489
FXUS65 KRIW 220933
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
333 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 today.

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. We will
issue a Special Weather Statement for tonight though. 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. And this was most challenging period, with
the the most difficult decision, 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 Big Horns with over
an inch or 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 or 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, at this time we decided 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 where 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 /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 953 PM MDT Tue May 21 2024

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

One weather system exits tonight as another moves in tomorrow.
Prevailing VFR conditions will continue tonight before the next
system moves into the area. Then, rain showers will move into
western Wyoming late Wednesday morning. This will impact KJAC, but
conditions will predominantly remain VFR even as showers pass
through. Steadier rain is possible (60%) 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 around KJAC late Wednesday
morning and continue through the TAF period.

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

One weather system exits tonight as another moves in tomorrow.
Prevailing VFR conditions will persist over the next 24 hours, but
wind will increase for all terminals late Wednesday morning. Wind
will be southwesterly sustained in the teens, gusting 20-30 knots
from late Wednesday morning through Wednesday afternoon. A shift to
northwesterly wind is likely (70%) Wednesday evening for
KWRL/KRIW/KLND.

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...Rowe