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fxus65 kcys 022133 
afdcys 


Area forecast discussion 
National Weather Service Cheyenne Wyoming 
235 PM MST Tuesday Dec 2 2008 


Short term...tonight through Thursday afternoon... 


Tonight...winds continue to subside thus have cancelled the High 
Wind Warning. Primary challenges will be the drastic change to cold 
weather and the long duration snowfall event beginning tonight. We 
are in the warm before the storm and the warm sector this afternoon. 
Strong cold front now poised across eastern Wyoming will quickly 
sweep south across our counties this evening based on impressive 
pressure rises in the Post frontal cold front airmass. Will see a 
healthy baroclinic band of light to moderate snow overnight. 
Snowfall accumulations on hard paved surfaces will be minimal 
initially due to warm ground. However...expect to see snowfall 
accumulations after midnight as ground cools and much colder airmass 
invades. Have issued a Winter Weather Advisory for all of our 
counties starting tonight...to alert our customers to the drastic 
change to colder temperatures and long duration light to moderate 
snowfall event. Expect strong north winds for a few hours after cold 
frontal passage then winds will subside. Will see areas of blowing 
and drifting snow across the mountains due to winds in brisk flow 
aloft. 


Wednesday...expect light to moderate snow to wane across our 
northeast counties as lift wanes. However...across most of southeast 
Wyoming...will see periods of light to moderate snow continue along 
the baroclinic band with lift aided by upslope...cyclonic flow aloft 
and weak shortwave trough ripples oozing southeastward in the 
northwest flow aloft. Have gone with colder guidance highs. 


Wednesday night...light to moderate snow will once again increase in 
areal coverage as next stronger shortwave trough dips southeast 
across Wyoming...enhancing upslope along The Spine of the Laramie 
range. GFS has been doing better with these snowfall events this 
season and thus have accepted its quantitative precipitation forecast amounts...as NAM looks too dry 
as is its typical tendency so far this winter...to be too scant with 
quantitative precipitation forecast. 


Thursday...light to moderate snowfall event will translate to the 
southern half of our counties with heaviest snow falling in this 
period from Rawlins to Laramie to Cheyenne and to Sidney. 
Again...colder guidance highs look best due to falling snow and cold 
airmass in place with 1000-500 mb thicknesses near 5250 meters. 
Rubin 


Long term...Thursday night through Tuesday 
the upper level pattern in the early part of the long term will 
develop into an upper ridge north of a closed upper low just off the 
Southern California coast. The upper level low closing off will be 
slightly farther west than a traditional Rex block and as a result 
northwest flow will be able to continue over the Rocky Mountain 
Front Range. Friday precipitation chances will diminish across the 
area with a warming trend beginning to occur. Temperatures will 
still generally be slightly cooler than normal...but quite a bit 
warmer than Thursday. Saturday and Sunday will have above normal 
temperatures as the warming trend continues beneath the upper ridge. 
Model differences emerge Sunday into Monday as the European model 
shows the closed low ejecting across The Four Corners region and 
combining with an upper short-wave in the northern stream. The GFS 
solution simply shows the short-wave aloft moving southeast across 
the area. With either solution a cold front will move across the 
area late Sunday or early Monday...with the European solution 
producing more widespread precipitation across the area than the 
GFS. Made very little change to the extended time-frame with this 
forecast package as inherited forecast handled the model differences 
well late in the period with some precipitation chances mentioned 
Monday and slightly lower chances Tuesday as both solutions show a 
return to at least weakly ridged upper level flow. 


&& 


Aviation...widespread LIFR and IFR through Wednesday night with 
ceilings and visibilities improving across east central Wyoming and 
western Nebraska during the day on Wednesday. Rubin 


&& 


Cys watches/warnings/advisories... 
Wyoming...Winter Weather Advisory from 6 PM Tuesday to 6 PM Thursday for 
wyz061-wyz062-wyz063-wyz064-wyz065-wyz066-wyz067-wyz068- 
wyz069-wyz070. 
Winter Weather Advisory from 6 PM Tuesday to 6 PM Wednesday for 
wyz059-wyz060. 
NE...Winter Weather Advisory from 6 PM Tuesday to 6 PM Wednesday for 
nez001-nez002-nez003-nez019-nez020-nez021-nez054-nez055. 


&& 


$$ 


Short term/aviation...Rubin 
long term...CArpenter