Weather Service Report

FXUS65 KRIW 202145

Area Forecast Discussion
National Weather Service Riverton WY
345 PM MDT Fri Jul 20 2018

.SHORT TERM...This evening through Monday

Dry conditions with clear skies so far this afternoon. Increasing
clouds from the south have started and this will be the focus for
isolated dry thunderstorms this afternoon. Overnight RH recovery
was poor last night over much of Sweetwater County northward
toward South Pass, with values in the upper teens to middle 20s.
This has led to critical RH values in these areas, along with the
Upper Green River basin, southern Lincoln County and areas east of
the Divide. A Special Weather Statement was issued earlier this
morning to highlight these extremely dry conditions, along with
the dry thunderstorms expected this afternoon and evening over
southern portions. Strong gusty winds and lightning will be the
main threats from these showers, as dewpoint depressions of 40 to
60 degrees are in place this afternoon. CAMs continue to trend 
downward this afternoon, as CAPE values range between 100-300 J/kg
and LI's over 0 to -2. 

Isolated showers will be possible over the Tetons/Yellowstone
early Saturday morning, in advance of the afternoon convection.
These showers could linger through 15Z, with additional storms
developing over the Absarokas/Wind Rivers after 18Z. A prefrontal
trough will enter northern portions by this time, moving out of
the forecast area by 06Z Sunday. PW's will increase substantially,
with a return of 0.75 to 0.80". Thus, thunderstorms will be more
wet than today. Storms will be isolated to widely scattered across
the forecast area and end by 06Z Sunday, with a few lingering over
southern Sweetwater County. There could be a chance for strong to
severe storms over eastern portions of Johnson County Saturday
evening, with dewpoints ranging between 50F to 60F, LI's -5 to 
-8, CAPE values over 3000J/kg and SE sfc winds. 

By 12Z Sunday, a cold front will progress southward across the
forecast area. This should suppress most of the convective
activity over southern portions and keep temperatures in the 80s.
Dry conditions will be in place for much of the area Monday, in
the wake of the cold front. Isolated chances for showers or storms
will mainly be over the higher elevations. 

.LONG TERM...Monday night through Friday

Monday night looks fairly quiet as a shortwave exits the region with 
maybe a few lingering showers over and on the lee side of the 
Bighorn Mountains. Tuesday will see increased ridging from the south 
but we will still have enough moisture for some diurnal 
thunderstorms, mainly influenced by higher terrain. The highest 
chance will be in the North Big Horn Basin and along the Bighorn 

Wednesday and Thursday become more interesting with a series of 
shortwaves overriding the ridge and bringing increased coverage of 
showers and thunderstorms east of the Continental Divide. Thursday 
night into Friday morning will see the arrival of another weak cold 
front and more showers and thunderstorms overnight. Friday afternoon 
expect lingering showers and thunderstorms in the mountains east of 
the divide.


.AVIATION...For the 00Z TAFS through 00Z Saturday
Issued AT 345 PM MDT Fri Jul 20 2018

VFR conditions will prevail through the period. Isolated showers and 
thunderstorms are expected to continue south of a KCPR-KLND-KBPI 
line from 00z to 03z Saturday. The main hazard near these showers 
and storms will be erratic wind gusts of 35-45kts. 

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for 
the latest information on icing and turbulence forecasts.



Although there will be less wind this afternoon, temperatures 
will remain hot along with RH values at or below critical values. 
Isolated thunderstorms may fire in southwestern Wyoming this
afternoon or this evening, otherwise expect dry weather. Any 
shower or thunderstorm could have a strong wind gust, due to the
low RH. Mixing and smoke dispersal will generally range from good
to excellent. Expect an increased chance of afternoon showers and
thunderstorms for Saturday. 





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