The verification for the 24-hour period ending about 15z on 22 August 2002 is shown below, taken from the KDVN radar estimates. Note that a band of precipitation in excess of 2" took place from west of Waterloo IA to east of Rockville IL. Within this band, precipitation estimates of as much as 10" or so can be found in a few grid-boxes (grid boxes are about 2-3 km on a side, depending on the distance from the radar site). Almost all of this rain occurred after midnight local time on 22 August.
Representative precipitation amounts for 22 August 2002 in the area of this precipitation band were 5.99" in Dubuque, IA and 5.85" in Rockford, IL.
Additional heavy rains took place the next night and morning (22-23 August), as shown in the graphic below on the right from the Climate Prediction Center 0.125° precipitation analysis for the 24-hour period ending 12z 23 August 2002. Between 35 and 100-mm (1.6"-4.0") of rain occurred over eastern IA and northwestern IL during this period. The CPC analysis for the previous 24-hour period, coinciding with the rains in the radar estimates above, appears on the left.
Synoptic evolution
The graphic below shows the initial conditions from the GFS analysis. At 12z on 20 August, high pressure was located over MI. A front extended from low pressure over southeastern MT along the high plains to the Red River valley of TX, then meandering eastward to the eastern Carolinas (upper left). Broad southerly flow was becoming established at the surface and in the lower troposphere over the central Great Plains with warm advection indicated at 850-hPa north and east of a 850-hPa warm front over northeastern KS extending across southern NE (upper right). Moist air was moving northeastward into IA, with a very strong moisture gradient at 700-hPa (lower left) and 850-hPa (not shown). Strong directional wind shear existed in the lower troposphere over and to the west of IA (See difference between 850 and 700-hPa winds.). At 500-hPa (lower right), a ridge-line was just west of IA, and a short-wave trough was upstream of the ridge with vorticity maxima over southeast CO and the west TX/east NM border.
Initial and evolving atmospheric conditions, 12z 20 August to 12z 23 August 2002
Clockwise from upper left: Sea level pressure and frontal surfaces; 850-hPa temperature (shaded), heights (dam, contoured at 3-dam intervals, winds (barbs); 700-hPa relative humidity (shaded), winds (barbs); 500-hPa heights (dam, red contours), vorticity (black contours, shaded above 12, units 10-5 s-1
To see how the atmosphere evolved over the next 72 hours according to the GFS analyses, click on the button below to see the animated graphic.
6-hourly MSLP, 850-hPa, 700-hPa, and 500-hPa fields from 18z 20 August to
12z 23 August 2002
In the SLP analyses we see the initial southeastern MT low move eastward and weaken to about 1014-hPa as it crosses WI and upper MI into Ontario over the 72-hour period, with a wavy front moving very slowly in its wake southeastward, to a position from PA across central OH/IN/IL to northern MO and KS after 72 hours.
At 850-hPa, a disturbance propagates eastward and also weakens from 144-dam to slightly less than 150-dam in depth as it tracks eastward into Canada from the upper Midwest. A 30-40 kt southwesterly jet lies southeast of the disturbance over the Midwest and central and southern Plains, weakening as the disturbance moves away, only to redevelop at 72 hours as a new 850-hPa trough develops over the northern Plains.
A southerly flow results in pooling of high relative humidity (RH) at 700-hPa along frontal boundaries set up over IA, IL, MN, and WI. This pooling is associated with the heavy rains from 21-22 August 2002 over the Quad Cities IA CWA. As the frontal boundary sags slowly southward during the night of 22-23 August, more heavy rains occur over Quad Cities area and east-southeastward through central IL, again with moisture pooling. Precipitable water (not shown) is from 1.75-2.25" over the areas near the frontal boundary receiving the heaviest rain.
At 500-hPa, we see a number of vorticity maxima scattered about the High Plains initially then propagating and weakening, as new maxima take their place and go through a similar life cycle. These analyzed vorticity maxima are generally associated with mesoscale convective systems (MCSs) producing heavy rain. These can be found between 00z and 12z 22 August over eastern IA, northern IL and southern WI, and over a similar area between 00z and 12z 23 August. The associated shortwave troughs have an amplitude of perhaps 2-3 dam.