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fxus65 kriw 050904 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
304 am MDT Sat Jul 5 2008 


Short term...today through Monday night 
ridge axis moved east this past evening as subtropical moisture 
plume over took the northern and central rockies. Has a few decent 
storms yesterday with one storm tracked from near diversion dam to 
just south of Ten Sleep producing dime size hail. The bulk of the 
activity was over Montana where the best jet dynamics were. 


For today...subtropical moisture will continue to move across 
Wyoming. Though it will be weaker as the source gets pinched off 
by a trough making land fall along the West Coast. Shortwave will 
move across Montana this morning...driving a cold front into 
northern Wyoming today. This will be a focus for thunderstorm 
development this afternoon and evening. The front will stall over 
southern Bighorn Basin and along the Johnson and Natrona County 
line. Will also have decent instability around as well...with the 
best cape over Johnson County...between 1000 and 1500 j/kg and 
<1000 j/kg over the Bighorn Basin. Some storms could be severe 
based on what has been happening over the past week. Not much 
instability elsewhere...though cant rule it out with that moisture 
around. 


Sunday afternoon and evening is looking quite active with some 
shortwave activity in the flow and very good divergence aloft. 
Right entrance jet will be off over the Dakotas...but some of that 
may influence thunderstorm development over the northeast 
County Warning Area...where the best instability lies. Southeast to east return 
flow over the central and east allowing moist air to advect in. 
Precipitable waters over the northeast around 1.25 inches and around and inch 
over the Bighorn Basin. Very favorable conditions for severe 
thunderstorms. Wouldn't be surprised to see severe storms over 
Fremont County as well. 


Drier air moves in for Monday behind the shortwave. Still enough 
moisture for thunderstorms. Best instability lies over the center 
of the state with convective available potential energy under 1000 j/kg. Will have the northeast 
flow over the area so should hang on to some low level moisture. 
Temperatures over the next few days will be cooler than Friday...with 
more normal readings in the 80s. 




Long term...Tuesday through Saturday 
shortwave that is forecast to move across eastern Montana and ND Monday 
night will likely push a cold front southward into our northern and eastern zones 
setting the state for at least some isolated thunderstorms Tuesday 
afternoon and evening. Will expand our coverage a little bit to 
include all areas east of The Divide based on this expectation. Northwest 
flow continues Wednesday with the GFS leaving a moist Ely flow 
east of The Divide. The Euro dries it out some but still leaves 
some moisture east of The Divide. For now...will expand our probability of precipitation 
to include our favored northwest flow thunderstorm development areas and see if 
these trends continue again tonight. Flat Ridge pushes in Thursday 
for decreasing thunderstorm threat. Will confine thunderstorms chances to just the 
northern mountains on Friday...a significant trough is forecast to dig into 
southwestern Canada and possibly the pacnw. Model solutions have changed 
several times on how strong this trough would be and how far S it 
would dig. The GFS and European model (ecmwf) are at least in reasonable agreement 
tonight. Friday may be a hot and breezy to windy day east of The 
Divide ahead of the associated front. The far west and northwest may see 
some strong storms with right entrance region jet streak dynamics. 
Other concern will be potential subtropical moisture being drawn 
northward in this pattern. Both models show this and it seems to be just 
a matter of timing. I lean more towards the GFS allowing this be 
drawn northward fast enough to impact at least our eastern zones and 
possibly the entire area by Saturday. There is even a chance that 
the beginnings of this moisture could impact the Red Desert by 
later Friday. Plenty of time to fine tune the details but it could 
be a very active thunderstorm period beginning as early as late 
Friday and then continuing through at least Saturday and perhaps 
longer. And as has been the case...the really hot temperatures that the 
GFS has been advertising for days keep getting delayed and the 
duration keeps shrinking. Luckily we've never bought into those 
hot GFS temperatures but I have trended some of our highs down even 
further. A few warm to very warm days will still likely occur but 
a dry...hot period doesn't appear very likely. 


&& 


Aviation... 
a cold front will move across the region today. This front will help 
develop widely scattered thunderstorms during the afternoon. Gusty 
winds can be expected near any storms. This includes Sunday 
afternoon and evening when even a better chance of thunderstorms 
exist...especially across central and northern Wyoming. 
MVFR conditions are possible around the storms. Otherwise...VFR 
conditions will prevail across west central Wyoming this weekend. 


&& 


Fire weather... 
a cold front will move into the north this morning and stall along 
The Owl creek mountains and along the Johnson/Natrona County 
border. This will be a focus for thunderstorms this afternoon and 
evening. Plenty of middle and high level moisture will be available 
as a plume of moisture from the Pacific remains over Wyoming. 
Thunderstorms will be most likely over the north...but could occur 
across the area. The greatest thunderstorm coverage will occur on Sunday 
as easterly flow east of The Divide brings in moist air form the 
plains. Monday looks to be a drier day...though storms are still 
possible over the central and east. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Short term...twb 
long term...skrbac 
aviation...kpl 
fire weather...twb