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Area forecast discussion 
National Weather Service Riverton Wyoming 
259 am MST Sat Nov 21 2009 


Short term...today through Monday night 


The baroclinic Leaf has moved into western Wyoming this 
morning...though cloud tops continue to warm based on current 
satellite imagery. Radar reflectivities noted over central and 
eastern Idaho this morning...with showers continuing to progress 
towards western Wyoming. Expect snowfall to develop over the western 
mountains and valleys this morning...and continue through the 
afternoon hours. The trough associated with the baroclinic lift sees 
its axis move into western Wyoming by late this afternoon...with 
shower activity weakening as the axis passes. The majority of 
moisture associated with this feature will drain over the western 
valleys and mountains...as well as a few snowflakes over the Big 
Horn Mountains. Not expecting much of any weather for areas east of 
the Continental Divide...other than continued cloudiness. 


The trough axis will move east of the County Warning Area late Saturday night into 
early Sunday morning...with a brief transitory ridge building over 
the area. On the heels of the exiting trough will be 
another...stronger area of low pressure which will move into the 
intermountain west by the middle morning hours of Sunday. This second 
trough has taken more of a negative tilt in recent model runs...the 
introduction of convectively based snowfall will allow for the 
potentially for brief...heavier snow showers. This system will also 
bring in continually cooling 700 mb temperatures...with cooling daytime 
high temperatures expected for the latter half of the weekend and 
into the work week. Snow and rain associated with this system will 
become more widespread Sunday evening...moving east of The Divide to 
potentially impact much of the state with at least brief light snow. 
The axis with this trough will move into central Wyoming late 
Sunday...and move through the County Warning Area by early Monday afternoon. Moist 
northwest flow will move over the area as this system departs...with 
some wraparound moisture continuing light snowfall over northern 
Wyoming into Monday evening. As wraparound moves further 
east...another ridge will build over the area...helping to weaken 
snow and rain Monday night. 


Long term...Tuesday through Saturday 
dry conditions with a warming trend are expected Tuesday through 
Thanksgiving day. The next Pacific storm system will bring a chance 
of snow to the west Friday and cooler conditions with chances of 
snow across central Wyoming Friday night and Saturday. 


Latest European model (ecmwf) showing a solution similar to last few Gem runs with a 
clipper diving down front side of intermountain west upper ridge 
into backside of upper low over upper Mississippi Valley Tuesday and 
Tuesday night...keeping system well north and east of Wyoming. 
GFS...showing a stronger clipper diving into the northern 
plains...is also trending in this direction. Forecast sides with 
European model (ecmwf) and Gem resulting in a dry forecast Tuesday with high 
temperatures around 10f warmer than Monday...or upper 30s to near 50 
most basins and valleys. Upper ridge expands across area Wednesday 
and Thanksgiving day with dry and unseasonably mild high 
temperatures though valley inversions will continue to keep 
temperatures 10f-15f below full mixing potential as has been the 
case with the last few days of clear skies/light wind. Nighttime 
lows will be more near normal with good radiational cooling. Model 
spread increases Friday and Saturday with GFS continuing to handle 
split flow of last few weeks poorly...too far north and too 
progressive with incoming Pacific energy. European model (ecmwf) splits next Pacific 
system along West Coast Friday morning with most of this energy 
diving into the Great Basin Friday night. European model (ecmwf) is likely not 
progressive enough...holding a closed low back over Arizona/nm into 
Sunday. Forecast brings in chances of precipitation into the west 
Friday...spreading east of The Divide Friday night and Saturday 
along with cooler temperatures more near seasonal normals. 


&& 


Aviation... 
a cold front will move through the area today. Windy weather will be 
common across much of the region...with speeds of 20 to 30 knots 
with gusts to 35 knots. Wind speeds will diminish to 10 to 15 knots 
after 00z sun in most locations. Snow will occur in the mountains 
and parts of the far west through 18z...then coverage will decrease 
after 18z. MVFR to IFR conditions will occur in areas of snow in 
western Wyoming through 21z...then VFR to MVFR conditions are expected 
with scattered snow showers through 00z sun. Expect widespread mountain 
obscuration through 21z...then mainly mountain top obscuration 
through 00z sun. East of the Continental Divide...VFR conditions 
will occur today through 06z sun. 


&& 


Fire weather... 
snowfall expected over the western mountains and valleys today. The 
passage of a cold front through the state will bring cooler 
temperatures across much of the area today...with increased cloud 
cover. Snow and rain are expected to weaken this afternoon...with a 
very brief period of high pressure anticipated tonight. The next 
approaching system for Sunday looks stronger than the current 
system...with forecast models indicating some instability and 
possibly convective snow fall over portions of the area. 
Temperatures will continue to cool through the weekend and into 
early next week. Wind will remain elevated today and are expected to 
weaken tonight. However...the arrival of the second system will 
again cause a tightened pressure gradient over central Wyoming 
resulting in increased wind Sunday evening. 


&& 


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


$$ 


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