Weather

276 
fxus65 kriw 070837 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
237 am MDT sun Sep 7 2008 


Short term...today through Monday night 


GFS and NAM showing strong continuity over the past 48 hours with 
shortwave moving southeast from southern British Columbia/southern 
Alberta. Both models bring the southern portion of the trough across 
northern Wyoming today before the base of the trough moves southeastward 
east of The Divide tonight. Right entrance region of a h25 jet will 
enhance lift across northern Wyoming today...where will carry likely probability of precipitation 
from the Absaroka Mountains eastward to Johnson County. As the 
trough moves southward tonight...models are pointing toward a 
coupled jet structure north of the surface cold front which will 
push into Sweetwater County. NAM trough axis is slightly farther to 
the west than the GFS tonight. Leaned toward the NAM tonight which 
has been more consistent in the path of the trough. Included likely 
probability of precipitation tonight as far south as the east slopes of the Wind River 
mountains and southern Natrona County with both the GFS and NAM 
showing fairly strong q vector forcing and steepening middle level 
lapse rates. Snow levels today and tonight in the Bighorn Mountains 
will range between 7000 and 7500 feet...and snow accumulations in 
the northern portion of the Bighorn Mountains of 3 to 6 inches are 
expected. With northwest flow aloft today...highs will continue to 
be 10 to 15 f below normal across northern and central Wyoming and near 
normal across southern Wyoming. 


Trough will move rapidly eastward Monday with weak shortwave ridge 
moving across the northern rockies. Highs Monday will moderate...but 
still remain slightly below normal. Flow aloft will back to the west 
Tuesday...and thermal ridge aloft will move into western Wyoming. Highs 
Tuesday are expected to be slightly above normal with 700 mb 
temperatures rising to above 10 c. GFS indicating middle level moisture 
moving northward from Utah across southern Wyoming...where isolated 
thunderstorms could develop. 


Long term...Wednesday through Sunday 


A more complex weather scenario is forecast by the GFS/European model (ecmwf) models 
for Wednesday through Friday. An upper low is now forecast to move into 
Idaho Wednesday. Then the GFS moves the low slowly east to western Wyoming at 
06z Friday...then moves it across northern Wyoming on Friday. The GFS ensemble 
members do have a few solutions similar to the operational GFS. The 
European model (ecmwf) is farther south with the upper low taking it to northern Utah at 
12z Friday...then it tracks the system east across northern Colorado on Friday. 
Have increased the precipitation chances over the region Wednesday and Thursday. For 
now will keep precipitation chance over the north Friday as upper system 
tracks across central Wyoming. Have taken the middle of the Road track 
with the system and see how it looks tonight. The weather system 
moves out of the region Friday night. Per GFS...the next shortwave in 
the northwest flow tracks across Montana on Saturday with northern Wyoming 
seeing some showers Sat PM. The European model (ecmwf) does not show much in the flow 
on Saturday. Have just low precipitation chances in the northern mts for 
now and can refine later. Otherwise dry on Saturday. Next Sunday 
looks dry as well. As for temperatures...mild on Wednesday ahead of the 
storm...then cooler on Thursday. Little change in temperatures on Friday with 
slight warming Sat and sun. 


&& 


Aviation... 


Middle-level clouds on the east side of The Divide will erode from the 
southwest this morning. A cold front will push south into north 
central Wyoming 18z-21z and into central Wyoming...from near a 
kcpr-kriw line around 21z...and to krks around 03z Monday. Areas of 
mfvr ceilings and light rain will fill in behind the front. Snow levels 
will drop to near 8000 feet Sunday afternoon behind the front and to 
near 7000 feet across the north 00z-06z Monday. Mountains will be 
obscured in IFR/LIFR conditions above the snow level. 


&& 


Fire weather... 


Strong upper level system will move from western Montana across 
northern Wyoming today and into southeast Wyoming by Monday morning. 
Significant rain and snow will be possible across much of northern 
Wyoming today and tonight from the Absaroka Mountains east...and 
light to moderate rain will move southward into the Wind River basin 
and Natrona County tonight. Highs today will continue to be well 
below normal. The upper level system will move rapidly into the 
Central Plains Monday with drier and slightly warmer air moving into 
Wyoming. Minimum relative humidities Monday will still be quite 
high...above 20 percent across the bulk of the region. 


&& 


Riw watches/warnings/advisories... 
none. 


&& 


$$ 


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