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Area forecast discussion 
National Weather Service Riverton Wyoming 
341 PM MDT Sat Jul 5 2008 


Short term...tonight through Tuesday 
on WV loop this afternoon main features are an upper low moving 
across Saskatchewan/Manitoba border with shortwave axis across 
northern High Plains and lagging trough in this split flow 
drifting across Utah. In between top of middle/high level moisture 
plume sheared off from northern stream trough leaving remainder 
across southern Wyoming. At the surface...trough/dryline stretched 
north to south across central Wyoming with very dry airmass and 
westerly wind across the southwest half of Wyoming and mainly a 
light northerly flow with higher dewpoints across the NE half. 
20z laps data shows highest convective available potential energy and lowest Li/S across Johnson 
and Natrona counties with Theta-E ridge stretching back westward 
just south of The Owl creek mountains. Visible imagery shows enhanced 
cumulus field just north of this Theta-E ridge on higher dewpoint side. 
This will be likely initiation area for storms late this afternoon 
and evening with storms strengthening in more moist unstable 
airmass to the east. These storms and storms to the north over 
Montana will help drive front with outflow boundary assist back to 
the east slopes of The Divide overnight along with higher 
dewpoints. This front will set the stage for an active day Sunday. 
Next split trough off the northwest coast will move across the 
intermountain west tonight into western Wyoming by 00z Monday. GFS shows 
60 knot jet streak shooting SW-NE across the area 18z sun-00z 
Monday over old frontal boundary. GFS continues to show moderate 
instability from the northern Big Horn basin and across Johnson 
and Natrona counties Sunday afternoon where the best thunderstorm 
coverage is expected along with isolated severe storms with large 
hail and damaging wind. This area also aligns well with timing of 
jet streak and area of best shear/upper divergence. West of The 
Divide will see weaker instability and a drier airmass and mostly 
isolated thunderstorms. Upper trough moves across the area Sunday 
night with lingering thunderstorms across east central Wyoming 
into the early Monday morning. 700 mb temperatures to be around 4c 
cooler Sunday and with high temperatures 5-8f cooler than today. 


On Monday...upper ridge and drier more stable air push into the 
west while stationary frontal boundary lingers near the east 
slopes of The Divide with weak instability behind the front. Not 
much support aloft with this instability...so thunderstorms should 
mainly fire off favored convergence regions in the high terrain of 
central Wyoming and move east-southeast...with northern Johnson County 
possibly seeing a few stronger or borderline severe storms. 
Upper trough moving across central Canada Monday and Monday night 
will help reinforce quasi-stationary front Monday night leaving 
mainly weak instability and precipitable waters  near 0.75 east of The Divide for 
a continued chance of late day thunderstorms while the west remains 
mainly stable and dry with above normal temperatures. 




Long term...Tuesday night through Saturday 
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... 
widely scattered thunderstorms will continue over western and 
central Wyoming through the early evening hours. Isolated MVFR 
conditions are possible in the stronger storms. Strong and gusty 
outflow wind will be the main aviation impact through this evening. 
Scattered to numerous thunderstorms are expected over the west 
Sunday morning and over the central and east during the afternoon 
and evening. Some of the storms over the central and east may become 
severe Sunday afternoon and evening with hail and damaging wind 
gusts. 


&& 


Fire weather... 
a surface trough/dryline stretching north to south across 
central Wyoming this afternoon will be a focus area for isolated 
thunderstorm development this evening. These storms should 
strengthen and become more organized as they head into a more 
moist and unstable airmass across Johnson and Natrona counties. 
A frontal boundary will push in higher dewpoints back to The 
Divide overnight setting the stage for scattered thunderstorms 
across north and central Wyoming Sunday afternoon and 
evening...with isolated severe storms from the northern Big Horn 
basin across Johnson and Natrona counties. Storms will be more 
isolated and dry across southwest Wyoming. An upper ridge will 
build in Monday bringing warmer and more stable air into the west 
with isolated storms over the high terrain of central Wyoming. 


&& 


Riw watches/warnings/advisories... 
none. 
&& 


$$ 


Short term...aem 
long term...skrbac 
aviation...church 
fire weather...aem