462
FXUS65 KRIW 212224
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
424 PM MDT Tue May 21 2024

.KEY MESSAGES...

- Showers and isolated thunderstorms continue this afternoon and evening.
  The majority of precipitation likely remains east of the
  Divide.

- Clearing skies will cause temperatures to drop into the mid
  30s overnight, but light winds should prevent widespread
  freezing, so the Freeze Watch has been canceled.

- Another round of widespread showers and high elevation snow
  for the second half of the week.

- Near to below freezing temperatures are possible for Friday
  morning.

&&

.DISCUSSION...
Issued at 158 PM MDT Tue May 21 2024

Only 4-5 days ago temperatures were in the mid to upper 70s and it
was feeling like summer was on its way. Unfortunately, the
rollercoaster that is Spring weather in WY continues, with winter
making a return to parts of the Cowboy State this morning. Parts of
Lander and Casper saw a brief period in which rain transitioned over
to snow this morning. These areas have now returned to rain, as the
strong May sun has warmed daytime temperatures. Snow is continuing
to fall across high elevation mountain ranges, such as the Bighorns.

The upper-level low that developed overnight and moved into the
Northern Plains will continue to bring chilly and unsettled weather
for today. Periodic showers will be possible throughout the day as
moisture from this disturbance wraps back around from the north. The
bulk of the precipitation will remain east of the Divide with a (40-
80%) chance of showers during the afternoon and evening.
Precipitation chances west of the Divide are less likely, with a (10-
40%) chance of a shower this afternoon and evening. A few isolated
thunderstorms are possible this afternoon/evening with much of the
state seeing about a 20% chance. The amount of sun that can peak
through the low clouds east of the Divide will be key to determining
if enough instability can develop to fuel thunderstorm development.
Overall, the threat for thunderstorms remains low, with the only
concern being gusty outflow winds from any that do form. Highs
across the Cowboy State today will be chilly with temperatures in
the upper 40s to low 50s. These temperatures are nearly 15 to 20
degrees below normal values for this time of the year. This leads to
concerning temperatures overnight into Wednesday morning. Tonight
skies are expected to gradually clear out. This all points to
efficient diurnal cooling occurring, with lows dropping into the 30s
for basins. Light breezes should mitigate frost concerns, and the
Freeze Watch has been canceled. That being said, overnight lows will
be below seasonal normals, so those not wanting to risk frost may
want to take appropriate actions.

Wednesday sees a quick moving and weak transitory ridge move through
the region. Behind this ridge is another shortwave that is barreling
its way south from British Columbia. Due to the Canadian origin of
this system, chilly temperatures return to the Cowboy State.
Precipitation looks to remain mainly across the northern and western
half of the CWA. These areas have a (40-80%) chance of precipitation
starting Wednesday afternoon into Thursday afternoon. The remainder
of the state may see a few showers but at a much lower likelihood,
with models showing a (10-30%) chance for precipitation. High
elevation mountain ranges across the northern half of the CWA may
see another round of snow with amounts nearing Advisory criteria. By
Thursday evening, precipitation will begin to wind down as the
shortwave moves out of the region. Depending on how quickly the
skies can clear, we may have another chilly morning come Friday.
Currently models are showing a (20-50%) chance of lows at or below
freezing. If this trend continues there may be another round of
Freeze Watches issued for low elevation basins.

The upcoming holiday weekend will be worth monitoring as models are
indicating a potent low moving through the region sometime during
the weekend. This may lead to some widespread unsettled weather
across the Cowboy State. However, conditions look to improve by
Monday with temperatures returning to near normal values. There
still is a lot of uncertainty regarding the forecast this far out in
advance, so its very possible things may change as we near the
weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 423 PM MDT Tue May 21 2024

A few showers will linger until 03Z, with brief reductions in vis to
MVFR if a shower impacts a terminal. Overnight will feature light
wind and clearing skies. After daybreak on Wednesday, wind will
slowly increase and become breezy for most by the late morning
hours. Locations like KJAC in far western Wyoming will see another
round of showers move through from late morning into the early
afternoon hours. Any mountain obscuration will end this evening
as skies clear.

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 this evening for WYZ002-
015.

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

&&

$$

DISCUSSION...Dziewaltowski
AVIATION...VandenBoogart/Rowe