Western Region/West Coast Techniques
(last updated 8/3/2005)
An example of using the "Satellite Fog Product" in predicting dense fog over south-central and southeast Montana and north-central Wyoming on November 28, 1999 (Canepa, Richard and Mark H. Strobin, NWSO Billings, MT, 2000. Western Region Technical Attachment No. 00-06, 4 pp.)
During the late afternoon and evening of November 28, 1999, dense fog moved from southeast Montana into the Billings area. The fog was dense enough to close Billings Logan International Airport. Dense fog, defined as visibilities of 1/4 mile or less, is uncommon across NWSO Billings County Warning Area (CWA). NWSO Billings CWA incorporates south-central Montana, southeast Montana, and Sheridan County Wyoming. Based on National Climatic Data Center normals, dense fog during the month of November across NWSO Billings county warning area occurs on average one day during the month. This Technical Attachment will show the utilization of satellite imagery, in particular the fog product, and the synoptic situation, which allowed forecasters to accurately predict the fog. Dense Fog Advisories were issued well ahead of the occurrence of dense fog in the Billings area.
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Mesoscale ETA forecasts of stratus surges along the California coast (Archer, Scott and David Reynolds, NWSFO Monterey, CA, 1996. Western Region Technical Attachment No. 96-03, 4 pp.)
With the introduction of the new NCEP Mesoscale Eta model (MESOETA), forecasters along the west coast can anticipate better forecasting of mesoscale phenomena. One such phenomena is the coastal stratus surge or "southerly surge" which occurs on the average of one to two days a month along the central California coast. Forecasting the onset of stratus along the California coast and adjacent valleys is a major priority at the NWSFO in Monterey, California. With the MESOETA now available, forecasters can view some of the larger mesoscale phenomena such as the Catalina eddy and the southerly surge, and better predict stratus onset/dissipation times. This Technical Attachment (TA) will discuss two cases in which the MESOETA, although indicating that surge conditions were possible, failed to turn the winds in the boundary layer from northerly to a southerly direction as was observed. AVHRR satellite imagery, coastal and buoy reports, the Fort Ord profiler, and the Oakland sounding will be used to confirm the presence of the southerly surges. This (TA) will not discuss the theory of orographically trapped waves, Kelvin waves, gravity waves, or density surges that are associated with southerly surges. Instead, this (TA) will focus on the ability of the MESOETA to forecast stratus surges by comparing 10-meter and 850 mb wind forecasts to observations. The MESOETA was able to predict the general synoptic scale setting and, in well defined surges, the 850 mb flow pattern, yet most often failed to predict the southerly reversal of winds at the surface.
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Meteogram analysis and interpretation (McNulty, Beth, NWSFO Glasgow, MT, 2001. Western Region Technical Attachment, No. 01-13, 11 pp.)
This technical application note describes how to make the best use of a meteogram. The forecaster's cookbook outlines the step-by-step technique for analyzing and interpreting meteograms with the best results. Other sections give the historical background and detailed descriptions of the analysis techniques shown in the cookbook.
Two main versions of forecast meteograms are discussed, the meteogram available from the Air Force Weather Agency (AFWA) using the MM5, AVN, NOGAPS, or MRF models; and the meteogram available from the Environmental Modeling Center (EMC) of the National Centers for Environmental Prediction (NCEP), using the ETA model. The AFWA meteograms use the same format regardless of the model used. Although previous Air Weather Service Technical Reports have mentioned the use of observed time trend charts for short-term forecasts, there exists no systematic discussion of the use, analysis, and interpretation of meteograms. This technical application note remedies that deficiency. The military forecaster's emphasis on a point forecast, whether airfield, drop zone, or battlespace, makes the meteogram especially useful.
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Clearing index verification study (Gibson, Chris , NWSFO Salt Lake City, UT, 2001. Western Region Technical Attachment, No. 01-01, 3 pp.)
A verification study of gridded forecasts of air quality parameters from the Forecast Systems Laboratory's Graphical Forecast Editor (GFE) has been completed. The study compared GFE surface weather grids and actual parameters calculated from soundings at Salt Lake City (SLC), UT and Grand Junction (GJT), CO. GFE grids of mixing height (depth of the well mixed boundary layer), transport wind (average wind in the mixed layer) and the Clearing Index (index of stability and transport) were evaluated. Eta forecasts for 18Z and 00Z valid times were examined and compared with the 00Z sounding. GFE surface temperature (T) fields and maximum temperature (MaxT) fields were utilized in calculating the mixing height. The calculations using MaxT proved superior to calculations using T grids.
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Using varied enhancement curves to better evaluate fog and stratus with RAMSDIS (St. Jean, Daniel, NWSFO Boise, ID, 1997. Western Region Technical Attachment, No. 97-40)
Many examples and uses of the fog-reflectivity product imagery derived from the GOES-8 and GOES-9 imagers have been recently documented. (See NWS Western Region TA-Lites 96-1, 96-4, 96-8 and 96-15, and TA 95-25.) Detection of fog and stratus using the RAMSDIS fog-reflectivity product has proven to be an invaluable tool for forecast operations, not only in detecting fog and stratus, but also in distinguishing cloud phase, forest fires and snow cover vs. stratus when used in conjunction with other imager channels. Furthermore, the ability to toggle RAMSDIS's enhancement curves on a given image loop can be significant in making optimum qualitative use of the fog-reflectivity product.
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Applications of a diagnostic wind model to stratus forecasting for aircraft operations in the San Francisco Bay region (Strach, Walter and F. L. Ludwig, 1997. Preprints of the 7th Conference on Aviation, Range, and Aerospace Meteorology, Long Beach, CA., pp 390-95.
San Francisco International Airport (SFO) is one of the five busiest airports in the country in terms of passenger travel. Its Instrument Landing System (ILS) equipment allows aircraft to land under most weather conditions, but the close spacing of the two major runways requires that the rate at which aircraft land be reduced by half whenever there is low level cloud cover such as stratus in the airport's approach zone. Summer stratus frequently fills the basin of San Francisco Bay during the night. It usually moves into the bay just after the evening peak aircraft demand for arrivals ends at SFO. Clearing generally occurs prior to the morning peak arrival period. If stratus arrives earlier or clears later than normal, air traffic flow becomes saturated, resulting in airborne and ground delays.
Since October 1995, the outputs from a diagnostic wind model have been used operationally as a forecast tool by National Weather Service (NWS) meteorologists of the Central Weather Service Unit (CWSU) at the Federal Aviation Administration (FAA) Oakland Air Route Traffic Control Center in Freemont, CA. The ready availability of detailed wind fields has proven to be quite helpful in forecasting the behavior of stratus in the region. This paper describes the wind analysis methodology and how it is used in forecast operations.
