIC MOUNTAIN       5420 11/1309 10/1309  23    55  32  59  18  00.00  21
AZ NIXON FLATS          6500 11/1302 10/1302  23    52  27  85  34  00.00  17
AZ NOON CREEK           4925 11/1337 10/1337  24    78  48  34  13  00.00  36
AZ O W SADDLE           7300 11/1319 10/1319  24    61  33  59  23  00.00  35
AZ OAK CREEK            4924 11/1303 10/1303  23    63  35  60  18  00.00  29
AZ OLAF KNOLLS          2900 11/1310 10/1310  23    65  44  57  20  00.01  40
AZ PARIA POINT          7235 11/1340 10/1340  24    52  32  72  17  00.00  41
AZ PAYSON               4975 11/1215 10/1215  22    69  37  54  15  00.00  30
AZ PINEY HILL           8102 11/1341 10/1341  24    62  27  66  21  00.00  34
AZ PLEASANT VALLEY      5050 11/1341 10/1341  25    69  32  53  24  00.00  31
AZ PROMONTORY           7800 11/1302 10/1309  23    53  28  74  28  34.38  38
AZ RED LAKE             6200 11/1359 10/1359  23    69  25  83  27  00.00  24
AZ RINCON               8240 11/1357 10/1357  23    66  32  50  20  00.00  33
AZ ROBINSON TANK        5560 11/1311 10/1311  23    57  32  78  22  00.00  30
AZ ROOSEVELT            2180 11/1111 10/1111  25    82  53  45  12  00.00  23
AZ RUCKER               5700 11/1308 10/1308  23    76  38  47  13  00.00  48
AZ SAGUARO              3100 11/1308 10/1308  23    89  46  41  10  00.00  26
AZ SAN CARLOS #1        2840 11/1314 10/1314  23    85  46  50  14  00.00  35
AZ SASABE               3500 11/1333 10/1333  23    82  38  73  21  00.00  33
AZ SCOUT CAMP           7554 11/1337 10/1337  24    66  36  37  17  00.00  26
AZ SELLS                2366 11/1314 10/1314  23    84  47  49  15  00.00  26
AZ SMITH PEAK           2500 11/1331 10/1331  23    72  53  42  11  00.00  26
AZ STANTON              3600 11/1312 10/1312  23    67  51  63  11  00.00  35
AZ STRAY HORSE          7020 11/1319 10/1319  24    70  37  58  18  00.00  17
AZ SUNSET POINT         2960 11/1312 10/1312  23    74  44  43   7  00.00  32
AZ TONTO PORTABLE #2    4905 11/1336 10/1336  24    64  33  64  14  00.00  92
AZ TRAIL CABIN          6279 11/1308 10/1308  23    76  41  47  15  00.00  27
AZ TRUXTON CANYON       5350 11/1314 10/1314  23    54  40  66  24  00.00  32
AZ TUSAYAN              6697 11/1308 10/1308  23    54  22  99  22  00.00  21
AZ TWEEDS POINT         5200 11/1313 10/1313  23    52  39  67  32  00.02  40
AZ TWIN WEST            5936 11/1316 10/1316  24    52  31  81  29  00.00  26
AZ UNION PASS           3520 11/1313 10/1313  23    66  48  34   6  00.00  25
AZ VERDE                3100 11/1305 10/1305  23    73  36  56   7  00.00  34
AZ WARM SPRINGS CANYON  8010 11/1323 10/1323  23    48  26  95  35  33.65  36
AZ YELLOW JOHN MOUNTAI  6160 11/1310 10/1310  23    51  23 100  28  00.00  28
NWS - Boise, Idaho

New Mexico RAWS 24hr Summary Ending:  Wed May 11 2005 14:03Z
Use with caution.  Data may be incomplete or contain errors.  Times are GMT. 
State                        Newest  Oldest   Total                       Peak
ID Name                 Elev Time    Time     Obs   Tx  Tn  Rx  Rn  Pcpn  Gust
==============================================================================
NM ALB PORTABLE #2      8140 11/1306 10/1306  23    74  35  54   7  00.00  18
NM ALBINO CANYON        7160 11/1322 10/1322  24    71  36  87  13  00.00  32
NM BARTLEY              8339 11/1304 10/1304  23    66  42  46  16  00.00  27
NM BATDRAW              4425 11/1345 10/1345  24    91  62  80   8  00.00  31
NM BEAR WALLOW          9953 11/1319 10/1319  24    61  30  58  18  00.00  31
NM BEAVERHEAD           6700 11/1321 10/1321  24    71  35  62  15  00.00  34
NM BLUEWATER CREEK      7624 11/1309 10/1309  23    70  22  95  14  00.00  31
NM BLUEWATER RIDGE      8289 11/1305 10/1305  23    68  29  63  15  00.00  24
NM BOSQUE               4500 11/1303 10/1303  23    88  40  67   8  00.00  31
NM BRUSHY MOUNTAIN      8789 11/1349 10/1349  24    70  31  64   7  00.00  35
NM CAPROCK              4210 11/1326 10/1326  23    89  61 100  15  00.00  34
NM CARLSBAD PORTABLE    3532 11/1340 10/1340  24    95  61  60   6  00.00  28
NM CHUPADERA            6520 11/1327 10/1327  23    81  52  25   5  00.00  34
NM CIMARRON             8744 11/1309 10/1309  23    67  40  48  15  00.00  27
NM COSMIC               9100 11/1315 10/1315  23    65  42  49  14  00.00  25
NM COYOTE               8800 11/1240 10/1240  22    67  35  52  12  00.00  23
NM CUBA                 6172 11/1329 10/1329  23    75  43  47   7  00.00  30
NM DATIL                8300 11/1311 10/1311  23    69  31  87  14  00.00  30
NM DEADMAN PEAK         8450 11/1307 10/1307  23    70  35  79   9  00.00   0
NM DRIPPING SPRINGS     6172 11/1314 10/1314  23    78  53  34   7  00.00  37
NM DUNKEN               5500 11/1327 10/1327  23    84  51  31   7  00.00  34
NM EIGHT MILE DRAW      3697 11/1318 10/1318  24    94  53 100   6  00.00  36
NM GILA CENTER RAWS     5600 11/1319 10/1319  24    79  32  60  12  00.00  30
NM GRANTS               8449 11/1316 10/1316  24    67  26  79  13  00.00  24
NM HACHITA VALLEY       4291 11/1319 10/1319  24    87  50  41   7  00.00  31
NM JARITA MESA          8803 11/1316 10/1316  24    67  36  69  15  00.00  19
NM JEMEZ                7999 11/1310 10/1310  23    71  36  69  15  00.00  25
NM LAGUNA               5773 11/1333 10/1333  23    80  46  49   8  00.00  32
NM LINCOLN PORTABLE     7102 11/1330 10/1330  23    73  32  31  13  00.00  21
NM MAGDALENA            8500 11/1249 10/1249  22    70  36  54   9  00.00  28
NM MALPIAS LAVA FLOW    7514 11/1321 10/1321  24    72  28  58   9  00.00  33
NM MAYHILL              6558 11/1301 10/1301  23    77  40  51  10  00.00  26
NM MESCAL               6227 11/1349 10/1349  24    74  43  49  12  00.00  34
NM MILLS CANYON         5856 11/1305 10/1305  23    79  49  91  12  00.00  42
NM MOUNTAINAIR          6500 11/1305 10/1305  23    76  43  52  10  00.00  34
NM OAK FLATS            7550 11/1250 10/1250  22    74  40  62  12  00.00  22
NM PADUCA               3510 11/1328 10/1328  23    95  59 100  15  00.00  31
NM PECOS                8600 11/1220 10/1220  22    73  43  33   7  00.00  34
NM PELONA MOUNTAIN      8080 11/1323 10/1323  23    69  38  44   8  00.00  34
NM QUEEN                5605 11/1313 10/1313  23    83  59  84  11  00.00  26
NM RAMAH                7038 11/1334 10/1334  23    72  28  63  14  00.00  36
NM ROSWELL #1 PORTABLE  3572 11/1344 10/1344  24    80  38  53  10  00.00  30
NM ROSWELL #2 PORTABLE  3572 11/1320 10/1320  25    93  54 100   6  00.00  31
NM SAN ANDRES           6138 11/1304 10/1304  23    79  54  32   7  00.00  29
NM SANDIA LAKES         5000 11/1342 10/1342  24    85  48  67  13  00.00  18
NM SIERRA DE LAS UVAS   5000 11/1330 10/1330  23    84  58  26   6  00.00  41
NM SLAUGHTER            8680 11/1319 10/1319  24    69  30  73  14  00.00  27
NM SMOKEY BEAR          6900 11/1400 10/1400  23    76  46  31  10  00.00  24
NM SOCORRO PORTABLE     7407 11/1343 10/1343  24    70  43  51  13  00.00  38
NM TAOS PORTABLE #2     9122 11/1348 10/1348  24    62  38  61  17  00.00  32
NM TOWER                6500 11/1321 10/1321  24    76  47  38  12  00.00  33
NM TRUCHAS              8340 11/1310 10/1310  23    65  31  85  19  00.00  25
NM WASHINGTON PASS      9370 11/1341 10/1341  24    58  23  76  25  00.00  29
NM zz STONE LAKE        7440 10/1200 09/1300  24    69  38  64  16  00.00   0
NM zz ZUNI              6320 10/1300 09/1400  24    74  37  68  14  00.00   0
NWS - Boise, Idaho

NWS/BLM

You have completed this section on Breakdown of the Upper Ridge pattern.

Moisture Surges

Lightning on the edge of a thunderstorm.

Moisture surges can produce both wet and dry lightning.

Dry lightning is frequently generated on the peripheries of moisture plumes as they surge northward from their source regions, the Gulf of Mexico and the Gulf of California. This section introduces two common moisture surge patterns and presents case study examples of each.

Overview

Introduction

The moisture surge pattern is important for the desert southwestern region of the US, not only for New Mexico and Arizona but also for the southern and central Rockies, southern plains, Great Basin, southern California, and occasionally the northern Rockies. This pattern type includes both Pre-monsoon and High Plains surges. These two phenomena typically impact the desert Southwest early in the Western fire season. As fire season spreads northward these two moisture surge patterns affect areas farther to the north and evolve into another pattern called the Hybrid (described in a separate section).

Map showing regional distributions of High Plains and pre-monsoonal moisture surge patterns.

Regional distributions of the two main categories of moisture patterns. The High Plains moisture surge occurs as a distinct pattern earlier in the season and contributes to the pre-monsoon pattern during the critical fire period across southern latitudes of the western U.S.

Seasons

The High Plains pattern can affect the western US fire potential between late February and mid-autumn, but is most common between April and October. Events generally last 2-5 days.

May is the month during which the High Plains surge, as a distinct pattern, generates the most significant lightning fire starts.

Critical lightning fire events are also highly correlated with the pre-monsoon period which extends from late May into early July. This period coincides with the time of year when fuels are generally driest.

Seasonal patterns for the pre-monsoon and High Plains moisture surge patterns.

Seasonal ranges for the moisture surge patterns.

Trigger

The key triggers are increasing boundary layer-700 hPa moisture combined with increasing instability. Moisture in High Plains surge events originates in the Gulf of Mexico.

Pre-monsoon surge events are a combination of the High Plains surge with additional moistening from the Gulf of California. Moistening, within the atmospheric profile, occurs from the boundary layer upward.

Reference

For more background on High Plains moisture surges see:
Monthly Weather Review, 1995: The structure and evolution of cold surges east of the Rocky Mountains. Vol. 123, pp. 2577-2610.

High Plains Moisture Surges

Increased fire danger from this pattern typically occurs in the southwest, but can also occur occasionally in the Northern Rockies.

ETA80 500hPa heights, 18 UTC, 13 May 2005.

This plot shows a shortwave at 500 hPa moving southeastward into the Great Plains, during the early stages of this pattern. Accompanying this wave is a cold front diving southward along the east slope of the Rocky Mountains. This front will travel all the way through the eastern New Mexico/western Texas region, setting the stage for subsequent return, or easterly, flow and moisture.

Stages of Development

First, a cold front attached to an eastward translating shortwave or upper-level trough pushes southward along the Front Range of the Rocky Mountains (see wind chart 1 below). Higher surface dewpoints advected by northeast to easterly flow follow the front. Moisture subsequently drives westward, penetrating the gaps found along the Front Range. This general advection pattern is often complicated by storm-scale convective outflows in the late spring and summer months.
Next, low-level moisture advection or “return flow” from the Gulf of Mexico reinforces the initial instability, creating a significant moisture convergence boundary (see wind chart 2). Numerous lightning strikes eventually develop along this boundary.
Finally, a dry cold front and upper-level trough from the northwest push the moisture convergence boundary eastward again. Gusty southwesterly winds behind the dry cold front bring clearing from west to east. The winds and lower surface humidity levels cause previously ignited lightning fires to flare up.

Wind flow patterns at different stages of a High Plains moisture surge.

This plot shows the boundary layer winds during the various stages of the high plains surges process: (1) postfrontal upslope, or easterly flow, (2) return flow from the southeast, and (3) dry intrusion after the return flow subsides. Approximate moisture convergence boundaries are indicated in orange.

Identifying the Pattern

Tracking low-level moisture advection is key to identifying this pattern. An example of such advection is shown in the following loop of surface observations. Moistening occurs from the surface upward, enhanced by daytime mixing.

METARs from 13-15 April 2005. Several important features are highlighted. Note the position of the back-door cold front at 21 UTC on the 13th. This push is further indicated by the easterly-component gap winds at Taos (SKX), Albuquerque (ABQ), and Alamogordo (ALM) 09 UTC on the 14th. Finally, notice the moisture convergence boundary draped across western New Mexico by the 15th of May. Such boundaries are important focus points for thunderstorm formation.

Also, monitoring the steering flow for thunderstorms can help you anticipate the direction of thunderstorm complex outflow propagation. Some storm outflows act to enhance the overall moisture advection pattern.

Pay close attention to thunderstorms found along the primary moisture convergence boundary. Drier storms are found along the western edge. The High Plains moisture surge sequence sometimes precedes a breakdown of the upper-ridge pattern, creating an extremely potent fire danger situation.

Pre-monsoon Moisture Surges

The pre-monsoon surge pattern occurs during a period when there is sufficient moistening to produce thunderstorms, but lesser amounts of precipitation are generated. Most common from June into early July, the pre-monsoon pattern can develop relatively quickly with a zonal jet stream oriented over the northern tier of the western US.

Stages of Development

First, the subtropical ridge progresses northward during late spring or early summer as meridional flow aloft increases over and west of the ridge axis. An example of this type of progression is illustrated by the following loop.
Moisture associated with thunderstorm complexes in the Gulf of California or Gulf of Mexico advects northward with the ridge. Moisture spreads downward from the middle and upper levels of the troposphere.
Mid-level moisture is eventually advected northward under this ridge, overspreading much drier surface conditions. In initial stages, storms are dry.
Large thunderstorm complexes driven by significant pooling of Gulf of California and Gulf of Mexico moisture develop over Mexico. These complexes propagate gradually northward over time. Drier storms are typically found along the periphery of this moisture surge.
As time progresses, deeper moisture is transported northward, increasing thunderstorm coverage and moisture content.

These development stages are illustrated by the following loops of 500 hPa heights and satellite precipitable water. Note the lightning patterns along the edges of the moisture plume between 20 UTC on 21 June and 02 UTC on 22 June.

Combined/Simultaneous Pattern

Both moisture surge patterns often occur simultaneously. Low-level moisture from the High Plains surge advects westward across New Mexico into Arizona, while mid- and upper-level moisture initially associated with the building ridge progresses northward. The combined effect can go on for several days and results in intensified lightning activity in areas that are dry and prone to fire starts.

Map showing moisture surges advecting from both the Gulf of Mexico and the Gulf of California.

Given the right setup, moisture surges can advect over dry areas from both the Gulf of Mexico and the Gulf of California.

The moisture surges abate as drying behind an eastward-propagating upper wave quickly scours the moisture out of the southwest and winds strengthen. Pre-existing fires may grow rapidly in this stage.

Flat cumulus along the horizon near Albuquerque, New Mexico.

An example of a drying trend across western New Mexico. Flat cumulus dissipate to the West of Albuquerque, New Mexico.

Note: The location of the subtropical high governs which moisture surge pattern dominates. In a typical year, the High Plains surge sets up the pre-monsoon pattern.

Fire Cessation

As atmospheric moisture from the pre-monsoon and high plains surge patterns deepens toward the north, storms become wetter and precipitable water increases. Unless the pattern is interrupted by a long duration drying event, a positive feedback response will develop between the fuels and weather.

Increases in daytime and overnight humidity combined with increasing wetting rain coverage moisten the litter and duff layers. Fire starts from lightning gradually diminish as does fire spread. Moisture levels also increase within the ladder and canopy fuels, reducing fire intensity. Moistening takes longer in timbered areas than those where fine fuels are predominant. Some perennial grasses may also green up. This generally occurs over a period of several weeks across southern California, the Southwest, the southern Rockies, and portions of the Great Basin.

As the moisture surges become progressively wetter, duff layers like this one become saturated, reducing the likelihood of fire starts. This photo was taken in New Mexico's Sandia Mountains at an elevation of 10,300 ft on 25 June 2007.

High Plains Surge: 13-17 May 2005

Set-up

This short case study represents an example of a High Plains moisture surge event that generated significant number of lightning strikes.

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following describes current conditions? (Choose all that apply then click Done.)

1. Moisture surge patterns normally do not occur in early May

2. High to very high fire danger is present in southern Arizona and New Mexico

3. Recent precipitation has decreased fire danger significantly in southern New Mexico

4. The 500 hPa trough is progressing southeastward from the northern plains

5. Low surface relative humidity was observed in southern Arizona and New Mexico

Answer

Answers 2,4, and 5 are correct.

Following a dry spell, which had cured fuels and increased fire danger, a strong wave moved southeastward from the northern plains.

Evolution

How did this event unfold? Examine the products in the data viewer (check images to see the data products) to determine the sequence of the following events. Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

...convective outflows across eastern New Mexico and strengthening return flow from the Gulf of Mexico deepened moisture levels westward across New Mexico, while drier southwesterly flow prevailed across Arizona.

...a wave moved in from west, bringing drier conditions and stronger winds to New Mexico.

...a cold front pushed southward along the Front Range, extending all the way into New Mexico.

...northerly to northeasterly winds developed over the eastern plains of New Mexico, increasing boundary layer moisture.

...drier storms formed along and west of the moisture convergence boundary while wetter storms were found to the east of the convergence zone.

...easterly flow pushed deeper moisture westward across the Southwest.

Answer

As the 500 hPa trough progressed southeastward from the northern plains, a cold front pushed southward along the Front Range, extending all the way into New Mexico. Northerly to northeasterly winds developed over the eastern plains of New Mexico and increased boundary layer moisture later on 13 May. Surface moisture was sufficiently deep to pass through gaps along the front range during the overnight period of 13-14 May, extending as far west as Arizona.

Return flow from the Gulf of Mexico, aided by convective outflows from eastern New Mexico, brought additional moisture into New Mexico on 14-15 May. A moisture convergence boundary developed, with wetter storms found to the east of this boundary and drier storms to the west. This boundary shifted eastward during the daytime period and westward during the overnight periods.

On 16 May, a wave moved in from the west, bringing drier and windier conditions to the region. The moisture convergence boundary translated further east across New Mexico and lightning was not observed as the upper levels were too dry. This type of clear-out can cause rapid spread of both pre-existing and newly ignited fires.

Fire Weather

What were the key fire weather elements?

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following statements accurately identify the key fire weather elements and transitions in this case study? (Choose all that apply then click Done.)

1. Most of the lightning activity was seen in regions of low precipitable water.

2. Drier thunderstorms were found along and to the west of the moisture convergence boundary.

3. Low-level RH increased from north to south and east to west during the onset of the event.

4. A shortwave on 16 May brought drier conditions that could support the spread of fires initiated by drier storms.

5. The return flow behind the main trough was too weak to bring moisture into New Mexico.

Answer

Answers 2, 3, and 4 are correct.

The cold front initially brought higher surface moisture to the Southwest area spreading from north to south and then from east to west. Southeasterly return flow from the Gulf of Mexico brought additional moisture into New Mexico. The moisture convergence boundary, generally found over western New Mexico during the event, provided a focus area for a mixture of drier and wetter storms. Wetter storms were found to the east of this convergence zone.

On 16 May, a critical fire weather pattern developed as a wave moved in from the west. Drier and windier conditions associated with the wave helped intensify and spread newly ignited fires throughout the region.

Outcome

Although no large fires were started by this lightning outbreak, it was the type of event that can generate numerous small fire starts.

Key Points

When tracking this type of event look for the following features and signatures:

Set-up

Pre-existing dry conditions and low fuel moistures in the threat region.

'Back-door Front'

A transition from moist north to northeast flow down the Front Range to an easterly flow of moist air originating from the southern Plains. The depth of the moisture surge associated with this flow needs to extend from the surface up to around 700 hPa. Deep outflows originating from thunderstorm complexes can aid low-level moistening from east to west.

Return Flow

Easterly or southeasterly “return flow” of moist air from Southern Plains and Gulf of Mexico. Deep thunderstorm complex outflows can aid low-level moistening from southeast to northwest.

Moisture Convergence Boundary

A moisture convergence boundary typically develops within the low- to mid-levels of the atmosphere. Drier storms are found to the west of this zone; wetter storms are found to the east.

Clear-out

Then look for the mix out as drier southwesterly winds prevail.

Pre-monsoon Surge: 18-21 June 2005

Set-up

This short case study represents an example of a pre-monsoon moisture surge event over the Southwest and Great Basin.

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following describe the situation on 18 June 2005?(Choose all that apply then click Done.)

1. The southwestern portion of the US had experienced dry conditions prior to June 18

2. GFS model forecast suggests a convective pattern is beginning to set up

3. Fuel beds were dry and very susceptible to any lightning-producing situation

4. Solar insolation is near a maximum during this time of year allowing for greater pre-heating (drying of fuels) as well as increased low-level instability (supporting thunderstorm growth).

5. GFS model forecast shows substantial rain predicted over the southwest, limiting potential for dry lightning

Answer

Answers 1-4 are correct.

The lack of precipitation over the area and the very low 100-hr fuel moisture values demonstrate how dry conditions were and how susceptible fuels were to ignition. In addition, mid June is when solar insolation is near a maximum.

The GFS forecast shows very little precipitation expected over the region. Strong height gradients over the area imply mid-level southwesterly flow. Remember the focus of the mid-level moisture surge will be steered by the location of the high over Texas and New Mexico.

Evolution

How did this event unfold? Examine the products in the data viewer (check images to see the data products) to determine the sequence of the following events. Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

...mid- and low-level flow originating from the Gulf of Mexico and southern Plains intensified, transporting additional moisture across portions of the Southwest and Great Basin.

...the upper ridge built northward and strengthened over the southern Plains; dry air was in place at the surface across portions of the Great Basin and Southwest.

...mid-level moisture gradually advected northward over portions of the Southwest and Great Basin.

...atmospheric moisture levels became sufficient to support significant shower and thunderstorm formation.

Answer

This pre-monsoon pattern example shows the importance of the location of the high in focusing moisture advection. In addition, the surface level wind flow acts to dry the low levels and decrease the amount of precipitation reaching the surface.

The moistening of storms was also supported by outflows from large thunderstorm complexes to the south and east in Mexico, eastern New Mexico, and eventually Arizona and western New Mexico.

Notice developing persistent southeasterly flow within Southwest METAR loop. Dewpoints jump into the mid 40s across portions of western New Mexico.

Notice, within the METAR loop, southerly to southeasterly surface flow developing during the nightime hours. This flow turns more southwesterly during the daytime hours due to mixing of winds aloft. A battle zone sets up as Gulf moisture is advected around the subtropical high over Arizona. Southwest flow pinches off the surface moisture advection. However, mid-level moisture advection continues to push westward and northward around the subtropical high, eventually creating a moisture layer sufficiently deep to produce higher based thunderstorms. Initially, the thunderstorms are fairly dry. But, they become wetter in places, depending on the mid-level moisture flow around the upper high. The storm moistening process was also dependent on outflows from large thunderstorm complexes to the south and east (in Mexico/eastern New Mexico and eventually Arizona/western New Mexico). Toward the end of the METAR loop, the pinch off of surface moisture pushed more northward across southern portions of Great Basin.

Fire Weather

What were the key fire weather elements?

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following statements accurately identify the key fire weather elements and transitions in this case study? (Choose all that apply then click Done.)

1. On 21 June, thunderstorms were driest across central portions of New Mexico.

2. On 21 June, thunderstorms were driest across central Arizona, southern Great Basin and the Four Corners region.

3. Fuel moisture most likely increased across central New Mexico due to an increase in precipitation and humidity.

4. On 21 June, fires starts were most likely across north central New Mexico because of the combined effects of storm dryness, low fuel moisture levels, and low relative humidity.

5. The effects of this pattern were limited to eastern New Mexico because of that region's proximity to the Gulf of Mexico.

Answer

Answers 2 and 3 are correct.

Moisture advection associated with the upper ridge helped set the stage for thunderstorm development. There was sufficient moisture transport to initiate widespread thunderstorms on 21 June. Thunderstorms were initially dry, but became wetter in central/southern New Mexico and extreme eastern Arizona, as moisture transport from the Gulf and southern Plains intensified. Abundant rainfall raised fuel moisture levels across central New Mexico. In contrast, fire danger was much higher in central Arizona, where fuel moisture and RH levels were low and thunderstorms were relatively dry.

Outcome

Extensive areas of dry lightning developed across the southwest. The SWCC morning intelligence briefing listed forty-seven new fires reported for 21 June. Some of intelligence briefing locations are indicated on the lightning strike chart.

Morning briefing from the Southwest Coordination Center for 22 Jun 2005.

Lightning map with Coconino, Tonto, and San Carlos highlighted.

Lightning map with Coconino National Forest, Tonto National Forest, and BIA San Carlos highlighted.

Key Points

When tracking this type of event look for the following features and signatures:

Set-up

Subtropical ridge with meridional upper-level flow
Low 100-hr fuel moistures (increases ignition efficiency)

Moistening

Advection northward of 700-400 hPa moisture from source regions such as the Gulf of Mexico and Gulf of California
Circulation of this moisture around the high
Significant dry layer below cloud base
Eventual low-level moistening from rainfall, thunderstorm outflows, and plains surges

Position of Upper High

Governs which areas receive both a broad and deep moisture surge

You have completed this section on Moisture Surge patterns.

Hybrid

Breakdown with a Twist

500 hPa heights with satellite precipitable water, 05 UTC 22 Jul 2005.

This pattern is a variant of the Breakdown of the Upper Ridge and an extension of the Pre-monsoon Moisture Surge critical fire weather patterns.

Introduction

The hybrid pattern is the interaction between the breakdown of the upper ridge and a significant moisture surge. This pattern typically affects the inter-mountain West from July through early September. A variant of this pattern also affects the interior of Alaska.

The catalyst for the upper ridge breakdown is a Pacific cold front/shortwave steered by the mid-latitude jet. There are two main initial breakdown mechanisms to this pattern:

The synoptic-scale wave originating from the Pacific translates eastward or northeastward along the periphery of the ridge
Weaker vorticity maxima move through the base of the parent Pacific trough and translate eastward or northeastward along the periphery of the ridge.

The strength of both the upper ridge and the parent Pacific trough shape the overall breakdown processes of these events.

Map showing the regional distribution of the post-frontal pattern.

Examples of three different prototypical 500 hPa height configurations associated with the hybrid critical fire weather pattern over the Pacific Northwest.

Map showing the regional distribution of the hybrid pattern.

Regional distributions of the hybrid pattern, shown in red.

Typical timescales for this pattern range from 1 to 3 days, depending on the strength of the Pacific trough and upper ridge.

Satellite IR with overlaid 500 hPa vorticity (top panel) and precipitable water with lightning strikes (bottom panel). A vorticity maximum has moved off the Pacific, impinging on the western edge of a moisture plume that extended from western Mexico into Canada. The key transitional zone between the moisture plume axis and the vorticity maximum is indicated by arrows.

The critical ingredients are:

The shortwave from the Pacific
The moisture plume (wrapped around the subtropical high over the continental interior). This is an extension of the pre-monsoon moisture surge pattern.

The key area to focus on is the transitional zone, defined as the drier area (mid level dry slot) between the plume axis and the area of highest cyclonic vorticity.

Weather indicators associated with this feature include:

Low relative humidity
Warm temperatures
Gusty daytime winds
Instability

Dry lightning often occurs along the edges of this transitional zone and even sometimes within it.

Alaska Variant

A variant of the hybrid critical fire weather pattern occurs in Alaska. During the set-up phase, an upper-level ridge builds from western Canada into the Alaskan interior. Hot dry weather beneath the upper ridge can allow fuel moisture levels to become very low.

Subsequently, a westward moving short wave (easterly wave) advects moisture and instability into the Alaska interior. These easterly waves typically result from mature occluded frontal systems sweeping northwestward from the Gulf of Alaska.

Generalized 500 hPa height configurations for the hybrid pattern over Alaska.

Either dry or wet thunderstorms can result, depending on atmospheric moisture, potency of the shortwave, and thunderstorm steering flow. Prolific lightning can be generated. There are typically several days every summer with more than 8000 lightning strikes. On July 4 2007, 12,388 strikes were detected.

Example Case: 21-24 Jul 2005

Set-up

Examine the data products in the box below, and familiarize yourself with the initial set-up for this case study.

Upper-level Configuration

An upper-level ridge was in place across a broad section of the western U.S., with a subtropical high centered over the Four Corners region. Also, as is typical for this time of year, a loosely defined trough was positioned off the West Coast.

Pre-conditioning Environment

Before the event, the strong ridge and subtropical high supported near-record high temperatures during a prolonged dry spell. Drying was enhanced by the combination of a thermal ridge and dry slot at mid-levels over portions of the Great Basin, California, Pacific Northwest and Rockies.

This pattern caused rapid drying of surface fuels throughout a large portion of the Great Basin, Pacific Northwest and Rockies. With drying, the fuels became more susceptible to lightning ignition and rapid fire spread.

Evolution

This case involved a complex set of interactions between the upper-level ridge, the loosely defined trough off the West Coast, and associated moisture fetches that advected from both Pacific and southwest monsoon sources. Examine the data products below (check images to see the data products), and try to find each of the key features and interactions described below.

Key Stages and Interactions

Initial Stage

Subtropical High - Thermal Ridge/Dry Slot - Monsoonal Moisture Interactions

A subtropical high developed and remained entrenched over the Four-Corners region and portions of the Rockies on the 20-21 July. Monsoonal moisture, steered by the subtropical high, flowed northward and spread across a large portion of the Great Basin, including the Sierra mountains of California.
A mid-level dry slot was evident on the northern extension of the upper-level ridge, generally affecting northern portions of the Great Basin, California, Pacific Northwest and Rockies. Warm temperatures, low humidity levels, and some breeziness occurred beneath this mid-level dry slot. Such dry slots are usually coincident with a stronger mid-level wind gradient found along the edge of the mid-level thermal ridge. Haines Index values of 5 to 6 can be typically found within this same general area.
Fueled by convectively induced waves, unstable air, and abundant moisture, wet thunderstorms affected southern portions of the Great Basin and Sierra mountains. Convective storms were drier in peripheral areas along the western and northern boundaries of the moisture push.

Pacific Trough - Upper Ridge Interactions

The breakdown began across the Pacific Northwest later on 21 July, continuing into 22 July as the synoptic wave translated eastward and northeastward along the periphery of the upper ridge.
A stronger mid-level wind gradient was found between the wave and the upper ridge over portions of California, the Pacific Northwest, and the northern Rockies on 20-21 July.
Mid-level dry slots existed:
- Between the wave and the monsoonal moisture moving northward under the ridge (this was the primary dry slot)
- South and west of the synoptic wave as it pushed inland across the Pacific Northwest on 22 July.

Breakdown Stage

The synoptic wave continued its translation northeastward across portions of the Pacific Northwest and northern Rockies on 22-23 July. This flattened the ridge and re-positioned the subtropical high further east over portions of the central Plains and mid-Mississippi valley.
A strong mid-level cold front strengthened surface winds across portions of the Pacific Northwest, Great Basin, Rockies and northern California during this same period. (This front can be inferred from the tighter packing of isotherms in those regions at 700 and 500 hPa.)
Thunderstorms (indicated by the hourly lightning data) associated with the synoptic wave translation affected portions of the Pacific Northwest during the evening of the 21 July and much of 22 July. Storms were drier along the periphery of this lightning area, as indicated by the observed precipitation graphics.
Drier storms also developed along the western and northern boundaries of the monsoonal moisture plume, generally affecting portions of the Great Basin and Rockies.
The amount of precipitation that fell on a given location was minimized by the speed with which these convective storms propagated, driven by the strong mid-level wind gradient and cold front. Thus, fires were likely started even in areas beneath the wetter convection.
Fire spread after ignition was aided by:
- Gusty surface winds
- Clearing in the wake of individual storms
- Vertical mixing of lower humidity air from the mid-level dry slot down to the surface
Idaho and western Montana were the main focus areas for lightning ignition and subsequent fire spread on 22 July

After the Breakdown

Clear skies and low humidity prevailed across portions of California, the Pacific Northwest, the Great Basin, and the Northern Rockies on 23-24 July, as the mid-level dry slot became fully entrenched over the region. Look for this in the IR and visible satellite loops.
Breezy conditions were also supported on these days by the stronger mid-level wind gradient over the Pacific Northwest, Northern Rockies, and portions of the Great Basin. You can infer this from the spacing of 700 and 500 hPa isotherms over these regions.
The mid-level thermal ridge became re-established beneath the subtropical high as the upper level ridge shifted back westward. Heightened Haines Index values coupled with warm surface temperatures affected portions of California, the Great basin, the Rockies, and the Pacific Northwest on 24 July.
Sleeper fire starts, ignited by lightning during the earlier stages of the event, began to flare up during the days following the primary breakdown phase. Such sleeper fires are especially likely in areas with heavier forest canopies.

Outcomes

During this event, several new large fire starts were reported across portions of the Pacific Northwest, Great Basin and Rockies over a period of one to three days following the thunderstorm passage.

Fires that show up a few days after the storm event are referred to as sleeper lightning starts. The time to detect these starts depends on weather conditions after the starts and how long it takes the smoke to be seen by human eyes. Lightning ignitions are usually harder to detect in forested areas with closed canopies and minimal human population. When dry air is advected behind a shortwave, these sleeper lightning starts become particularly important.

The case shown here for Oregon, Idaho, and western Montana was not a massive lightning outbreak but did produce some large fires. Coincidently, numerous pre-existing large fires were burning across the Southwest and far southern portions of the Great Basin. Rain produced by this event helped ease fire concerns in parts of the Southwest and southern Great Basin, but the lightning also created numerous new starts as well.

Practice Case: 13-20 Aug 2007

Set-up

This practice case study represents an example of an hybrid critical fire weather pattern over the western U.S. in mid-August 2007.

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following describe the situation on 13 Augutst 2007?(Choose all that apply then click Done.)

1. Fine fuels were extremely dry across the Great Basin and 100-hour fuel moisture levels were very low in a large portion of the inter-mountain West.

2. Lightning ignition efficiency values were very high along coastal areas of Washington, Oregon, and northern California.

3. A large upper-level ridge dominated a large portion of the interior West; beneath this ridge, monsoonal moisture was plentiful over an area stretching from southern California to the High Plains.

4. A synoptic-scale trough was positioned at mid-latitudes west of the Pacific Coast.

5. A mid-level thermal ridge, centered over the central Rockies, extended westward across eastern Nevada and northwestward over southeastern Idaho and southern Montana.

6. The primary dry slot extended from Oregon's Pacific coast southeastward into southeastern Nevada.

7. Breezy to windy conditions were mostly likely observed across northern California, Pacific Northwest, northern Rockies and portions of the Great Basin.

Answer

Answers 1, 3, 4, 5, and 7 are correct. The initial set-up for this case could be described as a classic synoptic trough-ridge configuration in the upper-level flow, with a dominant ridge over the central-western U.S. and a trough positioned off the west Coast. Between these two wave features, a prominent dry slot extended from the central California coast northeastward across eastern Oregon into central Idaho. The edge of the ridge was defined at mid-levels by both the enhanced wind gradient along its periphery and the mid-level thermal ridge beneath the upper ridge. Surface fuels found within this region, especially under sunny skies, would have dried out even more.

Within, and wrapping around the subtropical high, there is plentiful monsoonal moisture (as can be seen in the water vapor imagery). The western and northern peripheral areas, where this moisture interacts with the dry slot and stronger mid-level winds, will be of particular interest as this case unfolds.

Evolution

How did this event unfold? Examine the data loops below (check images to see the data products). Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

...the Pacific trough and associated wet cold frontal passage completed the breakdown of the upper ridge while shunting monsoonal moisture further east and south. At the same time, a mid level dry slot affected portions of California, the Great Basin, and the Rockies.

...the upper ridge began to break down as the Pacific trough translated eastward while monsoonal moisture extended further north.

...monsoonal moisture flow was steered northward by the subtropical high, affecting the eastern Great Basin and central Rockies. At the same time, the surface fuels continued to dry across the western Great Basin, Pacific Northwest, and northern Rockies.

...the Pacific trough deepened and edged closer to the West Coast, while monsoonal moisture wrapped around the subtropical high.

...the mid-level wind gradient strengthened due to passage of a cold front. Mid-level winds interacted with monsoonal moisture to drive fast-moving storms across portions of Idaho, Montana and Wyoming.

Answer

Well done. You have correctly ordered the sequence of events.

With a trough off the west coast and the subtropical high centered initially over the Four Corners region, monsoonal moisture flowed northward and affected portions of the Great Basin and Rockies on 13-14 August. At the same time, the interaction of the mid-level thermal ridge, dry slot and strong wind gradient continued to dry the surface fuels north and west of this moisture fetch.

As the pattern evolved on 14-15 August, the Pacific trough edged closer to the coast while monsoonal moisture progressed even further north.

The primary breakdown period initially started later on 15 August, continuing into 16 August, while monsoonal moisture was transported as far north as western Montana.

As the breakdown progressed on 17-18 August, a vigorous cold front pushed inland across the Pacific Northwest. The mid-level wind gradient strengthened and interacted with storms along the western and northern periphery of the monsoonal moisture plume.

The Pacific trough received a reinforcing shot of cooler air from the north Pacific on 19-20 August and completed the full breakdown of the upper ridge.

Fire Weather

Take a second look at the data loops for this case (check images to see the data products), then try to answer the questions that follow.

Map of Geographic Area Coordination Centers in the western U.S.

Question

Referring to the map of highlighted Geographic Area Coordination Centers above, choose the best answers to the questions below. Keep in mind, as you consider these questions, that many fire starts are not noticed or reported until several days after ignition. (Choose an answer for each question, then click Done.)

1. Which GACC do you think likely had the most new fire starts during 13-22 August, 2007?

2. Which GACC do you think likely had the fewest new fire starts during 13-22 August, 2007?

3. Which day was likely the day on which most new fire starts were reported?

Answer

As it turned out, the Eastern Great Basin GACC saw the most new fire reports with 268 new fires and 17 new large fires over the 10-day period. The Western Great Basin GACC had the fewest new fire reports, with 55, including 3 new large fires. The largest numbers of new fire reports were made on August 16 (127/5) and August 17 (105/9).

The following charts show total numbers of new fires and new large fires reported each day during the event. Keep in mind that fires reported by the individual Geographic Area Coordination Centers include both lightning and human-caused fires, but considering the time of year and patterns of occurence, it is safe to assume that most of the fires reported were ignited by lightning.

In forested areas with heavy canopies, such as are found across the northern Rockies, there is often a significant time lag between the ignition and reporting. This makes it difficult to pinpoint the exact days or critical weather events that produce fire starts in these regions. (More information about the fire weather and outcomes is given on the next page.)

New reported fires for selected Geographic Area Coordination Centers in the northwestern U.S.

Lightning ignition was highest in areas where there was the right mix of either drier thunderstorms or abundant lightning over a well-cured and very dry fuel bed.

Fire spread and intensity was highest in areas coincident with:

The mid-level dry slot
The periphery of the thermal ridge
Strong wind gradients at mid-levels

Outcomes

The hybrid pattern shown in this practice case was the culmination of a breakdown of the upper ridge combined with a northward extension of the monsoonal moisture plume.

Lightning was primarily associated with the monsoonal fetch but some lightning also affected portions of the Pacific Northwest and northern Rockies due to the Pacific wave translation. Surface fuel beds found under a large portion of the lightning-affected areas were dry and prepped for ignitions, as indicated by the 10-hr fuel moisture graphics. Fire ignitions were most numerous across the Eastern Great Basin due to fuel dryness and the interaction and residence time of the:

Mid level dry slot
thermal ridge
strong wind gradient
monsoonal plume.

Review some of the outcomes data loops for this case in the viewer below (check images to see the data products).

Lightning streaks, as indicated by the accumulated hourly lightning data, also occurred across this area on 17-18 August, and probably contributed to fire ignitions. Rapid storm movement can mitigate the effects of an overall wet environment by reducing the time individual storms linger over a given location.

During the primary breakdown phase, critical fire weather conditions, as indicated by SPC forecasts, included:

High Haines Index values
Low humidity
Strong winds ahead and coincident with the cold frontal passage

These fire weather factors caused both new fire starts and ongoing fires to intensify and spread more rapidly.

Cooler and wetter weather did eventually follow the passage of the vigorous Pacific cold front, increasing fuel moisture values across portions of the Pacific Northwest and northern Rockies.

You have completed this section on hybrid critical fire weather patterns.

Post-Frontal

A Critical Pattern for the East

A conceptual model of the post-frontal fire weather pattern.

The post-frontal fire weather pattern is the most common and important pattern for much of the eastern U.S. This section introduces this pattern and presents two case study examples.

Introduction

The post-frontal pattern is the most critical pattern for most of eastern North America. Regions experiencing critical fire weather in post-front situations include areas east of the Rocky Mountains. Critical fire weather can also occur behind fronts along the west coast of the US and Canada.

Regional distributions of the post-frontal pattern. This pattern affects fire weather over a significant portion of the eastern U.S.

Advection of dry air behind the front causes critical conditions associated with this pattern, which generally range from 6 to 48 hours. A key indicator of this pattern is northwesterly or northerly flow both in upper levels and surface flow. In the eastern portions of the US and Canada, this flow direction brings drier continental air into an area.

Changes in the fire weather parameters include below-normal humidity levels, and strong, gusty surface winds.

(Note: This pattern is discussed in the COMET module Introduction to Fire Behavior.)

Example Case: 15-16 Oct 2005

Set-up

This short case study represents an example of a post-frontal fire weather event over the Southeast in October 2005.

Set-up

Strong, persistent high pressure over the eastern U.S. supported above-normal temperatures and northerly surface flow, which brought drier air to the southeastern U.S. The area of focus for this case includes Alabama, Georgia, and northern Florida.

Examine the surface and upper-air charts for a quick snapshot of initial conditions (check images to see the data products).

Evolution

The morning of Oct 16th, foggy conditions in the southeast dissipated with daytime heating. Steady northeasterly flow was replaced by dry northerly flow as the front passed. The front was mainly tracked as a wind shift and dew point drop with little change in temperature.

The 925-700 hPa layer winds show steady flow from the north backing to northwesterly flow after frontal passage indicating that a deep layer of dry air moved into the area of interest. The loop of 500 hPa heights shows persistent northwesterly winds continuing to steer the dry continental air mass into the southeastern U.S.

Examine the data loops to better understand these processes (check images to see the data products).

Fire Weather

There was little temperature relief following the frontal passage. Instead, post-frontal areas experienced temperatures in the upper 70s-80s (24-27 C), lower relative humidity, and stronger winds. This combination created critical fire weather conditions. Relative humidity values of 20% are considered significantly low for this region of the country.

Outcomes

These dry, breezy conditions resulted in issuance of Red Flag Warnings by NWS offices in Alabama, Georgia and northern Florida. The next day, rising surface high pressure behind the front caused winds to become lighter, reducing the risk of critical fire weather.

Red Flag Warnings issued by the National Weather Service 16 Oct 2005.

Practice Case: 14-19 Apr 2005

This practice case focuses on conditions behind a cold front that moved through the northeastern U.S. two days earlier. The area of concern covers Vermont, New Hampshire and Maine.

Set-up

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Based on your quick review of the available data, which of the following descriptions is/are likely to be true?(Choose all that apply then click Done.)

1. Little precipitation fell in Vermont during the previous week.

2. Dry air was advected into the Northeast from the Ohio Valley and Appalachian region.

3. Strong mid-level northwesterly flow developed ahead of the building ridge.

Answer

Statements 1 and 3 are true. Leading up to this case, the northeastern U.S. had experienced very little precipitation over the previous week. Behind the front, dry air moved in from Canada as can be seen in the sounding data. High pressure building behind the front and the upper level ridge to the west helped funnel this dry air into the region of interest.

Evolution

How did this event unfold? Examine the data below (check images to see the data products). Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

...extremely dry air from Eastern Canada moved into New England.

...an extended period of dry weather occurred during the pre-green up season, drying fuels and increasing fire likelihood.

...a front passed through the area and high pressure moved in behind the front.

...clear skies were accompanied by progressively warmer temperatures and very low dewpoint temperatures.

Answer

Well done. You have correctly ordered the sequence of events. Pre-existing dry conditions were exacerbated by advection of dry air in the post-frontal region. With the high pressure, clear skies and greater warming caused very low humidity for this region (values are considered critical below 30%).

Fire Weather

Re-examine the data products in the box below (check images to see the data products), then answer the question about fire weather that follows.

Question

Based on your quick review of the available data, which of the following descriptions is/are likely to be true?(Choose all that apply then click Done.)

1. Temperatures warmed with clear skies helping to dry fuels.

2. Relative humidity dropped dramatically between 14 and 15 Apr.

3. Scattered showers developed with the approaching frontal system.

4. Strong, gusty surface winds of over 20 kt, greatly increased the potential for fire spread.

5. Dry air was limited to the lower levels of the atmosphere. This meant that fire danger would likely decrease with downward mixing of moisture from the middle and upper levels.

Answer

The greatest impact on fire weather was the advection of very dry air into the region behind the front. Values of relative humidity below 30% are considered critical. On 15 Aug, RAWS stations reported values in the teens, a significant drop from the previous day. Clear skies under the influence of high pressure helped raise the surface temperatures slightly, which also helped dry fuels.

Outcomes

On 15 Apr, numerous fires were reported in Vermont, New Hampshire, and Maine. A 16-acre fire was reported east-northeast of Waterville, Maine. In this part of the country, fires greater than 10 acres in size are significant and can impact availability of fire resources.

This pattern persisted in the region with the passage of another relatively dry cold front. Continued warming and persistent dry air supported new fire activity. However, the absence of strong winds kept the situation below Red Flag conditions. On 19 Apr, the approach of yet another front with stronger winds resulted in the issuance of Red Flag Warnings.

Key Points

When diagnosing this type of fire weather pattern, look for the following features:

Northwest or northerly flow both in upper levels and at the surface, bringing drier continental air into an area
Strong post-frontal winds
Changes in fire weather parameters, including above-normal temperature, below-normal humidity and strong, gusty surface winds

You have finished the section on the post-frontal fire weather pattern.

Tropical Storms

Satellite image of massive smoke plumes drifting southwestward over northern Florida.

Winds associated with subtropical storm Andrea drive large wildfire complexes along the Florida-Georgia border, 11 May 2007.

Although tropical storms are well-known for their potential to cause heavy precipitation and flooding, they can also ignite and intensify wildfires. This occurs when lightning in outer bands strikes dry fuel beds and steady, strong winds associated with the system's circulation extend beyond the cloudy core areas of the storm.

Learn more by reading through the following sections.

Introduction

Tropical storms can trigger fire events along the coastal areas of the U.S. However, these events occur infrequently because fuels in these areas tend not to be susceptible.

Map showing the regional distribution of the tropical storm pattern.

Regional distributions (in North America) of the tropical storm critical fire weather pattern, shaded in red. This pattern affects fire weather in coastal regions along the Gulf of Mexico and Atlantic seaboards.

As a tropical storm approaches the coast and later moves inland, fire starts can occur on the western and northwestern side of the forward-propagating storm. The threatened areas are outside the main cloud/precipitation shield and coincide with gradient tightening at the surface and aloft. Subsidence aloft adds to the compressional warming aspect of this event, while the near-surface wind flow advects drier continental into the fire zone.

Keetch-Byram drought index, mid-level wind field, and satellite water vapor images illustrating the tropical storms fire weather pattern.

Mid-level winds (upper-right), satellite water vapor data (lower right), and Keetch-Byram Drought Index (left, illustrating pre-existing drought conditions) from 21-22 Sep 2005. Strong offshore wind flow and dry air in place prior to landfall can increase the fire danger in areas north and northwest of the storm. Pre-existing drought conditions and dry fuels increase the probability of lightning ignition outside the main rain core.

Tropical storm-related patterns tend to develop during the period from May to November. Within this seasonal period, fuel conditions are typically driest during May-June and September-November. The patterns can last from several hours to several days, depending on the track of the storm.

Example Case: Andrea

Set-up

In spring 2007, the region including southern Georgia and northen Florida was in the midst of a historic drought. Significantly high fuel loading was present forest-wide, and abnormally low water levels in the Okeefenokee swamp exposed additional fuels.

The Sweat Farm Road and Big Turnaround fires in Georgia started in mid-April, 2007, followed by lightning ignition of the Bugaboo fire in northern Florida on May 7.

Evolution

Andrea reached subtropical storm status 9-10 May. The system developed off the Florida-Georgia-South Carolina coast but remained nearly stationary during its short life, trapped within a large middle to upper-level trough.

Andrea’s circulation produced northeasterly winds, which helped intensify the wildfires. Estimated burned acres for the Bugaboo fire increased from 68,000 acres on 8 May to over 242,000 acres on 14 May.

The following data images and loops illustrate conditions over northeastern Florida and southern Georgia. Notice that, between 9 May and 11 May, Andrea weakened into a thunderstorm complex, while the Bugaboo fire perimeter expanded significantly. The precipitable water loop indicates drying and subisdence associated with Andrea remained over the significant fire areas for a sustained duration. In addition, enhanced northeasterly to northwesterly winds carried dry continental air into the southeast throughout the event.

Fire Weather & Outcomes

During the May 2007 fire event in the Southeast, fire behavior was influenced by land-sea breeze circulations, with wind shifts and flare-ups occurring frequently in the evenings around dusk when daily fuel moisture was at its lowest. Although there is little variation in the terrain within the fire zone, tree stands and higher vegetation islands acted to channel winds in some locations.

Fires tended to be fuel driven at night in the absence of wind even as the inversion set in. Fires remained active many nights during the event, due to high fuel density and very low fuel moisture levels. This active behavior occurred even when relative humidity levels were near 90%.

During the period when the fires were influenced by subtropical storm Andrea, northeasterly winds combined with extremely dry conditions to produce extreme fire behavior and large fire growth. Observers reported that the class 7 palmetto fuels were burning similarly to class 4 chaparral fuels. Smoke plumes produced by the fires brought darkness during the afternoon, and firebrands as large as half dollars were reported falling over 10 miles away from the fire.

Lightning ignitions associated with Andrea's outer bands also affected resource management on the large fires as some crews needed to be pre-positioned for initial attack. On 10-11 May, the Bugaboo fire expanded 60,000 acres while the Big Turnaround Complex grew an additional 7,000 acres.

Large runs and extreme fire behavior resulted when subtropical storm Andrea's winds fanned the flames of pre-existing fires. But, this was a relatively short and intense phase during a significant and long-lasting fire event. Altogether, wildfires burned in northern Florida and southern Georgia from 16 April to mid-June. The Big Turnaround, Sweat Farm Road, and Bugaboo fire complexes burned over 600,000 acres, including about seventy-five percent of the Okefenokee National Wildlife Refuge.

Practice Case: Rita

Set-up

This practice case study represents an example of a tropical cyclone, Hurricane Rita, which enhanced fire danger in the lower Mississippi Delta region.

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Which of the following describe the situation during mid-September 2005? (Choose all that apply then click Done.)

1. Conditions throughout Louisiana, Mississippi, and eastern Texas were very dry.

2. Large fires were burning in the region.

3. Plentiful moisture was present over much of the southern tier states, stretching in a broad band from Texas to the Carolinas at middle and upper levels.

Answer

Answers 1 and 2 are correct. Moderate to extreme drought conditions were observed from south Texas to Arkansas, with favorable drought conditions for wildfire extending from eastern Texas and Oklahoma to southern Mississippi. In addition, wildfires were burning in Texas, Louisiana, and Mississippi. Several of these fires were significant events. A dry slot existed over much of the southern U.S., resulting in below-normal precipitable water values for Fort Worth, Texas and Lake Charles, Louisiana.

Evolution

Examine the data products in the box below (check images to see the data products), then answer the question that follows.

Question

Judging from the wind data and water vapor imagery, which time frame was likely the most critical period for fire weather during Rita's approach and landfall? (Choose the best answer.)

Answer

Rita rapidly gained intensity as it reached the warm Gulf of Mexico waters on 20 September, becoming a Category 5 hurricane on September 21. Prior to making landfall on September 24, it produced strong northeasterly winds that further dried fuels in the existing drought conditions.

The daytime hours of 21-22 September were a critical fire weather period. During this time, offshore winds strengthened, while the dry slot aloft remained in place over the region. In addition, diurnal heating and daytime convective mixing acted to increase surface temperatures and bring dry air to the surface. Fire danger is always intensified when stronger winds coincide with peak heating.

As the hurricane approached the coast during the day and evening of 22 September, lightning generated in the storm's outer bands could have caused multiple ignitions. Areas along the northwest periphery of the hurricane were especially susceptible to lightning fire starts, as fuel beds were very dry and precipitation amounts were limited.

The afternoon of 22 Sep was the most critical period for fire weather, because it was a time when enhanced offshore winds coincided with peak diurnal heating and increased lightning ignition potential.

Outcomes

Fire danger was moderate over much of the Gulf Coast by the later part of September. Winds and dry conditions preceding landfall by Hurricane Rita enhanced the fire danger in parts of Mississippi, Louisiana, and Texas. Because conditions were already dry, chances of fire ignition by lightning in the storm’s outer bands also increased.

You have completed this section on the tropical storm-related critical fire weather pattern.

Downslope Winds

Satellite image of smoke plumes trailing westward along the southern California coast.

Santa Ana winds drive 12 large wildfires in southern California, 23 Oct 2007. Fires stretched from the Mexican border to north of Los Angeles, forcing evacuation of more than 500,000 people. By 23 October, over 1300 homes had been consumed.

This is an important critical fire weather pattern for many locations situated in the lee of mountain ranges or other areas of elevated terrain.

Introduction

Downslope winds occur on the leeside of mountain ranges across the U.S. These events are characterized by strong, warm, dry winds resulting from adiabatic compression of air as it descends. Winds can reach 30-50 mph (14-23 m/s) and higher. Channeling through valleys and canyons can further strengthen the wind speed.

Smoke plumes over Los Angeles, 26 Oct 2003.

Smoke plumes blanket Los Angeles as the Grand Prix and Old wildfires burn near San Bernardino, California, 26 Oct 2003.

Some areas of the country have specific names for downslope wind events. These include Santa Ana winds in southern California, Chinook winds in the Rockies, East winds in northern Oregon and Washington, and Sundowner winds near Santa Barbara, CA.

The following map shows areas across the US and Canada where downslope winds impact fire weather. Downslope winds events can occur on time scales of 6 hours to 3 days, depending on the synoptic scale conditions driving the event.

Map showing the regional distribution of the downslope winds pattern.

Regional distributions of the downslope wind critical fire weather pattern.

Downslope Winds in Alaska

Critical fire weather can result in the Alaskan interior when there is strong southerly to southeasterly flow in the 850-500 hPa layer is channeled through passes in the Alaska Range to move down the northern slopes. This pattern, which can occur any time of the year, is the most common cause of Red Flag conditions in the interior.

Conceptual model of the downslope winds pattern in Alaska

This conceptual model illustrates an upper-level flow configuration, which can create the downslope winds and critical fire weather conditions in the central interior north of the Alaska Range. Blue arrows indicate upslope flow, while red arrows indicate downslope flow.

Example Case: Southern California

Set-up

During October 2003, downslope wind conditions contributed to devastating wildfires in Southern California. The fire environment was preconditioned by a multi-year drought resulting in large amounts of dead fuels.

Above normal precipitation in the February and March promoted growth of grasses that had fully cured and dried in the summer with above normal temperatures. This resulted in extreme fire danger. The last measureable rainfall prior to the fire event was in the late spring.

Record high temperatures were observed in southern California during mid-October, prior to the event.

Examine the data products in the box below (check images to see the data products), and familiarize yourself with the initial set-up for this case study.

Evolution

The evolution of this fire weather event was typical of a Santa Ana wind pattern:

A high pressure ridge built off the west coast of the U.S.
A surface high strengthened over the inter-mountain west and extended southward.
Dry air from the Great Basin and Mojave desert moved into coastal southern California.
A thermal trough moved off the west coast, extending the strong flow off shore.

Examine the surface and upper-air data loops in the box below (check images to see the data products), to review the evolutionary stages of this case event.

Fire Weather

Strong winds moved into the lower elevations of southern California on 25 October and lasted through 27 Oct with peak winds on 26 Oct. Observed wind speeds during the fire ranged from 15-25 kt with gusts greater than 50 kt.

Relative humidity values dropped below 10% during most afternoons; overnight recoveries reached only as high as 20-30%.

Outcomes

Thirteen major fires ignited between 21 and 28 October in Southern California and northern Baja, Mexico.

Large fire incidents active on 31 Oct 2003. Seven of these (Piru, Simi, Padua, Grand Prix, Old, Paradise, and Cedar) were associated with the Santa Ana winds event.

Together, the fires burned over 700,000 acres, destroyed 3600 homes and resulted in 22 fatalities. The Cedar Fire burned over 270,000 acres alone.

Astronaut photo of fires in the San Bernardino mountains, 26 Oct 2003.

Astronaut photograph taken from the International Space Station shows smoke plumes from wildfires in the San Bernadino Mountains, 11 LT, 26 Oct 2003. This photo, which looks toward the southeast, shows thick yellow smoke blowing toward the south, blanketing the valley below.

Satellite image of wildfires in southern California, 28 Oct 2003.

Santa Ana winds drive multiple wildfires in southern California, 28 Oct 2003. From top-left to lower-right, they include the Piru, Simi Incident, Verdale Fires to the northwest of Los Angeles, the Old, Grand Prix, and Mountain Fires in the San Bernardino Mountains, and the Paradise and Cedar Fires east of San Diego. By the time this image was gathered, 17 deaths were attributed to the fires, which had consumed over 1136 homes.

Heavy fuel loads and the extreme dryness of fuels allowed the fires to develop extreme behaviors. Fire whirls as wide as 2600 ft (800 m) were observed while fire brands were observed by pilots at altitudes up to 1500 ft AGL.

By 29 Oct, winds had shifted to westerly, bringing higher humidity levels and cooler temperatures into the fire zones. Tremendous smoke plumes spread across southeastern California into southern Nevada and western Arizona.

Satellite image of smoke over the interior southwest, 26 Oct 2003.

Smoke from the fires blanketed southeastern California and Nevada and eastern Arizona.

To learn more about this devastating fire event see:

San Diego Wildfires Education Project Fire Facts
San Diego 2003 Wildfires Photo Gallery
Web-based Mapping Services for the 2003 San Diego Wildfires
Climate, Santa Ana Winds and Autumn Wildfires in Southern California (an article by Westerling, A.L., D.R. Cayan, T.J. Brown, B.L. Hall and L.G. Riddle in the Aug 2004 edition of EOS.)

Practice Case: Front Range

Set-up

This case focuses on a downslope wind event that affected the Front Range of northern Colorado on 28-30 October 2003.

Take a quick look at the initial conditions for this case, as represented by the charts in the data viewer below (check images to see the data products). Then answer the question that follows.

Question

Which of the following statements best describe the initial setup for this case? (Choose all that apply then click Done.)

1. Fire danger along the western slope of the front range is extreme.

2. This time of year falls within the primary fire season for the region.

3. Hundred hour fuel moisture was in the 6-10% range.

4. Fuels during this time of year are very green, reducing the potential for fire ignition.

5. Development of downslope winds is monitored closely because of the drying affect on fuels.

Answer

Answers 3 and 5 are correct. The end of October normally falls between the primary fire season in the summer and the secondary season that begins after the killing frost has occurred. In this case, fire danger was very high, and 100-hr fuel moisture was relatively low, in the 6-10% range. So, the environment was conducive to the spread of fire. Under the added influence of strong, dry, downslope winds, fires would grow and spread even more quickly.

Evolution

How did this event unfold? Examine the data loops below (check images to see the data products). Then complete the exercise that follows.

Question

What was the sequence of events? (Decide the order for each event, then click Done.)

... a strong cold front progressed southward through the area during the evening of 29 Oct.

...strong downslope winds formed along the northern Front Range.

...behind the cold front, freezing drizzle, rain and snow occurred.

...upper-level flow shifted from northwesterly, becoming more zonal over Colorado.

...at the surface, a trough formed in the lee of the Rockies, and a strong pressure gradient formed.

Answer

This case unfolded with classic indicators of strong downslope events in the Rockies. In the upper levels, development of zonal (west to east) flow helped form mountain waves, which contributed to downslope acceleration of flow on the leeside of the mountains. At the surface, a leeside trough developed with a very strong east-west pressure gradient. Strong downslope winds did develop in this situation. However, a cold front moved in from the north and ended the downslope event while bringing precipitation to the area.

Favorable conditions for downslope winds were in place by late in the evening of 28 October. Specifically, a strong low-level trough was located just east of the Front Range with a strong east-west oriented pressure gradient over the area of interest. This setup intensified into the morning of 29 October, then persisted until a strong cold front moved southward through the area during the evening of 29 October.

Outcomes

Photographs of the wildfire looking north from Boulder, Colorado.

Top: An early afternoon photograph taken looking north from NCAR's Mesa Lab facility in Boulder, Colorado. The fire, driven by strong winds, was spreading rapidly.

Bottom: By evening, flames could be clearly seen backing down the hillsides near Boulder. Photo by Lynda Lester, UCAR/NCAR/SCD.

An explosive wildfire spread by high downslope winds began in the foothills just northwest of (about 10 miles from) Boulder, Colorado early on the morning of 29 Oct 2003. By late afternoon it had already spread to 3500 acres and was threatening 250 residential structures. According to on-site reports, high winds were causing erratic fire behavior. Helicopters, air tankers and ground crews were active at this time.

The initial rapid spread of this fire was driven by downslope winds and prefrontal conditions.

The frontal passage at about 7:00 to 8:00 pm on 29 October quickly put an end to the downslope winds, while cooling and moistening the lower atmosphere.

The Incident Status Summary for the afternoon of 30 October indicated that rapid fire spread ceased shortly after the afternoon report on 29 October, and that the fire had been completely contained. Surface temperatures at this time were near freezing, precipitation developed quickly behind the surface cold front late in the evening of 29 October.

Photo of smoke plume driven by strong winds downslope and toward he east.

Strong downslope winds drive plumes of smoke eastward over the Front Range. Photo by Lynda Lester, UCAR/NCAR/SCD.

You have completed this section on the downslope winds critical fire weather pattern.

Other Mesoscale Patterns

Thumbnail images of gap winds, outflow boundary, and land/sea breeze conceptual models.

Gap winds, outflow boundaries, and land/sea circulations are three important mesoscale critical fire weather patterns.

Any weather pattern that can influence fire weather elements (strong winds, high temperatures, low humidity levels, instability) can become a critical fire weather pattern if it occurs in an area where fuels and topography conditions support fire growth and spread.

Several of the more important mesoscale processes for fire weather are introduced in this section.

Gap Winds

Winds through Nuannu Pali Gap, Oahu, HI.

Gap winds can be highly localized.

Gap winds develop at low levels of the atmosphere in locations where there are gaps between or depressions within areas of higher terrain. They tend to be localized, and the horizontal extent can vary from hundreds of feet to over one hundred miles. The vertical extent of these winds is normally quite shallow, reaching only hundreds of feet to a few thousand feet above the surface.

Gap winds occur in numerous locations throughout North America. Knowledge of local terrain is essential to recognizing regions susceptible to the development of gap winds. The duration of these wind events depends on the timescale of the driving mechanism.

The main effect of air flowing through a gap is acceleration of the flow, resulting in wind speeds of 20-60 kt depending on the factors driving the acceleration.

Evolution

Gap winds are driven by two main mechanisms:

Pressure gradients associated with synoptic or regional-scale features
Pressure gradients associated with rapid changes in the depth of cool air at low levels with colder air masses on the upstream side of the gap

Synoptic pressure gradients can force acceleration through terrain gaps .

Surface and upper-air pressure cross-section across a terrain feature with cold air pooled to the high-pressure side.

This schematic cross-section illustrates pressure gradients caused by differences in air mass temperature and density on either side of the terrain feature.

The strongest winds occur in the gap exit regions, as winds accelerate through the gap. In mountain pass areas, gap wind mechanisms can combine with downslope wind mechanisms to generate winds stronger than 100 kt. For more detailed information on the development of gap winds and numerous examples, please see the Gap Winds module.

The strongest winds are found in the gap exit regions.

Gap types: Sloping gap in a mountain pass and level gap in a through a marine channel.

Gap wind effects can be generated both mountain passes and terrain cuts such as river channels or fjords.

Application to Fire Weather

Wind

The most obvious fire weather effect of gap winds is wind speed. Areas with gaps in terrain are likely to experience much stronger cross-slope winds. Since wind is a major driving factor in fire spread, forecasts of gap winds are critical to fire weather customers.

Grass fire at Yellowstone National Park, 1988.

Temperature

Gaps can link air masses of very different characteristics. In the case of gap winds driven by a pressure gradient due to low-level changes in the depth of cool air, the air flowing through the gap may be cooler than on the leeside of the gap. In sloped terrain, gap winds may act to accelerate air warmed by downslope flow.

Relative Humidity

Dry air may be forced through the gap by differences in air masses across the gap. Decreases in relative humidity can occur within the gap winds resulting in hot, dry, windy weather.

In other situations, cool, moist air can be pushed through gaps, undercutting a smoke plume. This can result in multiple spot fires as the plume collapses.

Summary

Understanding the forcing mechanisms of gap winds will help identify situations of potentially strong winds, wind shifts, and changes in humidity. Also, knowing where gaps in terrain exist, and those typically subject to the development of gap winds, will help you provide critical information to your customers.

Outflow Boundaries

Air cooled within a thunderstorm is mixed downward along the storm’s perimeter, creating a colder, more dense air mass that is separated from the surrounding air by an outflow boundary. Outflow boundaries can persist for 24 hours or more and travel hundreds of miles from the originating thunderstorm. The boundary passage is important to fire weather as it brings about a sudden increase in wind speeds with a wind shift and a drop in temperatures. Outflow boundaries can also trigger the formation of new thunderstorms. The pattern can occur anywhere thunderstorms occur so is an important consideration for fire weather throughout North America.

Outflow boundaries mark the edge of a cooler air mass being pushed outward by thunderstorms. In this radar image, a microburst west of Santa Fe produced outflow propagating toward the southwest. A new line of convection has developed along the outflow boundary, indicated within the yellow circle.

Example Case

Outflow boundaries result from the downward and outward flow of air cooled within a thunderstorm. On July 6, 2003, a band of high based showers (virga) formed over the upper reaches of the Rio Grande valley, northwest of the Encebado fire near Taos New Mexico.

Map of central-northern New Mexico, showing the fire location.

2:25 PM

The initial outflow boundary which eventually affected the fire area passed over the incident command post (northwest of the fire location).

A wind gust associated with an outflow boundary is indicated by the wind sock in this photo, taken 2:25 PM local time.

2:30 PM

Signs of an outflow boundary passage included an increase in wind speeds, which accelerated the fire activity. The outflow also suspended dust and ash above the surface, providing visual information about the height of the cooler air mass. Outflows that emanated from the band of high based showers located over the upper Rio Grande valley were drier in origin and provided only slight increases in humidity while wind speeds increased dramatically.

Smoke, dust, and ash at the Encebado, NM Fire, July 2003.

Passage of the outflow boundary is indicated by suspended dust along the base of the mountains and the movement of ash getting swept along the ridges. Photo taken at 2:30 PM local time.

2:35 PM

The band of virga continued to slowly propagate toward the south-southeast, emitting several outflow pulses.

Active burning at the Encebado, NM Fire, July 2003.

As outflow winds reached the fire, the smoke column began to shift direction. The fire also became more active. Photo taken at 2:35 PM local time.

2:40 PM

Affected by the relatively dry outflow pulses, the fire column shifted direction and the fire began to pick up. As a stronger outflow pulse reached the fire, flame lengths increased to near 200 ft (60 m) and the fire behavior became more extreme. Crews had sought refuge in their respective safety zones well before this point.

2:45 PM

Spot fires developed as a result of the strong outflow push.

Spot fires have started on mountain slopes downwind of the Encebado fire.

Spot fires have been ignited from firebrands falling out of the initial smoke plume. Photo taken at 2:45 PM local time.

Storms in the Moreno Valley

At the same time, wetter thunderstorms formed over the Moreno Valley just to the east of the fire area. These storms, positioned to the east of the dryline, filled the valley with cooler, more dense air. Outflows from these storms combined with the pressure difference between the Moreno Valley and upper Rio Grande valley forced cool, moist air westward through the terrain gap leading to the fire area. The wind direction shifted to the northeasterly and fire activity diminished.

METARs, 18 UTC, 6 July 2003. The observation for Taos, NM is highlighted.

Fire activity has diminished significantly.

Cool, moist gap winds affect the fire as smoke drifts towards the southwest. No visible flame can be seen. Photo taken at 3:20 PM local time.

4:30 PM

The band of virga moved just to the west of Taos and emitted another outflow pulse. The resulting wind shift caused smoke to be blown in a new direction, toward the northeast.

A subsequent outflow from the southwest of the fire area resulted in a second wind shift, which pushed smoke toward the northeast. No visible flame can be seen. Photo taken at 4:30 PM local time.

Outcomes

Several wind shifts impacted the Encebado fire over a two-hour period of time. Fire behavior was most intense during the outflow pulses that were generated by the virga band to the northwest of the fire location. These outflows were drier than the moist gap wind that later affected the fire area. Abundant cloud cover also helped reduce fire activity as the afternoon progressed.

Land/Sea Breezes

A conceptual model shoing land/sea circulations in late morning.

The strength of land/sea breeze winds is proportional to the magnitude of the land and sea temperature difference and dependent on the direction and the strength of the prevailing synoptic flow.

Satellite visible image with overlaid surface wind observations, 1915 UTC, 30 May 02.

An example of sea breezes over Florida in the late morning. Onshore flow is indicated at all coastal locations, while convection has been initiated over northwest Florida. The associated outflow at the surface has overwhelmed the sea breeze signature in this region. By 1915Z, deep convection has also developed rapidly over southeastern Florida, likely due in part to sea breeze convergence.

The sea breeze is most common in the spring through fall months. It results from a circulation that sets up between the ocean and shoreline due to surface temperature differences during the day.

Sea breezes are most easily evaluated under conditions of relatively weak synoptic scale winds.

Sea breeze winds are typically between 10 and 20 knots. The vertical depth of these winds reaches near 500 ft (~150 m), while the associated circulation typically tops out between 1,500 and 3,000 ft (460-915 m) above the surface.

A sea breeze normally begins in the late morning and lasts through the late afternoon, completely ending a few hours after sunset.

The effects of a sea breeze typically extend about 25 mi (40 km) inland, but sea breeze fronts have been known to push inland as far as 100-200 mi (160-320 km) under moderate onshore flow.

Similar types of winds occur near lakes and even rivers. However, the circulation and resulting winds are normally weaker than the sea breeze.

For more detailed information about sea breezes, please visit the COMET module on Sea Breezes.

Evolution

Sea breezes develop perpendicular to the shoreline. As they set up, a sea breeze front develops, separating the warm air over the land surface from the cooler, more moist air over the ocean.

Warm Ocean Water

In an unstable atmosphere, cumulus clouds often develop along the sea breeze front. Depending on the orientation of the coastline, convergence and cumulus formation may be enhanced. These clouds can develop into thunderstorms with lightning, gusty winds, and perhaps precipitation.

Cumulus clouds form over the sea breeze frontal boundary.

Convergence and divergence patterns are determined by coastline configurations.

Cold Ocean Water

With cooler ocean temperatures, such as along the west coast of the U.S., the sea breeze occurs under more stable conditions. Sea breeze winds advect cooler, more moist air inland, often including fog and low stratus.

As these example satellite vis images illustrate, onshore flow can bring fog and low stratus well inland.

Application to Fire Weather

Warm Ocean Water

An example of sea breezes over Florida in the afternoon. Onshore flow ranges from 10-25 knots around the coast, with several areas of deep convection, most of which are well inland. Coastal wind observations are likely influenced at this time in many areas by flows associated with the convective cells.

Wind

On-shore flow can bring a change in wind direction and increase in wind speed, causing a change in direction and speed of fire spread. If convective elements develop along the front, gusty winds accompanying these storms can contribute to erratic fire behavior.

Relative Humidity

RH increases as higher dewpoint air moves onshore, depending on the air mass in place over land.

Temperature

Temperatures typically decrease because marine air is usually cooler than air over the land surface.

Lightning

Convection along the sea breeze front may be strong enough to produce thunderstorms that can generate lightning and ignite new fires.

Cold Ocean Water

Wind

The wind direction becomes onshore. The strength of the winds is often sufficient to change the direction of fire spread. However, relatively stable conditions make thunderstorm development less likely, so winds tend to be more uniform in nature.

Relative Humidity

RH increases as the cool, moist air over the ocean moves inland. There tends to be a lag between the onset of the sea breeze and a substantial increase in relative humidity. So initially, the wind shift will be experienced, causing a shift in the direction of fire spread without a significant decrease in activity. Often behind the sea breeze front, coastal fog moves inland.

Temperature

Temperatures are cooler behind the front which can also help to decrease fire activity.

Satellite image of smoke plumes drifting inland from wildfires along the Southern California coast.

A satellite view of fires associated with the October 2007 Santa Ana event in Southern California. Marine stratus approaches the coast after Santa Ana winds have subsided and the sea breeze pushes smoke plumes eastward. Image acquired 25 Oct 2007.

Example

Dr. Harry D. Johnson of the San Diego State University Department of Geography created 3D fire spread animations of the devastating Cedar, San Diego County, and Paradise Fires, which occurred in October, 2003. Toward the end of the Cedar Fire animation, you can observe the effects of the wind shift. Fire spread reverses direction, and begins moving upslope toward the east. Although onshore flow is associated with less active fire behavior, the fire continues to move inland and up the topographic features. The onshore flow did help firefighters contain the fires.

Summary

Thermally forced land/sea circulations can have tremendous impacts on fire spread and intensity in coastal regions. These mesoscale processes can produce strong, gusty winds, temperature and humidity shifts, and trigger thunderstorms. Depending on sea temperatures, coastal configurations, and diurnal phase, sea breezes can cause fires either to intensify or diminish. You should be alert to these potential impacts whenever forecasting fire weather in coastal regions.

You have finished this section on other mesoscale critical fire weather patterns.

Resources

Fire Regions

Interactive Map of Wildfire Regions

This reference map shows the various fire regions in the United States and Canada. These regions are defined in terms of their typical seasonal periods, rather than by fuel types. To see a map of fuel type distributions, see the USFS-WFAS National Fire Danger Rating System (NFDRS) Fuel Model Map.

Note, when using this tool, that the geographic boundaries are approximate. There are many within-region variations, such as those dependent on elevation or micro-climate, which are not reflected in this broad brush overview.

Check the regions to see notes about regional/seasonal characteristics. (For more detailed information on seasonal characteristics, see the Fire Seasons reference.) Click the Regional Patterns buttons to see where the main synoptic-scale fire weather patterns typically occur.

Breakdown of the Upper Ridge

Post-frotal

Hybrid

Moisture Surge

Tropical Storm

Downslope winds

Fire Seasons

Welcome

This reference section provides two interactive tools for exploring regional variations in fire seasons.

Calendar Grid

This interactive grid lets you display and compare fire seasons for many different climatological regions. This calendar is also cross-referenced with the regions references for this module.

Fire Season Map

This interactive map allows you to view seasonal variations in geographic context. A calendar slider lets you scroll to different times of year, providing a snapshot view of fire seasons in different regions.

Click Calendar Grid and Fire Season Map (above) to access these tools.

Calendar Grid

This reference tool lets you compare wildfire seasons for different regions.

Note, when using this tool, that the seasonal boundaries are approximate. Fire seasons have many yearly variations, and no two fire seasons are exactly alike.

Fire Seasons Map

This reference tool gives you a geographic perspective on wildfire seasons. Click and drag the slider to see seasonal variations for all the wildfire regions at once. (Prescribed fire seasons are not indicated by this reference.) Notice that the seasonal transitions are not sharply defined, as the shift between fire seasons usually a gradual process. You can see this reflected in the color transitions between red, yellow, green, and gray.

Note, when using this tool, that the geographic and seasonal boundaries are approximate. Fire seasons have many yearly variations, and no two fire seasons are exactly alike. Also, there are many within-region variations, such as those dependent on elevation or micro-climate, that are not reflected in this broad brush overview.

January

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March

April

May

June

July

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October

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December

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Glossary

Glossary of Wildland Fire Terminology

Produced by The COMET® Program

Source Regions

Introduction

Pacific High

Northwest Canadian High

Hudson Bay High

Significance

Exercise

Lightning

Lightning Terminology

Terms

Lightning Bust

Dry Lightning

Wet Lightning

Wet/Dry Lightning

Regional Variations

Example

Distinguishing Wet from Dry Lightning

Question 1

Answer

Question 2

Answer

Lightning Ignition

Fuel Status

Fuel Types

Lightning Ignition Exercise

Where are lightning starts most likely?

Question

Fire Spread After Ignition

Question

Compare your answer with our answer:

Spread Factors

Weather Patterns Conducive to Lightning Ignition

Question

Answer

Breakdown of the Upper Ridge

A Critical Pattern for the West

Overview

Introduction

Regional Distribution

Key Points

Ridge Axis Location and Strength

Breakdown Catalyst

Dry Slot-Thermal Ridge-Wind Gradient Alignment

Example Case: 4-5 Aug 2004

Set-up

Set-up

Evolution

Ridge Strength and Position

Southwesterly Winds

Dry/Moist Air Distribution

Frontal Push

Outcomes

Practice Case : 15-19 Jul 2005

Set-up

Question

Evolution

Question

Answer

Outcomes

Synoptic Patterns

Fire Danger and Fire Activity

Practice Case : 10 May 2005

Set-up

Question

Answer

RAWS Summaries

Evolution

Question

Fire Weather

Question

Answer

Outcomes

24-Hour RAWS Summaries

Moisture Surges

Overview

Introduction

Seasons

Trigger

Reference