Weather

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fxus65 kriw 292030 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
230 PM MDT Friday Aug 29 2008 


Short term...tonight through Monday 


GFS and NAM are in good agreement Saturday with a digging trough 
moving to the Pacific northwest coast by 00z Sunday. As the strong upper ridge 
over the northern plains shifts slowly eastward Saturday...a core of 
warm air aloft will move northeastward from the Great Basin over Wyoming 
and Colorado. Both the GFS and NAM project 700 mb temperatures in the 15 c to 
18 c range...and highs Saturday 12 to 18 f above normal. High 
temperatures east of The Divide Saturday will range from the middle 80s 
to the middle 90s...while highs west of The Divide will be in the 80s 
to around 90. The trough will dig sharply southward Saturday night 
and Sunday with the base of the trough moving into northern 
California/central Nevada by 00z Monday. Plume of subtropical moisture will move 
northeastward from Arizona/southern Utah across Wyoming Sunday with GFS and NAM 
projecting precipitable water values between 0.75 and 1.0 inch 
across the entire County Warning Area by 18z Sunday. Weak vorticity maxima in the 
southwest flow at 500 mb and weak instability combined with abundant 
moisture will produce isolated showers and thunderstorms east of The 
Divide and scattered to numerous showers and thunderstorms west of 
The Divide. NAM and GFS continue to trend toward a slower 
progression of the trough to the east Monday with the trough axis 
running from central Utah across eastern Idaho by 00z Tuesday according 
to the GFS and further west per the NAM. This will allow the deeper 
layer moisture across Wyoming to persist through Monday evening. With 
increasing dynamic forcing across Wyoming Sunday night and 
Monday...scattered to numerous showers and thunderstorms will 
continue west of The Divide with scattered showers and thunderstorms 
east of The Divide through Monday evening. The potential for flash 
flooding will increase across western Wyoming Sunday night through 
Monday...as stronger showers and thunderstorms could produce one 
inch of rain or more per hour. The slower eastward movement of the 
trough to the east will prevent colder air from moving in aloft 
Monday. Snow levels across western Wyoming are expected to be above 10000 
feet Monday...except for the northern absaroka and Yellowstone 
National Park where snow levels could fall to 8500 feet Monday 
afternoon. 


Long term...Monday night through Friday 


GFS and European model (ecmwf) remain in good agreement this morning beginning the 
period with high amplitude pattern with deep trough over the western 
U.S. And strong ridge over eastern U.S. With gradual deamplification 
through the period. Upper low reaches its most southern 
extent...closing off over NE Nevada Labor Day morning before lifting 
NE. GFS and European model (ecmwf) in better agreement in lifting low further west 
across western Wyoming Monday night per previous European model (ecmwf) runs. This 
further westward track will put more of County warning forecast area in warmer airmass with 
more dry-slotting possible across the east. Other change noted with 
GFS is further northward placement of the trowal over Montana. This 
track will result in the heaviest precipitation swath setting up 
over the west half of the County warning forecast area Monday. Warmer airmass also will keep 
snow levels between 9k-11k feet across the most of west through the 
day on Labor Day with the exception of Yellowstone National Park and 
the Beartooth Pass area where snow levels will be between 8k-9k 
feet. Thus...Major Mountain passes...Teton...togwotee and South Pass 
will see mainly rain or a mix of rain and wet snow with wet roads. 
Main concern will be the possibility of heavy rain and resulting 
mudslides over blackened areas at The Gunbarrel fire. 
Otherwise...widespread precipitation amounts of one-half to one inch 
of water over the west will go a long way to containing remaining 
wildfires and reducing the fire danger across the west. 


Upper low lifts into eastern Montana Tuesday. Colder air and 
wraparound precipitation will likely bring several inches of 
snowfall across Yellowstone National Park and the northwest 
mountains Monday night into Tuesday morning with snow levels 
lowering to 8000 feet. Otherwise precipitation will decrease from 
the south as drier westerly flow prevails. Kicker system expected 
to follow out of the Gulf of Alaska into the northern rockies 
Wednesday and Wednesday night spreading chances of precipitation 
back into the northern mountains. NCEP ensemble mean shows a 
rather flat zonal flow across the northern U.S. Late in the period 
as first upper low moves east and flattens ridge over eastern U.S. 
So confidence only fair on how far south this second low will dig 
before lifting out. After a rebound in temperatures Wednesday 
forecast brings in cooler air across northern Wyoming Thursday and 
Friday along with chances of precipitation...keeping the south dry 
with near normal temperatures. 


&& 


Aviation... 


High pressure will keep VFR conditions over the forecast area 
through Saturday. Smoke from The Gunbarrel...dinwoody canyon...and 
New Fork Lake fires will continue to impact the central basins...and 
may reduce visibilities to MVFR levels at times. Saturday afternoon 
will see increased southwest winds of 15 to 20 miles per hour across western 
Wyoming as the next Pacific storm system moves towards the state. 
This system will also introduce some high level clouds across the 
western mountains and valleys. 


&& 


Fire weather... 


High pressure aloft will shift to the east Saturday...allowing warm 
core of air aloft to move northeastward from the eastern Great Basin 
into Wyoming. High temperatures Saturday will be 12 to 18 f above 
normal with minimum relative humidities in the lower teens in all of 
the lower elevations. Winds aloft Saturday will still be fairly 
light on the western edge of the upper ridge...and wind gusts should 
remain below 20 miles per hour. Even though winds will keep the region below 
critical fire weather conditions...hot temperatures and low relative 
humidity will create borderline conditions. Daytime relative 
humidities will increase Sunday...as middle and low level moisture 
moves across the area from the south. Increasing clouds and much 
cooler temperatures are expected Sunday with scattered to numerous 
showers and thunderstorms across the west and isolated activity 
across the east. 


&& 


Riw watches/warnings/advisories... 
none. 


&& 


$$ 


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