Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180424

Area Forecast Discussion
National Weather Service Riverton WY
1024 PM MDT Tue May 17 2022

.SHORT TERM...This afternoon through Wednesday night
Issued at 1015 AM MDT Tue May 17 2022

A weak cold front associated with Monday`s trough is making its way
southward through the Bighorn Basin and Johnson County this morning.
A line of weak showers extend from northern UT to the Rock Springs
area to Casper Mountain. These showers are expected to continue
through the day, gradually shifting eastward and extending from
Flaming Gorge to southern Carbon County this afternoon. Additional
showers and thunderstorms will develop across areas east of the
Divide after 1 pm this afternoon. Likely locations will be a line
from Casper to Thermopolis/Worland, Cody and northern portions of
Yellowstone. This activity will move to the east-northeast through
the afternoon. Small hail and strong gusty winds will remain to be
the main threats, although dewpoint depressions will not be as
strong as Monday (especially east of the Divide, where dewpoints
will continue to be in the 40s). Temperatures will also be 5 to 10
degrees cooler than Monday, due to the cold front and cloud
cover. Showers will subside this evening, becoming isolated across
the north through midnight.

A more zonal flow pattern, with west-northwest flow aloft, will be
in place by tonight into Wednesday. Drier air will begin to
infiltrate southwestern portions this evening, pushing through
Sweetwater County through the overnight hours. Additionally, a weak
cold front will enter far northwestern portions late tonight. The
front will push through the Bighorn Basin and Johnson County after
sunrise Wednesday morning. Drier air will accompany the front across
areas east of the Divide, removing the Gulf moisture that has been
in place for the last several days. The front will gradually wash
out through the afternoon. This will result in mainly dry conditions
and elevated fire weather for much of the CWA. Above normal
temperatures will return, similar to Monday. Middle to upper 70s
will occur for much of the area, with a few locations reaching 80
degrees. RH values will drop into the teens, as a result of the
drier air. West to northwest winds, with gusts of 30 to 40 mph, will
occur across southern and western portions. These winds will spread
into the Bighorn and Wind River Basins, as well as Natrona County.
late in the afternoon. Isolated showers will be possible late in the
afternoon as well, over far northwestern portions, as diffluent flow
aloft develops ahead of the approaching storm system. These
showers will become more likely through the overnight hours over
northwestern portions, as the storm system gets closer. Snow
levels will be around 8500 ft or higher, before quickly dropping
to around 7000 ft by 12Z Thursday.

.LONG TERM...(Thursday through Tuesday)
Issued at 218 PM MDT Tue May 17 2022

A large potent trough barrels down from the northwest into WY early
Thursday. This trough will bring a strong cold front through during
the day on Thursday with a tight pressure gradient ahead of and
along the front. This will result in strong, likely high winds for
many, across the area Thursday. The southern half of the area will
see gusts of 35 to 55 mph with upwards of 60 mph possible in wind
prone areas along the Wind Corridor from Rock Springs to Casper. The
timing of the front remains a question. Model guidance has a spread
of anywhere from Thursday morning to Thursday evening for the
passage of the cold front. It may struggle to push south as quickly
as some guidance might suggest, due to a northwest to southwest
movement and diurnal heating during the day on Thursday. On the
contrary the front will be very strong with strong cold advection
pushing it forward. With those competing factors in mind am leaning
toward a middle of the road solution with the front moving into
Wyoming Thursday morning and moving through southern WY Thursday
evening. Temps should have some opportunity to warm diurnally
Thursday afternoon with highs around 60F north to low 70s south.
Very cold air will filter in behind the front noted by 700mb temps
as low as -10C Thursday night. Initial ptype will be rain with the
precip along the front, but this will change to snow showers behind
the front Thursday night due to the cold temps aloft and sfc temps
near freezing. Precip intensity and duration is not expected to be
enough for accumulating snow, especially due to warm ground temps.

Convective showers likely continue through Friday and Saturday with
weak instability in cyclonic upper-level flow. Below average temps
can be expected as well with cold air aloft remaining in place
through Saturday. There exists better model agreement today with the
pattern Sunday into Monday. A quick hitting shortwave passes through
Sunday with rain and thunderstorms possible across the area. Zonal
flow should develop on Monday once this shortwave passes, but it
does not look like it will last long with the next trough poised to
impact the area on Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1030 PM MDT Tue May 17 2022

VFR with some mid to high-level cloudiness increasing Wednesday
afternoon. Westerly winds with gusts 20 to 30kts are expected to
develop at most terminals Wednesday afternoon. The challenge will
be how much winds diminish at area TAF sites Wednesday evening.
Some sites such as KBPI, KCOD, and KLND could have low-level wind
shear develop Wednesday night. Some shower activity is expected to
push into northwest Wyoming after 06Z Thursday, and could impact
the KJAC terminal. KCOD and KJAC will have lowering ceilings late
Wednesday night. For now will keep both terminals VFR through 12Z

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


Issued AT 1015 AM MDT Tue May 17 2022

A line of weak showers from Flaming Gorge to Casper Mountain will
move eastward this morning and be mainly over southern portions of
zone 279 this afternoon. Showers and thunderstorms will develop
from Casper, Thermopolis/Worland, to Cody and zone 140 after 1 pm
this afternoon and move to the east-northeast through the
afternoon. Small hail and gusty winds will be the main threats.
Convection will end across the area by midnight. Drier air will
infiltrate across the area tonight, leading to RH values in the
teens Wednesday. Critical RH values will be possible across the
southern zones, as well as zone 280 and portions of 283. West to
northwest winds, with gusts of 30 to 40 mph, will also occur
across southern and western portions, spreading into the central
basins and zone 280 late Wednesday afternoon. Thus, elevated fire
weather conditions will occur for much of the forecast area.





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