756
FXUS65 KRIW 171138
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
538 AM MDT Sat May 17 2025

.KEY MESSAGES...

- An increased chance of showers and thunderstorms today, most
  numerous in western Wyoming.

- Turning much cooler Sunday in the west with mountain snow,
  thunderstorms possible again East of the Divide.

- Much cooler Sunday night and Monday with rain and mountain
  snow continuing East of the Divide.

&&

.DISCUSSION...
Issued at 340 AM MDT Sat May 17 2025

Other than a few sprinkles, things are largely quiet across Western
and Central Wyoming as a shortwave ridge exits the area stage east.
this will not last though. Our next weather maker will move toward
the area today, an upper level low dropping out of British Columbia
and moving southeastward toward the Cowboy State over the next few
days. Most areas will start dry but increasing moisture will
introduce the chance of showers to southern areas starting later
this morning with the chance of showers spreading across roughly the
western two thirds of the area this afternoon. The best coverage of
showers will remain in western Wyoming, with anywhere from 2 in 5
chance to a 7 out of 10 chance at any given location. In areas East
of the Divide, the chance drops to 1 in 3 at the most. In addition,
in areas East of the Divide a vast majority of the day will be rain
free. Southwest flow will warm temperatures as well, with most areas
being within a few degrees of normal. As for stronger thunderstorms,
the Storm Prediction Center has dropped the marginal risk in
southwestern Wyoming this morning. There could be a few stronger
storms but this would be the exception rather than the rule.

By tonight, showers should end East of the Divide, but continue in
the west. And now brings us to Sunday, and we have a few concerns.
First, we will address thunderstorms. This day looks like a decent
likelihood for them, especially East of the Divide, The next
question is, will they be strong? There is a bit more CAPE on this
day, especially in our most favored location, Johnson County, where
values as high as 750 J/KG may exist. Lifted indices only go to
minus 2 as well. There will be a bit of shear though. The big X
factor will be cloud cover. If we can get some clearing to
destabilize the atmosphere a bit, we could get a few stronger storms
in the afternoon, especially in Johnson County and the northern
Bighorn Basin. This is far from certain but we will have to watch
it.

And, with the schizophrenic nature of weather with spring
weather in Wyoming, this reminds of an old Monty Python quote, "Now
for something completely different." And that is the cold side of
the storm and potential for snow. Some of the higher peaks West of
the Divide will see some through tonight. However, 700 millibar
temperatures should remain around 0 celsius or higher through
Saturday night, which would keep snow largely above pass level.
Colder air then arrives in earnest on Sunday across the west,
spilling East of the Divide Sunday night into Monday. At the same
time, the upper level low will be moving across Wyoming. Lee
cyclogenesis will be occurring over Colorado Sunday into Sunday
night, with the low moving into Nebraska on Monday. Flow will likely
turn northeastward and enhance upslope along the eastern slopes East
of the Divide. As for snow levels, areas in the west may see 700
millibar temperatures fall as low as minus 4 to minus 5, which could
bring some snow even down into the valley floors on Sunday. East of
the Divide, the lowest would be minus 3, which would put snow levels
at 7000 feet by Sunday night. As for accumulations, NBM ensemble
guidance gives at least a 1 in 2 chance of 6 inches or more across
the higher elevations of the Tetons, Absarokas, Wind River Range and
Bighorns. As for the chance of a foot, the main concern here would
be the higher elevations of the Bighorn Range, with a 1 in 2 chance
above 9500 feet. As for highlights, none will be issued at this
time. The heaviest snow for the Tetons would fall during the day on
Sunday, and with borderline temperatures and the strong mid May sun,
it would likely have trouble sticking to roads. In the Absarokas,
the heaviest snow would fall in areas with few to no people. In
addition, the one impacted road, Beartooth Pass, is still seasonally
closed. In the Wind River Range, the heaviest would fall in the
western peaks away from travel routes. So, that leaves the Bighorn
Range. I do have more concerns here. There will be favored upslope
flow. The heaviest snow will also likely fall on Sunday night, when
there is a better chance for accumulation on roads. We won`t issue
any highlights right now since impacts wouldn`t begin until
Sunday night, this would be the most likely location. All areas
will see well below normal temperatures Monday, with places
like Buffalo likely remaining in the lower 40s.

The other concern is heavy rain. There has been decent consistency
in showing the heaviest QPF in the western mountains as well as the
northern half of the area. The Weather Prediction Center has issued
a marginal risk for excessive rainfall for the northeastern half of
the area for Sunday. Precipitable waters do rise to around 150
percent of normal for the Bighorn Basin as well as Johnson County on
Sunday. As for flooding, there is some risk but not a large one. The
air is not very warm ahead of the system and cooler air moving in
Sunday night should put an end to any snowpack melting in the
mountains. The main threat would be from any convection that
develops and puts down heavy rain in a short amount of time.
Probabilistic guidance gives at least a 1 in 2 chance of a half an
inch of QPF from Sunday through Monday night for the northwestern
half of the area. The favored north to northeasterly upslope areas
have at least a 2 in 5 chance of 1 inch of QPF or more. This
includes the Lander Foothills, near Thermopolis  and especially
Johnson County where the chance rises to as high as 3 out of 4. Most
of the rain will fall west of the Bighorns through Sunday night,
with Johnson and Natrona Counties seeing rain and snow continuing
through Monday and even Monday night before tapering off.

Quieter weather should move in after this. Some lingering moisture
may bring a few showers Tuesday or Wednesday but it will be a milder
day. Ridging should then build into the area later in the week,
bring drier and warmer weather as we head toward the holiday weekend.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 537 AM MDT Sat May 17 2025

VFR conditions with mostly scattered cloud decks are expected
through the rest of the morning. Focus then returns to convection as
showers and thunderstorms become widespread this afternoon as the
leading shortwave from an approaching trough pushes into Wyoming.
This activity will move from southwest to northeast through the
afternoon, with the main aviation concern of gusty winds to around
40 kts. Brief heavy rainfall is also possible with the
strongest cells, which could bring short periods of MVFR
conditions to terminals. While shower and thunderstorm coverage
will generally decrease late this evening, the arriving trough
will bring more persistent precipitation chances to western
Wyoming (KJAC) near the end of the period.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Hattings
AVIATION...Myers