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fxus65 kriw 212214 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
314 PM MST Sat Nov 21 2009 


Short term...tonight through Tuesday 


First wave of the split trough moved through by middle morning with an 
inch or two of snowfall over the far West Mountains and valleys. 
Second shortwave moving across late this morning and middle afternoon 
with some light snow falling mainly in the western mountains. The 
snow will be ending here in the early to middle evening hours as the 
wave moves through and shortwave ridge builds in. Had some 
borderline high winds around Casper and southeast Fremont County. 
Winds are still quite windy over Natrona and southeast Fremont. The 
winds will taper some in the evening and in the overnight hours 
before picking back up again as the next system approaches. This 
next system currently moving quickly towards the Pacific northwest. This 
system will move on shore after midnight and begin to spread 
moisture into the west shortly before noon. This as the nose of 
cyclonically curved jet moves across southern Idaho and into western 
Wyoming. The jet digs southeast into SW Wyoming and into NE Utah and northwest Colorado 
in the afternoon and evening. Cold front moves through in the late 
afternoon and early evening...with the snowfall winding down to 
mainly the mountains. This will bring a 6 to 9 hour period of 
snowfall of perhaps 0.5 inch per hour snowfall rates in the western 
mountains. Light snow showers will likely persist in the western 
mountains...possibly in the valleys. Meanwhile northwest winds will 
increase in the evening over areas east of The Divide...and even the 
Upper Green basin. 700 mb wave moves through north central Wyoming 
Sunday night as well...bringing a decent chance of snow to the north 
and eastern bighorns and to Johnson County. The northwest flow 
continues over the County Warning Area Monday...with some lingering snow showers. 
Things stabilize and dry out in the afternoon. Northwest winds to 
persist Monday as well then tapering in the evening as the trough 
departs. Large ridge builds in over the Great Basin Tuesday with a 
clipper moving southeast from the Alberta/Saskatchewan region of 
Canada. Some light snow will approach far northern Wyoming. As a 
strong jet on the base of the clipper digs into central Montana. 700 mb 
temperatures of -15c moves into the west tonight and -10c or so over 
the central. With clear patches overnight the far west valleys such 
as the Upper Green could drop into the single digits and close to 
zero. Expect these near zero temperatures to be more likely Sunday 
night with even some sub zero temperatures. 


Long term...Tuesday night through Saturday 


Dry conditions with a warming trend are expected Tuesday night 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... 


VFR to MVFR conditions are expected west of The Divide with isolated 
snow showers continuing through approximately 06z sun. Expect mainly 
local mountain top obscuration through this same time. East of the 
Continental Divide...VFR conditions continue through 18z Sunday. 
Locally strong southwest wind from 20 to 30kts with gusts to 50kts 
is possible between 09z-14z Sunday vicinity of kcod. 


&& 


Fire weather... 


Another trough will approach for Sunday bringing another round of 
snow in the west in the afternoon and evening. Winds will also be 
on the increase with areas of strong winds in the typical spots. 
Northwest winds will increase in the evening east of The Divide and 
the Upper Green basin Sunday evening and persist through Monday. 
Light snow showers will dissipate Monday as high pressure builds 
in. Another system could clip the northeast Tuesday night with some 
light snow. Otherwise dry and milder conditions expected through the 
upcoming week. 


&& 


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


$$ 


Short term...twb 
long term...aem 
aviation...church 
fire weather...twb