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Area forecast discussion 
National Weather Service Riverton Wyoming 
202 am MST Sat Nov 22 2008 


Short term...today through Monday night 
transitory short-wave ridge axis over western Wyoming early this 
morning. Southwest flow aloft will be on the increase late this 
morning and continue through the day ahead of a weak but deepening 
upper level short-wave trough. Latest global models digging this 
feature farther southward late tonight into Sunday morning. Increased 
snow chances over Lincoln...Sublette and western Sweetwater County 
somewhat to account for better qg-forcing and a strong middle-level 
baroclinic zone there. Middle and low level moisture lacking with this 
feature so not expecting much in the way of measurable snowfall. 1 
to 2 inch amounts in the higher mountains at best. Any light 
snow...however along Interstate 80 in southwest Wyoming can be a 
major headache for travelers so future shifts will need to watch 
this possibility closely. Otherwise...digging short-wave trough moves 
rapidly into northwest Colorado Sunday afternoon with strong 
subsidence moving over the County Warning Area as a high amplitude ridge translates 
eastward over western and central Wyoming Sunday night and Monday. 
Moderately strong valley and basin inversions will develop Sunday 
night/Monday morning and continue through at least Monday night. 
Lack of surface snow cover should keep the inversions from 
becoming very strong but did Lower Valley temperatures down 
several degrees and raised mountain temperatures somewhat for 
Sunday night through Monday night with 700 mb/~10k mean sea level temperatures prognosticated 
to approach 3c/37f Monday afternoon. 


Long term...Tuesday through Saturday 
a large high pressure ridge aloft over the western U.S. Will 
dominate the weather across western and central Wyoming Tuesday 
with dry conditions and above normal temperatures resulting. 
Tuesday night 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 showers in and near the mountains. 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 Wednesday through Thursday with the chance for 
precipitation spreading east. With the spread of cloud cover and 
precipitation over the area...temperatures will decrease through 
Friday. 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 the models are quite 
chaotic...and have gone more with persistence lowering precipitation 
chances but keeping them over the area with cooler temperatures 
continuing. The model differences bring a high degree of 
uncertainty to the extended forecast...especially Wednesday night 
through Saturday. These model differences and problems are also 
highlighted in HPC discussions. 


&& 


Aviation... 
VFR conditions with increasing high and middle clouds from the west 
will prevail through 00z Sunday. A Lee-side surface trough will 
result in gusty southwest wind between krks-kcpr...and along the east 
slopes of The Divide and Big Horn Mountains. A Pacific storm system 
will move across Idaho into Wyoming Saturday night spreading snow 
showers in from the northwest to near a kjac-kcod line by 06z with areas of 
MVFR and local IFR conditions along with mountain obscurations. 


&& 


Fire weather... 
high pressure aloft will hold on briefly this morning ahead of a 
weak upper level Pacific trough that will bring scattered light 
snow showers to many areas of the west tonight through Sunday morning. 
Moderately strong valley inversions will develop in the wake of 
this trough Sunday night and Monday as strong high pressure moves over 
the state. Also...very dry air at mountain top level will keep relative humidity 
recoveries in the higher elevations in the poor to very poor range Sunday 
and Monday night/S. The strong upper level ridge begins to 
transition eastward late day Tuesday with increasing southwest 
flow developing Tuesday night and Wednesday. 


&& 


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


$$ 


Short term...church 
long term...c.Baker 
aviation...aem 
fire weather...church