262
FXUS65 KRIW 221048
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
448 AM MDT Wed May 22 2024

.KEY MESSAGES...

- Showers return to western Wyoming today; breezy to windy,
  warmer and mainly dry East of the Divide.

- Snow returns to the western mountains tonight.

- Much cooler Thursday with snow in the northern mountains, the
  heaviest in the northern Bighorns.

- Unsettled weather expected for the holiday weekend, although
  details for timing and placement of precipitation remain
  uncertain.

&&

.DISCUSSION...
Issued at 329 AM MDT Wed May 22 2024

Things are quiet as I write this discussion around 2 am this
morning. However, the quiet will not last. The next round of
precipitation will arrive in the west, courtesy of an upper
level low now circulating over the Pacific Northwest. Moisture
will advect into northwestern Wyoming starting this morning, but
precipitation amounts should remain on the light side through
the daylight hours. Dry and warmer conditions will be found east
of the Divide, but it will become windy as the pressure
gradient tightens ahead of the advancing system. High wind is
not expected today, but the favored areas in this pattern, like
Casper and Rock Springs, will get quite gusty.

Deeper moisture begins to move into the west as well as better upper
level dynamics in the form of an 100 knot jet streak to enhance
upper level divergence. This will bring steadier precipitation to
the western mountains. Snow levels will remain around 7500 feet or
higher through the evening though before falling to the valley
floors later at night. Most guidance is fairly consistent in keeping
the heaviest precipitation at night largely between 6 pm and
midnight. There could be some decent rates though, around 1 in 2
chance of 1 inch an hour at times. Probabilistic guidance gives
around a 2 in 5 chance of over 6 inches of snow in the higher
elevations of the Tetons. However, this is over a longer period.
So, we will not issue any Winter Weather highlights but will
issue a Special Weather Statement for tonight. If the cold air
can get in fast enough, there is a chance (about 1 out of 3) of
some light accumulations of snow in the western valleys as well.
The other area that could see decent snow tonight is the
Absarokas, but this would fall over areas with almost no people
or roads, so no highlights here.

The concern tonight east of the Divide is wind, mainly in the
southwesterly flow areas. There are a few 50+ knot wind barbs in
the favored areas of southeastern Fremont and Natrona Counties
later tonight. But this is for a short period of time around 12Z
before it lessens. There could be a few wind gusts of over 58
mph, but this would be mainly in the higher elevations where
impacts would be few. The best chance in the lower elevations
would be in places like Wyoming Boulevard and Hat Six Road near
Casper, but these look isolated. A Special Weather Statement
would probably cover this as well. As for precipitation, some
showers will spread into the area, mainly across the northern
half of the area. But any decent precipitation would hold off
until Thursday.

And now for Thursday. This was the most challenging period,
with the most difficult decision and the potential for
highlights. There were several factors that went into this
decision. First was QPF. This system does have a decent amount
of moisture to work with. However, the deepest moisture looks to
remain over Montana. Some guidance does show a bullseye over
the northern Bighorns with over an inch of QPF. This is a very
small part of the our area though. The other concern is that
there could be a very tight gradient in regards to the amounts.
Southern portions of the range may see little to no
precipitation. The second concern is wind. There will be a lot
more wind with this system on Thursday, especially in the
afternoon as the low moves to the east and the favorable
northwest flow / cold advection pattern sets up. Wind could get
quite strong, with around a 2 in 5 chance of wind gusts to 50
mph around Buffalo. There is not quite enough mid level wind for
High Wind Warnings though. The third consideration was timing.
Guidance is in very good agreement in the bulk of the
precipitation occurring during the day. And with the strong late
May sun, snow may have trouble sticking to the roads at this
time. So, with a greater than 2 in 3 chance of 6 inches of snow
or more north of Powder River Pass, we went with a Winter
Weather Advisory. Chances of 12 inches or more are less than 1
out of 2 except for the far north and highest peaks, so we have
not gone with watches or warnings. As for the rest of the area,
this looks like a blustery and cool day with showers scattered
through the area. The vast majority would fall across the north
with less of a chance in the south. For the most part, wind
should fall short of high wind criteria. Temperatures will
average 10 to 20 degrees below normal across the area.

More settled weather returns Friday with shortwave ridging over the
area, bringing a warmer and more settled day. As for the holiday
weekend, things are more uncertain. The third in the series of upper
level lows will cross the area and bring a chance of showers for
Saturday and Sunday and possibly into Monday as well. There is a
larger spread in guidance here though. So, although we have a
high confidence in unsettled weather for Saturday and Sunday,
confidence in the details on timing and placement of
precipitation remains rather low. Memorial Day is also up in the
air depending on the speed of the low. Ridging should then
build into the area and bring drier and warmer weather for the
middle of next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 440 AM MDT Wed May 22 2024

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

Prevailing VFR conditions will continue this morning, as high
clouds stream over the area from the west. Rain showers will
move into western Wyoming by midday, but will be mostly confined
over the mountains. This will also impact KJAC, but conditions
will predominantly remain VFR even as showers pass through.
Steadier rain is expected at KJAC Wednesday evening after 01Z,
so reductions in vis to MVFR are possible (40%). Isolated
thunderstorms will develop Wednesday afternoon into Wednesday
evening, mainly in the mountains around KJAC. Not enough
confidence in thunderstorm coverage for VCTS at KJAC at this
time.

Other terminals will remain dry, although there is a 20-30% chance
of rain at KBPI/KPNA Wednesday evening. Wind will increase at all
terminals late Wednesday morning from the southwest, with gusts of
20-30 knots at all terminals through the afternoon hours. A wind
shift from the northwest will occur late Wednesday evening.
Mountaintop obscuration will begin late this morning as clouds
continue to increase and continue through the TAF period.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Prevailing VFR conditions will persist over the next 24 hours,
but wind will increase for all terminals late this morning as
another storm system approaches the state. Wind will be
southwesterly sustained in the teens, gusting 20-30 knots from
14/16Z through the afternoon, gradually decreasing between
00Z/02Z. A shift to northwesterly wind is likely (70%)
this evening for KWRL/KRIW/KLND.

Rain will spread over northern portions of the Bighorn Basin
after 06Z. Snow is not expected at this time, but the rain could
change over to snow if the cold air arrives faster than
expected.

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...
Winter Weather Advisory from 3 AM to 6 PM MDT Thursday for
WYZ008-009.

&&

$$

DISCUSSION...Hattings
AVIATION...LaVoie