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fxus65 kriw 102156 
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Area forecast discussion 
National Weather Service Riverton Wyoming 
356 PM MDT Friday Oct 10 2008 




Short term...tonight through Monday 
complex winter storm continues to come together with the brunt of 
the system expected to hit Saturday and continue into at least 
Saturday night in most areas and into Sunday morning across the 
north. In the short term...jet streak moving northward across western 
Wyoming is aiding in cooling tops over the northwest part of the 
area. Isentropic upglide is also gradually increasing across the 
central zones along with the north to south upslope flow. Models 
not handling the cooling tops in the northwest that well but since the 
warning is already out for that area and heavier snow will be just 
fine this evening. Other area to watch will likely be from the Red 
Desert northward into the Wind River basin and possibly as far east as 
Natrona County. The combination of the eventual right entrance 
region of the aforementioned jet streak will combine with 
increasing qg forcing and isentropic upglide to bring another 
round of moderate snow or greater to this area for 3-6 hours this 
evening. After a possible break then for a few hours late tonight 
the storm comes in with even more ferocity Saturday morning with 
the peak occurring in the afternoon through Saturday night in the 
central zones and into Sunday morning across the far north. 
Isentropic upglide maxes late Saturday along with impressive jet 
streak dynamics centered on the central zones. Upslope gradient 
continues to intensify into Saturday with possible blizzard 
conditions across the Red Desert and Johnson County. Other areas 
from the Big Horn basin...through the Wind River Canyon will see 
north winds of 20 to 30 miles per hour with gusts around 40 miles per hour. Even the 
Wind River basin and Natrona County can expect north to NE winds of 15 
to 25 miles per hour with gusts to 35 miles per hour. Bottom line is that several waves 
of snowfall should hit the warning areas through tonight before 
the main attraction arrives during the day Saturday and continues 
for 12 to 18 hours with snowfall rates of an inch an hour possible 
across the lower elevations and 1 to 2 inches in the mountains. 
The best lower elevation snowfall amounts still appear to be 
centered between Lander Riverton and east into western Natrona County 
along with the Cody foothills eventually. Even Thermopolis could 
get quite interesting Saturday and Saturday night. Buffalo should 
see increasing wind and snow into Saturday night. The snow should 
generally be light most of the time until Saturday afternoon. In 
Casper...have issued the Winter Storm Watch for Saturday afternoon 
and night when best dynamics and colder air hit along with gusty 
northeast winds. Middle level flow is still not favorable but 
combination of snow and wind and first snowfall warrants a watch 
for this timeframe. Other highlights look good and have just 
extended our central zones through the night Saturday night and 
until noon Sunday up north. Quite a storm brewing and details will 
still have to be worked out as the storm unfolds. 


Long term...Monday night through Friday 
high amplitude pattern in the short-term transitions to more of a 
zonal flow during this period resulting in a dry pattern with a 
gradual warming trend. 


On Monday...main upper low lifts across northern plains into central 
Canada...with baggy trough lingering back across the southern 
rockies. Cool northerly flow remains over Wyoming with high 
temperatures 15 to 20 degrees below normal. 


Tuesday...kicker shortwave races across western Canada into the 
northern plains. Precipitation with this system is expected to stay 
well north of Wyoming. Temperatures will recover but highs will 
still be around 10 degrees below normal. 


Wednesday...GFS is slightly faster bringing a shortwave across the 
northern rockies than the European model (ecmwf). The main impact of this shortwave 
will be to kick up the winds but temperatures will remain similar or 
slightly milder than Tuesday with better mixing. 


Thursday and Friday...both GFS and European model (ecmwf) show a ridge of high 
pressure building over the western U.S. With the European model (ecmwf) placing this 
ridge further west than the GFS...the European model (ecmwf) keeping Wyoming under a 
northwest flow. Forecast dry for this period with temperatures returning 
to near normal. 


Aviation... 
large upper level low pressure system will move eastward across the 
northern Great Basin tonight and Saturday. A south to southwest flow 
will continue to transport abundant moisture over Wyoming tonight 
and Saturday. MVFR and IFR conditions are expected in light snow 
across all of central and western Wyoming tonight..except at kjac 
where downsloping northeast winds will dry out the lower layers of 
the atmosphere. Conditions are expected to deteriorate Saturday east 
of The Divide with IFR conditions likely in most locations east of 
The Divide and MVFR/IFR conditions west of The Divide. As deep 
surface low pressure moves across central Colorado Saturday...wind 
gusts will increase to between 20 to 25 knots across most of Wyoming. 
Wind gusts Saturday across southern Wyoming...including 
krks...however...are expected to gust to between 35 to 40 knots with 
blowing snow and whiteout runway conditions possible. 


Fire weather... 
snow...windy and much colder for most of the area tonight through 
at least Saturday night. Some downsloping will limit snowfall in 
the far west valleys. All of the southwest...northwest mountains and 
foothills and areas east of The Divide will have the threat of 
heavy snow and in most cases windy conditions. Temperatures will 
only be in the 20s and 30s the next couple days with colder 
overnight lows. 


&& 


Riw watches/warnings/advisories... 
blizzard watch from Saturday morning through Saturday evening 
for wyz027>030. 


Winter Storm Warning until 6 am MDT Sunday for wyz014-015- 
017>019-022. 


Winter Storm Watch from Saturday afternoon through Sunday 
morning for wyz010-011. 


Winter Storm Watch from Saturday morning through late Saturday 
night for wyz016. 


Winter Storm Watch from Saturday morning through Saturday 
afternoon for wyz024>026. 


Winter Storm Warning until 12 PM MDT Sunday for wyz001>003- 
007>009. 


Winter Storm Watch from Saturday afternoon through late Saturday 
night for wyz020. 


Winter Storm Watch from Saturday morning through Sunday morning 
for wyz004>006. 


&& 


$$ 


Short term...skrbac 
long term...aem 
aviation...Smith 
fire weather...skrbac 

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