929
FXUS65 KRIW 210401
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1001 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 959 PM MDT Mon May 20 2024

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

A slow moving weather system will pivot through the region over the
next 24 hours, which will bring rain chances to KRKS/KPNA Tuesday
afternoon. At KJAC/KBPI mostly dry conditions are expected, but an
isolated rain shower is possible (20-30%) Tuesday afternoon.
Westerly/northwesterly wind will increase Tuesday afternoon and
gusts will be 20-25 knots through the afternoon hours, quickly
fading Tuesday evening. Overall conditions will remain VFR, but a
drop to MVFR is likely (80%) if a heavier shower moves directly over
a terminal. Mountaintop obscuration will be prevalent through
tuesday afternoon.

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

A slow moving weather system will pivot through the region over the
next 24 hours, which will bring rain to all terminals tonight into
Tuesday morning. Steady rain tonight will give way to rain showers
late Tuesday morning through Tuesday afternoon at KRIW/KLND. A dry
period is likely (70%) at KCOD from 15Z-20Z/Tues before more rain
showers develop in the afternoon. A steady rain will hang on at KCPR
the longest, until 23Z, and then rain showers hang on for a few
hours before rain ends Tuesday evening. Wind increases at KCPR late
in the afternoon and will remain breezy for a few hours. At KWRL
rain ends at 20Z/Tues. As rain occurs MVFR/IFR conditions will
prevail at terminals. Mountain obscuration will develop and
persist through Tuesday afternoon.

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...Rowe