Weather

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afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
901 PM MDT Wednesday Aug 27 2008 


Update...winds have diminished and rh's have climbed above 15 
percent. Therefore the red flag warning has been allowed to expire. 


Short term...tonight through Saturday 
high clouds...especially east of the Continental Divide...have 
hindered warming temperatures this afternoon. This has delayed 
mixing in the Wind River basin...but most other locations have 
seen gusty wind since at least late morning. Current red flag 
warning looks on track despite cooler readings than yesterday. 
Strong middle-level wind coupled with passing jet stream has 
certainly produced plenty of wind to necessitate this warning. 
Winds will gradually diminish overnight as upper trough tracks 
into the northern plains and jet stream lifts north of the state. 
Dry west-northwest flow will prevail for Thursday and Friday with 
no chance of precipitation. 700 mb temperatures will gradually 
moderate these two days. For temperatures...have mainly adjusted 
overnight lows given the cool start to today in very dry airmass. 
Saturday looks to be the next day with significant weather 
occurring. Again...fire weather will be the concern as next trough 
moves into the intermountain West. Ridge axis will traverse the 
area on Friday night which will allow for increasing warm... 
southwest flow on Saturday. Temperatures will climb to above 
seasonal normals and gusty southwest wind will increase. The 
warming temperatures and gusty wind will also help to lower 
relative humidity values. Have adjusted temperatures up slightly 
for Saturday given current model trends and increasing confidence. 
Any moisture that creeps north into far West Wyoming Saturday 
afternoon will be too limited and high based to generate 
convection. Theta-E ridge across southeast Wyoming will remain 
well to the east of our County Warning Area confining convection to far eastern 
sections of the state. 


Long term...Saturday night through Wednesday 
the models continue to indicate monsoon moisture moving north on 
the west side of the ridge across the Great Basin and into the 
western mountains of Wyoming Saturday night and Sunday. This trend 
has been shown the past four nights so have increased confidence 
in this solution. The moisture surge strengthens on Sunday with 
moisture spreading into central...northern and eastern areas. The 
models continue to be much stronger with this trough with more 
widespread chances of precipitation and much more significant 
cooling looking more like an autumn storm than a Summer one. This 
puts likely to scattered showers isolated thunderstorms across the 
area. Local coverage could be much higher but pin pointing 
convection on days 5 and 6 is problematic. The moisture continues 
over the area on Monday with some indications that a Pacific 
trough will begin to push colder dry air into the west Monday 
afternoon. Tuesday is problematic with indications that drying 
will increase but also that convection will remain isolated over 
the area. Have started to dry thing out on Tuesday and Tuesday 
night. Looks cold Monday night with freezing temperatures across 
the west. Tuesday looks somewhat warmer during the day. Possible 
freezing temperatures again in the west on Tuesday night then 
warmer and dry on Wednesday. Models show quite a bit of spread on 
Sunday and Monday with the European model (ecmwf) cutting off a 500 mb low now over 
the Great Basin. This is much more bullish than previous runs. GFS 
shows deep open trough. Precipitation coverage highly uncertain at 
this time. 


&& 


Aviation... 
a cold front will move across northern Wyoming late this 
afternoon and early this evening. The front will then weaken as it 
moves into central and southern Wyoming tonight. West to northwest 
winds will gust to between 25 and 35 knots through 02z...as strong 
boundary layer mixing along and ahead of the cold front will bring 
higher gusts to the surface. Winds will decrease to 10 knots or less 
after midnight across the region....as surface high pressure 
builds in rapidly from the west. High pressure will remain over 
the area Thursday with west winds at 8 to 12 knots with a few gusts 
up to 20 knots possible. VFR conditions are expected tonight through 
Thursday. 


&& 


Fire weather... 
update...winds have diminished and rh's have climbed above 15 
percent. Therefore the red flag warning has been allowed to expire. 


The pressure gradient will relax on Thursday as the trough moves 
to our northeast and the jet stream shifts north. This will allow 
for much less wind on Thursday. This trend will continue into 
Friday. Improved humidity recovery will occur tonight...especially 
in the thermal belts. Dry conditions will prevail through Saturday. 
However...temperatures will warm to above seasonal levels on 
Saturday as gusty winds occur in warmer southwest flow ahead of the 
next storm system. The combination of wind...lowering humidity 
values...and warm temperatures may create critical fire weather 
conditions. This situation certainly bears watching over the coming 
days. This storm system will bring much cooler temperatures... 
cloud cover...and increased precipitation chances on Sunday and 
Monday. Temperatures may cool enough to generate snowfall in the 
higher mountains by Monday. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Update...twb 
short term...cnj 
long term...c.Baker 
aviation...Smith 
fire weather...cnj