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Area forecast discussion 
National Weather Service Riverton Wyoming 
121 PM MDT Wednesday Mar 17 2010 


Short term...tonight through Saturday 
major change back to winter is still on track for much of central 
Wyoming Thursday night. 12z runs have generally trended stronger 
again compared to the 06z runs. Details are still different in 
each model but they all agree in solid upslope combined with qg 
forcing and upper jet streak dynamics. The timing is also 6 hours 
faster than yesterday. The brunt of the storm is expected to 
impact Fremont and Natrona counties...eventually extending down 
into at least eastern Sweetwater County. The NAM has a longer 
duration of right entrance region jet streak dynamics Thursday 
night. The GFS initially has the jet in a favorable position 
around 00z before shifting south and allowing the left exit region 
of the northerly jet to spread fairly strong upper divergence into the 
west and as far east as lnd and riw during the evening...spreading 
east overnight. Upslope looks like it will extend up to around h6 
with speeds of 20-30kts at 700 mb...up to 40kts in the Red Desert. 
Combination of moderate to strong upslope component...optimal 
temperatures through the evening...jet streak dynamics potential and 
qg bulls-eye is enough to warrant a Winter Storm Watch for our 
central zones for Thursday night through Friday morning. Main 
brunt looks to be Thursday night but some snow will linger into 
Friday morning. -33c core at 500 mb in north-northwest flow aloft should keep snow 
showers going through the day Friday...especially downwind of the 
big horns...southern winds...Owl Creek...bridgers and maybe even the 
southeastern end of the absarokas. Surface gradient should be sufficient 
for windy conditions behind this front with areas of blowing snow 
as temperatures drop Thursday night...especially open areas of eastern 
Fremont...western Natrona...over South Pass and into the Red Desert. 
Roads will likely be very dangerous by Thursday evening as wet 
roads freeze over and then get coated with fresh snow. 


Conditions will improve and clear off Friday night for a clear and 
cold night with areas of fog. Temperatures will slowly rebound 
Saturday with late March sun. 




Long term...Saturday night through Wednesday 
Saturday looks dry and cool as trough exits the area. Northwest flow aloft 
on Sunday with dry weather expected and milder highs. Upper ridge 
axis over the region Monday with another dry day expected with 
warmer highs. In the far West Mountains...there may be enough 
moisture in the impending SW flow for a few snow showers Monday 
afternoon. Moisture increases in the west Monday night and Tuesday with 
increasing chance of precipitation as next trough takes shape over the 
western U.S. And have adjusted forecast accordingly. East of The 
Divide Monday night will be dry with a slight chance of precipitation on Tuesday. 
GFS model more potent with trough Tuesday night and Wednesday as it moves 
through The Rockies than the European model (ecmwf). Will keep a chance of precipitation in 
the forecast for all areas Tuesday night and Wednesday. Temperatures will be cooler 
next Tuesday and Wednesday. 


Aviation... 
a cold front will move south across Wyoming tonight with some rain 
and snow showers moving into the north and west after 12z 
Thursday...spreading into the central in the late morning/early 
afternoon. Expect MVFR ceilings and some visibilities behind the front. The 
rain and snow will become widespread over the Wind River 
basin...Natrona County and southern Bighorn Basin by Thursday 
afternoon. The rain will change to snow as well in these areas 
through the afternoon and will continue overnight with significant 
accumulations expected. North to northeast winds will increase as 
well behind the front. 


Fire weather... 
a major change in the weather from mild and dry to sharply colder 
and snowy is in store for the much of the area Thursday into 
Thursday night. A strong cold front will begin to push south into 
northern Wyoming later tonight into Thursday morning and then into 
central Wyoming Thursday morning. Rain will change over to snow 
late in the day and may become heavy at times across portions of 
central Wyoming Thursday night. Significant snowfall is quite possible 
from Lander and Riverton over to Casper. Strong north to northeast 
winds of 20 to 30 miles per hour with gusts to 40 miles per hour will accompany this 
front. 


&& 


Riw watches/warnings/advisories... 
Winter Storm Watch from Thursday evening through Friday morning 
wyz015-017>020-022. 


&& 


$$ 


Short term...skrbac 
long term...Arkansas 
aviation...twb 
fire weather...skrbac