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fxus65 kriw 210852 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
252 am MDT sun Mar 21 2010 


Short term...today through Tuesday night 
decided to remove fog from the forecast for this morning with wide 
temperature/dew point spreads and lack of low level moisture on the 
soundings. In addition...a bit too breezy across portions of the 
Wind River basin for fog formation this morning. An apparent 
Gravity wave occurred at the Riverton ASOS where an isolated wind 
gust of 38 miles per hour from the west occurred at 1214 am this morning. We can 
expect one more dry day across the entire County Warning Area today as a flattening 
ridge controls the weather today. Some Lee enhanced clouds will 
likely spread south out of Montana today and clip northern 
Wyoming...particularly to the Lee of the Absaroka Mountains and over the 
northern Big Horn basin. 


The strong circulation located at 50n/135w is prognosticated to ride up and 
over the ridge and track southeast in the form of a strong vorticity lobe 
across southwest Wyoming by 12z Tuesday. This is the same track that the 
previous storm took Friday. The models have been fairly consistent 
in this scenario. As this trough approaches Wyoming...the gradient will 
tighten and the upper ridge will flatten. Southwest winds will 
increase from the Red Desert to cpr coincident with a developing Lee 
side trough. A lead vorticity lobe within the main trough will track east 
across northern Wyoming Monday which will initiate valley rain and snow 
and mountain snow mainly in northwest Wyoming Monday. Will keep isolated lt in northwest Wyoming 
as lifted indices will be between -1 and -2 ahead of the approaching 
Pacific front. In the mean time a Canadian cold front will be 
swinging south across Wyoming Monday night. The leading edge of the front 
will likely be pushing south across northern Wyoming by 21z Monday 
afternoon. The front should then reach the Wind River basin and cpr 
by 03z Tuesday and then The Divide by 06 to 09z Tuesday. Increasing boundary 
level moisture will follow just behind the front. As 700 mb temperatures cool 
from around -3c to -11c the rain will turn to snow from west to east 
and down into the valley floors by 06z Tuesday which will be coincident 
with the onset of increasing precipitation just behind the Canadian front. 
The low level upslope pattern behind the front does not look as 
impressive as previous runs but will increase snow/quantitative precipitation forecast amtns to 
advisory amnts for now based on the favorable track of the storm. 
However any snow that falls Monday will be localized heavier amounts 
due to the convective nature of the precipitation. Decided to decrease probability of precipitation 
Tuesday afternoon from northwest to southeast and kept some likely probability of precipitation in the 
southern absaroka...southern Wind River and Big Horn Mountains with northwest 
difluent flow behind the troughpa. The best qg forcing will remain 
to the south of US over Colorado. Precipitation will decrease Tuesday nt. 


Long term...Wednesday through Sunday 
Wednesday and Wednesday night look dry with upper level ridge moving 
toward the area. There are model differences from Thursday through 
Saturday with timing and strength of the next trough. The GFS is 
faster with the next trough and does not dig it as much with 
tonights run. The GFS has the trough passage on Friday and Friday night 
with the axis to our east at 12z Sat. The European model (ecmwf) is slower by about 
12 hours with the trough and digs it toward The Four Corners by 12z 
Sat...similar to many of the past trough passages over the past few 
weeks. European model (ecmwf) moves the trough through the area Saturday and east of 
our area Sat night. The Canadian model timing more like the GFS 
through 00z Sat but is stronger with the trough...then by 00z sun it 
is more like the European model (ecmwf) with the trough timing. Bottom line moisture 
increases over the west and north Thursday PM and Thursday night with a 
chance of precipitation. The Friday through Saturday forecast will feature 
a chance of precipitation all areas while using a model blend. Precipitation 
should end Sat night leading to a dry day next Sunday for most 
places. However...the GFS/European model (ecmwf) bring moisture into the western mts 
next Sun afternoon and Sun night. Thus will have a chance of precipitation 
in the forecast for that area. As for temperatures...warmer highs Wednesday and 
Thursday then cooler Friday and Sat...then warmer highs next Sunday. 


&& 


Aviation... 
a ridge of high pressure and VFR conditions will prevail today. 
Southwest wind at 20 to 30 kts will occur vicinity kcpr through today 
and this evening. A cold front will push into the west between 
03z-06z Monday with snow showers increasing over the northwest 
mountains and vicinity kjac overnight...frequent mountain obscurations 
and areas of MVFR/IFR conditions expected vicinity and west of a 
kpna-kjac-bzn line by 12z Monday. 


&& 


Fire weather... 
a temperature inversion will be in place this 
morning. Dry weather will be the rule today with a flattening ridge 
overhead. Lee enhanced clouds to the Lee of the Absaroka Mountains 
may have an affect on the West Slope prescribe burn. The next 
Pacific weather system will deliver precipitation to northwest Wyoming Monday 
spreading south and east to the rest of the forecast area Monday 
night as a cold front pushes south across Wyoming. Isolated 
thunderstorms are a possibility in northwest Wyoming Monday afternoon. On 
Tuesday precipitation will gradually decrease from northwest to 
southeast. 


&& 


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


$$ 


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