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Area forecast discussion 
National Weather Service Riverton Wyoming 
255 am MDT sun Jul 20 2008 


Short term...today through Tuesday night 


Large upper level high pressure centered over western OK/western Kansas 
this morning will shift slightly to the northwest today. This 
movement of the upper high will allow middle level moisture over 
central and northern Utah to push slowly northeastward into 
southwestern Wyoming this afternoon and evening. GFS and NAM both project 
that precipitable water values across southwestern Wyoming will increase 
later today with the GFS showing around 0.75 of an inch by 00z and 
the NAM slightly less. The atmosphere below 500 mb however will remain 
very dry this afternoon and evening...and isolated thunderstorms 
which form across southwest Wyoming will be high based storms with strong 
gusty outflow winds and any rain amounts of only a few hundredths of 
an inch expected. Otherwise...the atmosphere across the rest of the 
County Warning Area will be stable with increasing middle level moisture producing 
scattered cumulus. 700 mb temperatures today will range between 14 c and 
18 c with high temperatures expected to be well above normal in the 
upper 80s to the upper 90s in the lower elevations. The upper level 
ridge will gradually drift northward Monday...and moisture will 
continue to move across the region from the southwest. GFS and NAM 
precipitable water values increase to 0.75 of an inch to one inch 
Monday. Although instability will increase Monday it will only be 
marginal with lifted indices near 0 c and cape values between 200 
and 500 j/kg. Included scattered mountain showers and thunderstorms 
in the mountains and isolated convective activity in the lower 
elevations...except in the Wind River and Bighorn basins where the 
lower layers of the atmosphere will be too dry. Scattered to broken 
middle level clouds will cause high temperatures to be 2 to 4 f lower 
than highs on Sunday...but highs Monday will still be slightly above 
normal. Precipitable water values will increase to around one inch 
across the entire County Warning Area Tuesday with slightly lower lifted indices and 
increasing cape values...as the upper ridge axis moves to the east 
and a trough moves inland into the western Great Basin and interior 
Pacific northwest. Included scattered shower and thunderstorms for western and 
northern Wyoming given falling 500 mb heights and higher instability with 
isolated convection for the remainder of the County Warning Area. Thermal ridge at 
700 mb will still be located over Wyoming Tuesday...and above normal high 
temperatures will continue. 


Long term...Wednesday through Sunday 


Water vapor imagery early this morning shows a shortwave trough 
off the central California coast. This feature will gradually be 
picked up by the westerlies and push across the region early 
Wednesday. This is a slower than yesterdays model runs...and will 
keep precipitation chances in the forecast Wednesday...before 
drier air works into the area in the wake of this system. This 
system will also help suppress most of the monsoonal moisture 
Thursday and Friday...with the best chance of convection across 
the east where better moisture...instability and some gentle 
upslope conditions are expected. By Saturday and Sunday...monsoonal 
moisture is expected to return to most of the area which should be 
sufficient for at least isolated to locally scattered convection everywhere 
during the afternoon and evening. Temperatures through the 
extended are expected to be seasonal to slightly above average. 


&& 


Aviation... 


High clouds will invade from the southwest this morning across the 
entire area. A few high-based thunderstorms are possible over the 
southwest in the afternoon and early evening...producing gusty winds 
and some lightning. Otherwise winds will be lighter today over 
yesterday. 


&& 


Fire weather... 


Another mainly dry and hot day is expected across the region 
today...as high pressure to the south will move slowly northward. 
Increasing middle level moisture in southwestern Wyoming this afternoon 
and evening will generate isolated high based thunderstorms with 
gusty outflow winds up to 50 miles per hour and a few dry lightning strikes 
possible. Minimum relative humidities in the lower elevations are 
expected to range from the upper single digits to lower teens across 
southern Wyoming and in the teens in the lower elevations to the 
north. Winds should be lighter than Saturday afternoon with winds 
speeds generally 15 miles per hour or less this afternoon. Moisture will 
continue to move northeastward into the region Monday and 
instability will increase slightly as well. This will increase 
daytime relative humidity across the entire region into the middle 
teens to 20s. Isolated showers and thunderstorms will develop in the 
lower elevations Monday...except in the Wind River and Bighorn 
basins which will remain too dry...while scattered activity is 
expected in the mountains. The chance of wetting rain with these 
showers and thunderstorms will be slightly higher...as the lower 
levels of the atmosphere gradually become more moist. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Short term...Smith 
long term...wm 
aviation...twb 
fire weather...Smith