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fxus65 kriw 230935 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
235 am MST Monday Nov 23 2009 


Short term...today through Wednesday night 
a strong Pacific cold front plowed across the County Warning Area earlier this 
evening leaving in its wake 2 to 3 inches of snow across much of 
Sweetwater County. The associated trough axis will track across the 
area this morning along with the associated q vector forcing. Some 
boundary layer moisture will spill over east of The Divide but most 
of this moisture should remain north and east of The Owl creek mountains 
this morning. Presently...a large area of virga is evident on radar 
east of The Divide that seems to be associated with the southbound 
Canadian cold front as the more southern reflectivity elements are 
moving east and the northern most elements are moving south causing 
frontal convergence. Some of this snow could be reaching the ground 
over eastern Fremont County where slightly stronger echoes are being 
detected. The threat of snowfall should persist the longest in the 
Casper area where q vector forcing will persist the longest. A 
secondary shortwave trough on the back side of the main long wave 
trough will push south across Wyoming today. Isentropic lift in 
western Wyoming ahead of this next feature should provide western Wyoming with 
a brief period of light snow this morning. Will keep low chances of 
light snow going for the remainder of the day today mainly across 
western Wyoming along with lesser chances in the Big Horn Mountains to account 
for this secondary trough to move through with deep moisture 
lingering across western Wyoming today. Clouds will linger for most of 
the day in the Casper area today along with a possible flurry as 
deep moisture will linger there through late this afternoon. 
Significant drying will take place elsewhere. 


Tonight will feature a cold clear night with some below zero 
readings in the west. Should see some patchy fog early Tuesday 
morning in the far west valleys once warm air begins to advect into 
the area aloft from the west. Tuesday morning will feature a 
transitory ridge before the next upstream trough now located at 150w 
off the Pacific coast travels up and over the ridge now...along the 
Pacific coast...and comes diving southeast out of Alberta as an 
Alberta clipper and clipping out northeast zones by Tuesday night. 
The models continue to oscillate between missing our County Warning Area altogether 
with this feature to affecting northern Wyoming and most areas east of 
The Divide and this will probably continue until the trough rides 
over the ridge. Will go with the idea of giving the Big Horn Mountains 
the highest chance of snowfall with this feature and will up the 
winds in Johnson County Tuesday night through Wednesday. Another 
clear calm night for Wednesday night is in store with more air 
advecting back into the area aloft once again for strengthening 
temperature inversions. 


Long term...Thursday through Monday 
the extended will begin with a ridge over the forecast area Thursday 
and Thursday night. This will result in dry conditions and above 
average temperatures Thursday. Medium range models are in good 
agreement in a longwave trough pushing into the West Coast Thursday 
night...and then east into the intermountain west/Great Basin 
Friday. A major cold front is also expected to push through the 
forecast area late Friday/Friday night. However...medium range 
models are having much difficulty in determining where the best 
energy/shortwaves will move across The Rockies...and thus where the 
bulk of the precipitation will occur. 00z GFS is much more bullish on 
developing upslope conditions Saturday/Saturday night while Gem and 
European model (ecmwf) keep the flow more northwest...with much less precipitation. 
Having said that...have increased probability of precipitation slightly for this 
system...especially for Friday night west...and then everywhere on 
Saturday. Will continue with slight probability of precipitation most everywhere on 
Sunday...due to the uncertainty of this system. Have cooled 
temperatures down for the weekend...as European model (ecmwf) and GFS have been 
consistent in bringing in at least -10 degree celsius 700mb temperatures for 
Saturday and Sunday...which roughly translates to around 40 degrees 
fahrenheit at 5000 feet if atmosphere is fully mixed. Thus...if 
cloud cover and/or precipitation becomes more apparent...the current 
forecast maximum temperatures for this weekend will need to be lowered. 




&& 


Aviation... 
a weather system will continue move through the area today. Snow 
showers will occur over the mountains with MVFR conditions at times 
along with mountain obscuration. The snow showers will be ending 
around 00z Tuesday. The lower elevations will see VFR conditions today 
into this evening. Some MVFR conditions will occur in east central 
Wyoming due to snow showers...from kbyg to kcpr...until 18z...then VFR 
conditions will prevail through 00z Tuesday. 


&& 


Fire weather... 
some snow will linger mainly in the Casper area this 
morning before the Pacific weather system exits to the east. Some 
light snow will persist in western Wyoming today as a secondary cold 
front pushes south across the area. High pressure will build into 
the area tonight for clear and cold conditions along with strong 
basin and valley inversions. Dry weather will be the rule through middle week 
with the exception of an Alberta clipper affecting mainly Johnson 
County and the Big Horn Mountains with a chance of light snow and 
increasing north winds Tuesday night and Wednesday. Strong 
inversions can be expected Thursday morning east of The Divide 
and Wednesday and Thursday mornings west of The Divide. 


&& 


Riw watches/warnings/advisories... 
none. 
&& 


$$ 


Short term...lipson 
long term...wm 
aviation...Arkansas 
fire weather...lipson