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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Thursday
• 
• High: 59 °
• Low: 39 °
• T-Storms

• Friday
• 
• High: 61 °
• Low: 34 °
• T-Storms

• Saturday
• 
• High: 73 °
• Low: 45 °
• T-Storms

• Sunday
• 
• High: 61 °
• Low: 39 °
• Partly Cloudy

• Monday
• 
• High: 73 °
• Low: 48 °
• Clear

Area forecast discussion...corrected for time 
National Weather Service Riverton Wyoming 
1135 PM MDT Wednesday may 23 2012 


Short term...Wednesday night through Saturday 


Quite an active pattern over the next three days along with much 
mashing of teeth as many of the details remain difficult to nail 
down. Some showers and thunderstorms have been firing along a 
boundary across the southern Big Horn basin across into northern 
Natrona County. We expect this boundary to slowly sag southward into 
tonight to a position near the Wind River mountains to near Casper 
Mountain. Here is lies the first forecast quandary. As weak low 
pressure tries to develop near the Montana-Colorado-Nebraska 
border...middle level flow will northerly and set up an upslope flow 
situation across the favored locations such as Casper Mountain and 
the Wind River Range. Models are showing a bullseye of 
precipitation over eastern Wyoming with as much as 0.40 inches 
tonight near Casper. We think this is overdone...but we have 
bumped up probability of precipitation a bit as well as quantitative precipitation forecast. The next problem is what 
precipitation type...models show wet bulb zero levels falling to 
between 6000 and 6500 feet later tonight. So...it appears the 
S-word will re-enter the forecast for places like Casper Mountain 
and the Lander foothills. Accumulations appear to remain light at 
this time due to very warm ground temperatures. However...the next 
shift will have to monitor this closely as if any heavy snow does 
set up it could bring the snow down to as low as 5000 to 5500 for 
a time later tonight so have added the possibility of some snow 
for places like Casper later tonight. 


The steady precipitation looks to shift out of the area for Thursday 
as flow turns more westerly. On Thursday...some models..especially 
the NAM...have trended drier. So we cut back probability of precipitation a bit although 
left the general idea of showers across the northern two thirds of 
the area with most areas south of the green and rattlesnake ranges 
mainly dry. Temperatures look to be below average across the entire 
area with almost all areas remaining in the 50s or below for high 
temperatures. 


Could be quite a contrast across the area for temperatures on Friday 
with the boundary stating to lift northward across the state. 
Temperatures should rise into the 70s across southern areas with 
areas from around the Wind River Range and northward remaining in 
the cool air with highs in the 50s and even 40s with more cloud 
cover. The best moisture has been trending further north so we cut 
probability of precipitation a bit...especially in the morning as models have trended a bit 
slower with the system. However...given the travel expected for the 
Holiday weekend we have keep at least chance probability of precipitation for most areas. 


As for Friday night and Saturday as the main system moves in from 
the south and west...most models have been trending the bulk of the 
dynamics and moisture northward into Montana. Models have also been 
bringing most areas into the warm sector of the storm so we raised 
temperatures for Saturday and future shifts will likely have to 
raise them some more if the trend continues. As a result snow levels 
should continue to rise through Friday night and into Saturday with 
the broad southerly flow developing. Of course...this is dependent 
on how much of the area breaks into the warm sector. Models continue 
to show a good amount of moisture over the western 
areas...especially the northwest so it looks like a soggy start to 
the Memorial Day weekend for those locations. East of The 
Divide...expect a windy day with a scattering of afternoon showers 
and thunderstorms developing ahead of the advancing low. 


Long term...Saturday night through Wednesday night 


Medium range models remain in agreement for the most part today for 
the extended period. The European model (ecmwf) and Canadian are the slower 
models...keeping the storm center over western Montana Sunday and 
gradually moving into southern Saskatchewan by Monday morning. The 
GFS is the fastest...ejecting the system into southern Manitoba 
during the same time. The impacts to the cowboy state will be the 
same...with the cold front exiting the state late Sunday morning. 
Moist northwest flow will mainly impact western areas through early 
Monday...before a short wave ridge builds in the wake of the storm. 
The rest of the period will be somewhat unsettled...especially 
across western and northern portions...as zonal flow sets up and 
brings weak shortwaves across the area. 


&& 


Aviation.../06z issuance/ 


Rain will continue over Natrona County through 12z with some MVFR 
conditions at the kcpr Airport. Conditions will improve by 15z in 
this area with VFR prevailing. Elsewhere isolated showers will occur 
through 12z with some snow showers in the far West Mountains. This 
activity will dissipate around 12z. Widely scattered showers and 
isolated thunderstorms will reform over the region after 18z Thursday. 
This activity will end by 03z Friday. VFR conditions will prevail 
with local MVFR conditions in thunderstorms. 


&& 


Fire weather... 


Fire danger should be somewhat reduced through Thursday night as a 
boundary drops slowly southward through Wyoming and brings cooler 
temperatures...somewhat higher humidity...cooler temperatures and a 
chance of showers. There will be a chance of showers across much of 
the area with the most numerous coverage across areas west of The 
Divide and the Big Horn Mountains. Fire danger may increase somewhat 
on Friday and Saturday in Sweetwater County with warmer temperatures 
and a gusty wind. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Short term...hattings 
long term...lavoie 
aviation...Arkansas 
fire weather...hattings