499 
fxus65 kriw 062039 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
239 PM MDT Sat Sep 6 2008 


Short term...tonight through Tuesday 
moist northwest flow continues today with just enough instability 
to fire afternoon showers and even an isolated thunderstorm near byg 
earlier. Laps analysis showing some weak instability from the eastern 
Big Horn basin over to Johnson County and south to Natrona County 
so have added isolated thunderstorms through 00z across the north 
and eastern areas. Showers will be most concentrated in the high 
country and northern Johnson County through this evening. The next 
disturbance in this northwest flow is heading south along the eastern 
British Columbia...western Alberta border. It appears this system will be similar to 
Labor Day system with widespread showers...along and east of The 
Divide. These showers will first impact the northern zones Sunday morning... 
gradually spreading south through the day and arriving in the 
central zones during the evening. Some showers will precede the 
main band but widespread showers and mountain snow should follow that 
timing. Have increased all of the north to likely probability of precipitation during the 
day Sunday and all of the central zones Sunday night after 00z. 
Snow levels will likely drop to 6500-7000 feet as this system 
drops southward. 700 mb temperatures fall to around -4c on the tail end of this 
system per the NAM and local WRF. The big horns seem to be most 
susceptible to some heavier snow due to favorable northwest flow...jet 
streak dynamics along with good qg and frontal forcing. Have 
indicated 3 to 6 inches with localized amounts of 8-10 inches. I 
wouldn't even rule out the need for a Snow Advisory for Sunday and 
Sunday evening up there. However...these good dynamics are not 
just limited to the far north as both the GFS and nam12 show 
decent frontal forcing across the central zones Sunday night 
combined with a brief jet couplet...steep lapse rates and qg 
forcing. System will all but exit the area by Monday morning but 
the low clouds will likely linger into the morning all the way to rks. 
Brief respite in the northwest flow later Monday into Tuesday before some 
monsoonal moisture is shown rapidly moving north through Utah on 
Tuesday. The GFS and European model (ecmwf) are more supportive of isolated late 
day storms than the NAM. Have left our isolated coverage in across 
the west for now. After cool to very cool temperatures Sunday and Monday 
east of The Divide...it should warm considerably Tuesday with 700 mb 
temperatures warming to at least 10c per all models. That may be the only 
warm day with the next system poised to move in but that's getting 
into the extended period. 


Long term...Tuesday night through Saturday 
a new storm system dives into Montana Wednesday with the trough moving 
into Wyoming Wednesday PM and evening. Precipitation should develop over northern Wyoming Wednesday 
PM. Then the system has the potential for rain and mountain snow 
Wednesday night into early Thursday morning across much of the area. 
Still plenty of time to see how deep the trough evolves. Trough 
exits early Thursday with more sunshine in the afternoon as clouds 
decrease. Friday looks dry with a northwest flow aloft. Next Saturday 
also looks dry for now. As for temperatures...a warm day on Tuesday. Cooler 
over northern Wyoming Wednesday as the front moves into that area...still 
mild in the central and south. A cool day Thursday...then 
moderation next Friday and Sat. 


Aviation... 
increased winds will continue this evening across many area airports 
under a northwest flow. General VFR conditions are expected across 
much of the area...the exception being in the northern basins and 
mountains where isolated to scattered shower activity is possible. 
Showers over the mountains will keep higher terrain obscured this 
afternoon and through the evening. Another upper level disturbance 
will impact the area Sunday...bringing another round of scattered to 
likely showers with isolated thunder possible across central Wyoming. 
Shower activity may cause periods of MVFR conditions through the 
afternoon...and will again keep higher elevations obscured... 
especially over the Bighorn and absaroka mountain ranges. 


Fire weather... 
unsettled northwest flow aloft will keep the area along and east 
of The Divide unseasonably cool with an increasing chance of 
showers again Sunday into Sunday night. A weaker system combined 
with instability will keep showers and mountain snow going across 
the northern district through tonight with lesser showers in the central 
fire zones from the Wind River basin to Natrona County. Showers will 
be likely on Sunday in the north...spreading to the Wind River mountains 
..Wind River basin and all of Natrona County Sunday evening. The 
associated cold front will push across the area during the day Sunday 
and make it all the way to Rock Springs by Sunday night. Showers 
should all but end by Monday morning. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


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