490 
fxus63 kunr 211746 
afdunr 


Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota 
1046 am MST Sat Nov 21 2009 


Discussion...17z surface analysis shows low pressure over central 
Saskatchewan with a leading trough extending across western North 
Dakota and central South Dakota...with a cool front extending from 
western North Dakota to near kgcc in northeastern Wyoming. A band 
of cirrus was located over western South Dakota moving slowly 
east. The tight pressure gradient associated with the trough and 
front has created windy conditions across much of the forecast 
area this morning. Have updated forecast to raise winds and high 
temperatures and to adjust the cloud cover. 


&& 


Aviation...gusty southwest winds will become northwest today. VFR 
conditions will prevail through the forecast period. 


&& 


Previous discussion... /issued 243 am MST Sat Nov 21 2009/ 


Discussion...09z surface analysis had weak low over northeastern 
Wyoming. Fairly tight gradient near low with gusty winds in Wyoming and 
portions of the Black Hills above the inversion. Will leave 
advisory in place until 600 am. For today...upper trough over The 
Rockies moves east dragging cold front through the County Warning Area...washing 
out late. Most highs will occur this morning with weak cold air 
advection in the afternoon. For tonight...weak high slips through. 
Have dropped probability of precipitation given dry nature of lower troposphere. 


Sunday...very transient shortwave ridge over the County Warning Area will give way 
to fast moving shortwave Sunday night. Surface high quickly shifts 
south with return flow over western portions of the County Warning Area. Dry 
during the day...but enough forcing/moisture present for low probability of precipitation 
Sunday night. Temperatures near guidance. 


Monday...some differences in model handling of shortwave trough. 
00z NAM/ECMWF/Gem trending south and east. 00z GFS more northerly 
track. Will go with model trends at this point...which will keep 
best precipitation away from the County Warning Area. 


Extended...aforementioned system will continue its departure on 
Tuesday...with some precipitation possibly lingering over south central 
South Dakota. GFS brings next wave through the region Tuesday 
night...though has had some trouble with the placement of this 
system over the past few runs. Therefore...have limited low probability of precipitation 
to the eastern/northern County warning forecast area. Ridge starts building in for middle-week 
bringing a return to dry conditions. 


&& 


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


$$