. To advance one frame, use the Plus 1 button 
. To resume looping, use the Forward button 
. Loop speed is increased by repeatedly clicking the Fast button 
and decreased by doing the same to the Slow button 
.Through the day 3 time frame, the medium-range ensemble forecasts (MREFs) can be used to determine the large scale flow regime (and given the likely flow, the risk of winter weather in that particular regime). In highly predictable regimes, the large scale flow can be forecasted well as far as 10-14 days prior to verification time. Toward the day 3 end of the medium range, specific features that may result in significant winter weather can be assessed.
The purpose of this case is to give the forecaster an example of how to:
1.Use the MREFs to assess the large-scale flow regime and detemine the risk of winter weather.
2.Understand limitations of the MREFs with respect to details of significant winter weather that may take place, including known ensemble system deficiencies.
In going through this case, when you open a separate graphic, an image or loop will pop up in a separate window. If one is already displaying, it will be replaced by the one you just selected. Loops automatically start looping. To freeze a frame, click the Stop button . To advance one frame, use the Plus 1 button . To resume looping, use the Forward button . Loop speed is increased by repeatedly clicking the Fast button and decreased by doing the same to the Slow button .
Medium range ensemble forecast systems (MREFs) are of rather coarse resolution because of computational limitations at operational forecast centers. This coarse resolution, coupled with the unpredictability of smaller-scale phenomena at longer time ranges, limits the use of MREFs generally (though not always) to a "heads up" for the possibility of significant weather in the county warning area (CWA) of interest to a NWS weather forecast office (WFO).
In the eastern U.S., phenomena such as cold air damming, coastal cyclogenesis (augmented by land-sea contrast), jet streak circulations, and other relatively small scale phenomena are important in determining exactly where severe winter weather will have an impact. Certain large-scale circulation patterns are conducive to the occurrence of these smaller scale phenomena, and the MREFs are helpful in determining their likelihood.
Click on the next menu item to see a specific case where the MREFs indicated large-scale circulations that would increase the likelihood of winter weather in the eastern U.S.
by Dr. Bill Bua UCAR/COMET at NCEP
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