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Area forecast discussion 
National Weather Service Riverton Wyoming 
218 am MST sun Nov 23 2008 


Short term...today through Tuesday night 
snowfall appears to have decreased across portions of western 
Wyoming where one to two inches of snow appears to have fallen 
according to area web cams. At least a couple of inches appears to 
have fallen at Teton Pass and around an inch or so of wet snow at 
Alpine Junction. As the shortwave tracks toward central and 
eastern Wyoming later this morning and the associated cold front 
pushes south to central Wyoming...snow showers are expected to 
spread across northern Wyoming behind the front. Snow showers are also 
expected to spread across Sweetwater County this morning as well 
where left front quadrant dynamics will influence lift. Isolated 
activity is expected elsewhere. 


High pressure will then build into the area later this afternoon 
with warming temperatures expected through Tuesday...especially in the 
thermal belts. High temperatures are expected to be in the low 50s in 
Natrona and Johnson counties by Tuesday. 


A digging trough of low pressure out at 143w will close off and 
move across Southern California and Arizona by Tuesday night according to 
the GFS. The NAM is farther north with this feature tracking it 
across central California. Enough of a southern stream of moisture may 
spread far enough north ahead of this Pacific weather feature to 
keep light snow in the forecast for western Wyoming for Tuesday night. 
However it now appears that we may stay in a split flow. We will 
see what the model trends do in the next 24 hours. 


Long term...Wednesday through Sunday 
southwest flow ahead of a large Pacific low pressure trough will 
develop over the west as the trough moves inland. This should 
bring moisture, instability and lift into the western mountains 
supporting some showers in and near the mountains Wednesday and 
Wednesday night. The timing of this transition is problematic with 
differences in the models with both the locations of the exiting 
ridge...the incoming trough and the moisture in the southwest 
flow. The chance for showers should increase on Thursday with the 
chance for precipitation spreading east. With the spread of cloud 
cover and precipitation over the area...temperatures will decrease 
through the extended. Thursday night and Friday may see more widespread 
showers over the area...but the extended models start to have 
significant differences during this time frame. Saturday through 
Sunday the models are quite chaotic...and have gone more with 
persistence lowering precipitation chances but keeping them over 
the area with cooler temperatures continuing. Much of the Pacific 
trough may dig so far south that Wyoming gets into a series of cold 
Canadian troughs bringing modified Arctic air south into the 
northern plains and northern rockies. Moisture would be limited 
but cloud cover and light precipitation would be likely to occur 
with each cold baroclinic band aided by upslope flow. The model 
differences bring a high degree of uncertainty to the extended 
forecast...especially Thursday through Sunday. These model 
differences and problems are also highlighted in HPC discussions. 


&& 


Aviation... 
a passing upper level trough will bring low ceilings and 
visibilities to the west and north today with mountain obscurations 
over the absaroka and Bighorn Mountains. MVFR to IFR conditions can 
be expected in the heavier areas of precipitation. Most of the 
lower elevations will have higher clouds with VFR conditions. The 
Big Horn basin and eastern Johnson County will have local MVFR conditions 
this morning. As the trough passes...clouds and precipitation will 
decrease with improving conditions over the north and west as colder 
and dry air moves in behind the trough with prevailing VFR 
conditions through tonight. 


&& 


Fire weather... 
a Pacific weather disturbance will be moving east across western 
and central Wyoming this morning. Snow showers will spread east of 
The Divide this morning...especially across northern and southern 
Wyoming. Clearing skies are expected this afternoon as the Pacific 
weather system exits to the east. High pressure will then build 
into the area for Monday and Tuesday with warming temperatures...especially 
in the thermal belts. High temperatures are expected to be in the low 50s in 
Natrona and Johnson counties by Tuesday. Strong inversions are expected 
to persist Monday and Tuesday keeping the basins and valleys 
cooler...especially at night and during the morning hours.The 
minimum relative humidity will be in the 20s at many lower elevation sites Monday 
and Tuesday. A Pacific low pressure system will track across the 
southwest United States Tuesday night and may spread enough 
moisture far enough north to produce some snow showers in western 
Wyoming Tuesday night. 


&& 


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


$$ 


Short term...lipson 
long term...c.Baker 
aviation...twb 
fire weather...lipson