155
FXUS65 KRIW 202136
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
336 PM MDT Mon May 20 2024

.KEY MESSAGES...

- Wet and cool today and tomorrow. Highest rain totals east of
  the Divide.

- Mountain snow, heaviest Monday night through Tuesday morning. Expect
  winter travel conditions along the mountain passes.

- An active pattern continues through the rest of the week. A
  couple more weather systems impact the area through the end of
  the week and this weekend.&&

.DISCUSSION...
Issued at 1259 PM MDT Mon May 20 2024

A trough centered over Idaho is currently moving eastward. This is
the main weather driver today and tomorrow, as it crosses through
Wyoming. Today, noticeably cooler temperatures have already made
their way into the state. Highs peak in the 50s for most, or about
10 to 15 degree below normal for mid-May.

Besides the temperatures, precipitation has also moved in the area.
Current radar shows a swath of rain extending from southwest
Sweetwater County into Natrona County. This is where the best rain
occurs today, but rain chances spread to much of the area through
the afternoon. The last place to get rain chances is western
Wyoming. In summary, everywhere has a shot to see rain today, with
some thunder possible. For the mountains, snow levels are 7000 to
8000 feet today, with light snow accumulation for most of the higher
elevations. Up to a few inches is possible for the highest terrain
for the Absarokas, Bighorns, and Winds through this afternoon.

The trough moves eastward tonight, centered over Wyoming. This
brings a more enhanced period of precipitation Monday night through
early Tuesday afternoon, primarily focused over locations east of
the Continental Divide. Latest models have the low slightly
northward of previous runs, influencing locations of highest
rain/snow totals slightly.

Starting with the lower elevations, rain totals Monday through
Tuesday are highest across Fremont, Natrona, and Johnson Counties.
Totals here are roughly 0.50 to almost an inch. HREF indicates about
a 20% chance for locations to exceed an inch. A few locations may
get repeated heavier showers resulting in over an inch, but for the
most part, amounts are less than an inch. There is a bit uncertainty
with amounts towards Natrona and Johnson Counties, with some models
showing a heavier band of rain in that area. This would result in
higher totals than currently forecast. Elsewhere, generally under
half an inch is forecast through Tuesday, with a few spots of higher
totals possible for Sweetwater County.

For the mountains, the highest snowfall will occur where the best,
favored upslope occurs. For the projected track of the trough, this
is the eastern sides of the Absaroka, Bighorn, and Wind River
Mountains. Due to the track looking to be a touch more north,
amounts in the Absarokas and Winds have decreased a little since
yesterday. The eastern Wind River Range could see up to 10 inches of
snow. For the Absarokas, mainly the southern range will get the
better snow amounts, with a little over a foot for the higher
elevations. The Bighorns are a bit higher due to favored flow, with
near a foot and half. Looking at impacts, expect winter travel
conditions on mountain passes late Monday through Tuesday, the
heaviest snow falling Tuesday. Winter weather Advisories are in
effect for the Wind River and Absaroka mountains. Heaviest snow
falling during the day will somewhat limit road impacts. A Winter
Storm Warning is in effect for the Bighorns for the higher
forecasted amounts.

Precipitation comes to an end Tuesday evening as the trough exits to
the east. Tuesday night/Wednesday morning will be cold, with
forecast lows in the lower 30s. For locations that have started
their growing season, like the Bighorn Basin, freezing temperatures
are possible. Though the current forecast is for many locations
there to be around 34 or 35 degrees, cold prone/low areas could be a
few degrees cooler. Right near the surface also tends to be colder
than the forecasted temperature due to radiational cooling, so plants
in the ground may develop frost. A Freeze Watch is in effect for the
Bighorn Basin Tuesday night/Wednesday morning to cover this
potential.

Slight, and brief, ridging builds for Wednesday. This should help
bring temperatures back towards normal. The next weather system,
however, is quick to approach. Rain/mountain snow chances increase
across western Wyoming as early as the afternoon.

This upper low`s main impact comes Thursday/Friday and looks to move
through quickly. Temperatures drop a little during its passage.
Favored location for precipitation is northern Wyoming, but given we
are still a few days out, have at least a small chance everywhere in
the forecast. Winds also increase across the area these days,
strongest across the Wind Corridor.

The weekend stays active, with yet another low impacting the area
Saturday, potentially into Sunday. Past that, it looks like ridging
may be in the cards, but its difficult to give confidence 9 days
out.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 335 PM MDT Mon May 20 2024

Showers will continue this evening with a few isolated thunderstorms
possible. All terminals will have at least a (30-70%) chance of
showers this evening into Tuesday morning. MVFR conditions are
likely to prevail at times during the period, especially at KCPR,
KRIW, KLND, and KWRL. Prevailing light rain develops late this
evening through Tuesday afternoon at KCPR, KRIW, KLND, and KWRL.
Isolated IFR conditions cannot be ruled out this evening and
overnight, mainly after 06Z Tuesday. KJAC will see showers dissipate
during the late evening hours and remain mostly dry overnight.
Showers and a few isolated thunderstorms will be possible across all
terminals during the afternoon Tuesday. Light winds of 10-20 knots
will continue through the majority of the TAF period. Mountains will
likely be obscured throughout the TAF 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...
Winter Weather Advisory until 6 PM MDT Tuesday for WYZ002-015.

Freeze Watch from late Tuesday night through Wednesday morning
for WYZ003>006.

Winter Storm Warning until 6 PM MDT Tuesday for WYZ008-009.

&&

$$

DISCUSSION...Wittmann
AVIATION...Dziewaltowski