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fxus63 kunr 212251 
afdunr 


Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota 
248 PM MST Sat Nov 21 2009 


Discussion...21z surface analysis shows low pressure centered over the 
Saskatchewan/Manitoba border with a leading north/S oriented trough 
across the central Dakotas...and a trailing cold front currently 
crossing western South Dakota. 


For tonight...winds will diminish early this evening as a weak surface 
high slides into the area. Temperatures will fall to near normal. 


On Sunday...a weak shortwave ridge will quickly cross the area in 
advance of a fast moving shortwave on Sunday night. The surface high 
will quickly shift south with southerly return flow developing over 
western portions of the County Warning Area. There will be enough forcing/moisture 
present to include low probability of precipitation on Sunday night. Temperatures will 
continue near guidance. 


On Monday...some differences remain in model handling of shortwave 
trough. 00z NAM/ECMWF/Gem continue to trend south and east...with 
the GFS continuing to indicate a more northerly track. Will go with 
model trends at this point...which will keep best precipitation away 
from the County Warning Area...but will still include a chance of snow or rain with 
the best chance of precipitation over our south central South Dakota 
zones. 


Extended...upper low is expected to be located over eastern Nebraska 
into western Iowa on Tuesday...bringing a chance of rain and snow to 
portions of south central South Dakota. Several GFS ensemble members 
indicating an upper low developing across this area. Gem and European model (ecmwf) 
also leaning toward this solution. System will exit the area 
Tuesday afternoon. Shortwave will sweep across the County Warning Area early 
Wednesday morning. At this time it appears County Warning Area will remain dry through 
midweek. Expect ridge to build across the western Continental U.S. Late next 
week. Rmop indicates high predictability with this scenario...so 
will keep forecast dry through late next week. 


&& 


Aviation...VFR conditions can be expected through the forecast 
period. 


&& 


Unr watches/warnings/advisories... 
South Dakota...none. 
Wyoming...none. 
&& 


$$