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fxus65 kriw 202126 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
325 PM MDT Sat Mar 20 2010 


Short term...tonight through Monday night 


High pressure will be over western and central Wyoming tonight 
and Sunday with cold night time temperatures over fresh snow and 
mostly clear skies, a Lee side surface low pressure trough will 
increase across eastern and central Montana and eastern Wyoming 
with southwest winds increasing along and near the east slopes of 
the mountains and through the wind corridor from Rock Springs 
through Casper. The wind will aid mixing with warming temperatures. 
The basins and western valleys will have lighter wind and cooler 
temperatures. The ridge will break down Sunday night with a 
trough of low pressure coming in from the Pacific bringing 
increased chances of precipitation. The air mass will become 
unstable in the afternoon with a chance of isolated thunderstorms. 
During the day rain or a rain snow mix is expected below 8500 
feet. Snow should increase Monday night. A cold front will move 
south out of Montana Monday afternoon. The timing of this front is 
somewhat in question with the spread of precipitation timing along 
and behind the cold front. Precipitation/snowfall amounts are 
quite questionable with convection driving the heavier 
precipitation amounts. 


Long term...Tuesday through Saturday 


The upper trough digs into Utah on Tuesday and into New Mexico Wednesday. 
Cold front moves across the area as well Tuesday. Will maintain the 
precipitation chances Tuesday. Tuesday night will see precipitation decreasing from north 
to south and have adjusted forecast to show this trend. There may be 
lingering precipitation in the south zones Wednesday so went with slight 
chance. Dry weather expected elsewhere Wednesday. Dry Wednesday night and 
Thursday for most places. A few rain and snow showers may spread 
into the northwest mountains and northern Bighorn mts Thursday afternoon as 
moisture increases ahead of the next weather system. GFS is faster 
than the European model (ecmwf) model with this precipitation on Thursday PM. This weather 
system moves across Montana Thursday night and Friday. The 00z GFS 
is about 12 hours faster than the 00z European model (ecmwf) at 00z Sat with the 
system moving across Montana. Will keep precipitation chance across the far 
north Thursday night and Friday siding with European model (ecmwf) model here. Have brought low 
precipitation chance to the western mts Friday afternoon as moisture returns 
to that area ahead of another storm system. Will keep the forecast 
dry on Friday for the rest of the area. Next upper trough is to impact 
the area next Friday night and Saturday. The European model (ecmwf) is stronger with 
the trough than the GFS. Will side with the European model (ecmwf) model for now as 
it has been better at showing these troughs digging into the Great 
Basin over past few weeks while the GFS catches up with time. As for 
temperatures...colder highs on Tuesday then warmer highs Wednesday and Thursday. Slightly 
cooler Friday and next Sat with the impending unsettled weather. 


&& 


Aviation... 


High pressure will build into western and central Wyoming from the 
northern Great Basin tonight. Winds will be generally light across 
the region tonight...however weak low pressure in western Nebraska 
and western South Dakota will generate a west wind between 10 and 
15 knots at krks and a southwest wind at kcpr between 10 and 15 knots 
overnight. Low level moisture from recent snowfall across the Upper 
Green river basin and the central Wind River basin will contribute 
to the formation of fog after midnight. Areas of IFR visibilities in fog 
are expected to develop in the pre-dawn hours at kbpi and kpna...and 
IFR to LIFR visibilities in fog are expected in the central Wind River 
basin....including kriw. High pressure at the surface and aloft will 
move across Wyoming Sunday. As the high pressure center moves across 
western Wyoming Sunday...west southwest wind at krks will gust to 20 
to 25 knots. Southwest wind at kcpr Sunday will range from 15 to 20 knots 
with gusts up to around 30 knots. VFR conditions are expected tonight 
and Sunday with only isolated to scattered high clouds anticipated. 


&& 


Fire weather... 


High pressure aloft will be over western and central Wyoming 
tonight and Sunday. A Lee side surface low pressure trough will 
increase across eastern and central Montana and eastern Wyoming 
with southwest winds increasing along and near the east slopes of 
the mountains and through the wind corridor from Rock Springs 
through Casper. The wind will aid mixing with warming temperatures 
with mixing heights increasing to around 3500 feet in the 
afternoon with good smoke dispersal. The basins and western 
valleys will have lighter wind and cooler temperatures with fair 
smoke dispersal in the afternoon. The thermal belts will be 
noticeable with warm windy and dry conditions. Melting snow will 
provide a source of low level moisture moderating relative 
humidity. 


&& 


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


$$ 


Short term...c.Baker 
long term...Arkansas 
aviation...Smith 
fire weather...c.Baker