008
FXUS65 KRIW 222159
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
359 PM MDT Thu May 22 2025

.KEY MESSAGES...

- Afternoon showers and thunderstorms return to northern and
  central WY today with the possibility of some strong gusty
  outflow winds.

- Gusty winds of 25-30 mph and low humidity may create elevated
  fire weather conditions across southern WY this afternoon.

- Friday looks quite seasonable with isolated showers and
  thunderstorms during the late afternoon and early evening
  across northern WY and far southern WY.

- The weekend storm system is still expected to impact the state
  with cooler temperatures and scattered to widespread
  precipitation. Heavier rain is now likely to fall more
  eastward across east-central WY into parts of central WY, and
  start during the late afternoon going into Sunday morning.
  Gusty northeast winds will create chilly conditions.

- Memorial Day looks to be quite nice with temperatures from the
  mid 60s to low 70s, though some late day showers and
  thunderstorms are expected, especially over northwest WY.

&&

.UPDATE...
Issued at 110 PM MDT Thu May 22 2025

The shortwave ridge over Wyoming today will move eastward during
the afternoon and evening hours as a shortwave trough moves from
eastern Oregon through central Idaho and into southwest MT
overnight. Clouds ahead of the trough are streaming over
northern WY, increasing moisture. Current models are indicating
that instability will increase between 3-6pm to give rise to
showers and thunderstorms over northwest WY. Confidence is not
particularly high since half of the high res models have fairly
good coverage from about 4pm to 9pm, with the other half only
indicating a handful of storms. The most likely impact from the
storms should be gusty outflow winds of 40-50 mph along with the
brief rain. Storms should initiate over the mountains and then
move over the northern half of the Bighorn Basin during the
evening. With southeasterly flow along and east of the divide,
and gusty southwest winds west of the divide, also anticipating
a few storms to kick off along the boundary and take advantage
of the increased moisture. Models are uncertain of specific
areas, but there are low chances around and east of Rock
Springs heading ENE toward Casper. The northern half of the
Bighorn Mountains should then see moderate precipitation from
sunset to midnight as the area of showers move toward SE MT.

Temperatures on Friday will be about the same as today with
mostly clear to partly cloudy skies. Isolated late day showers
and thunderstorms are possible over far southern WY, and across
far northern WY. Conditions are still likely for a stronger
storm from Saturday into Sunday. The biggest changes from the
previous forecast are 1) the area of the most impact appears to
now shift more to east-central WY, though some areas across
central WY could see heavy precipitation from Saturday evening
into Sunday morning; and 2) the timing for the larger impacts
appears to be above 6 hours later to start late Saturday
afternoon instead of early Saturday afternoon. Minor changes in
reality of the storm track will cause large local changes with
the impacts from this storm. Hopefully details will become
clearer tomorrow.

&&

.DISCUSSION...
Issued at 227 AM MDT Thu May 22 2025

Similar weather to Wednesday is expected across the Cowboy State
today. Showers and thunderstorms return to northern WY this
afternoon and evening with most seeing a 10-40% chance. Portions of
central WY have a chance of seeing a brief shower or thunderstorm as
well with chances ranging from 10-30%. Hi-res models look to paint
the best chances across parts of Natrona and eastern Fremont
Counties. While conditions are not the most favorable there is still
come favorable dynamics in place to possibly produce a strong storm
or two. If any strong storms do develop the main concerns will be
strong gusty winds and small hail. High temperatures will be mild
across the state with values in the mid 60s to low 70s. Winds will
be breezy today especially across southern WY where periodic gusts
of 25 to 35 mph will be possible. Near elevated fire weather
conditions are possible mainly in parts of Sweetwater County where
RH values will range from 15 to 20%.

A shortwave is expected to move across the region Friday bringing
another day of showers and thunderstorms. The track of the shortwave
looks to bring the best chances for showers and thunderstorms to far
northern/eastern WY. Most will remain dry Friday with precipitation
chances ranging from 10 to 30%. Mild temperatures persist with highs
ranging in the mid 60s to mid 70s. A more potent disturbance takes
aim at the state for the upcoming weekend. Model guidance shows it
develop and track across the Great Basin Saturday. The disturbance
gradually shifts east during the day Saturday bringing increased
chances for showers and thunderstorms for much of the state.
Saturday evening will see showers become more prevailing as upper-
level flow will create a brief period of favorable northeasterly
upsloping flow. This timeframe which looks to be around late
Saturday into early Sunday will likely see moderate to heavy showers
develop. Locations which favor this flow such as portions of the
central and eastern WY can expect to see the most rainfall. One
thing to monitor will be temperatures as models suggest they may be
too warm to support snowfall in the higher elevation. The only
exception to this may be elevations above 10000 feet which could see
wet heavy snowfall. However, this may be the first event of the
Spring in which rain will fall on the lingering snowpack above 9000
feet. While the forecast does not call for a concerning amount of
liquid precipitation as a result of this system, it will be worth
monitoring the possibility of some minor river rises.

The bulk of the precipitation as a result of this disturbance
gradually comes to an end Sunday. Showers and thunderstorms will
linger across the region into the start of next week. The good news
is this likely wont result in a washout of a Memorial day as
precipitation is looking rather isolated. Temperatures will begin to
warm as a ridge start building in over the western CONUS. Highs on
Monday will be mild ranging from the low to upper 70s. These
temperatures persist through the first half of the week with highs
in the mid 70s to low 80s across the state. Daily chances for
afternoon showers and thunderstorms do look possible for the first
half of the week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 359 PM MDT Thu May 22 2025

VFR conditions will prevail at all terminals through the period. A
shortwave passage will bring slight chances (20 to 40%) of showers
and isolated thunderstorms across northern WY generally through 03Z,
though storms that have occurred through the afternoon have been
weak, and mostly have been just showers and even virga showers.
Most likely chances will occur at KCOD for thunderstorm impacts,
which could include brief MVFR conditions and gusty and erratic
outflow winds. The gusty outflow winds are also possible at KWRL,
but chances for precipitation or thunderstorms passing over the
terminal are even more unlikely there. There are also slight chances
for showers (10 to 20%) and possibly a thunderstorm (less than 10%
chance of TS) for KRKS and KCPR through 00Z as well, as CAMs and
radar continue to indicate weak convection developing off the Uinta
Mountains and spreading northeastward through the late afternoon and
early evening. Still not enough confidence to include mentions of
these showers in the TAFs.

Winds will be the other notable aviation concern during the period.
Winds will diminish by 02/03Z as diurnal heating is lost and nearly
all sites will see light winds prevail through the rest of the
period. The exception would be the northwest drainage flow at KRIW,
which could continue at 10 to 15ts through much of the night. Winds
are expected to again increase at most terminals by early to mid
afternoon Friday.

At KLND, there continues to be hints at a weak mountain wave. Have
left a TEMPO group and extended it through 04Z to account for this.

At KJAC, most guidance indicates winds will remain
south-southwesterly through the period, and LAMP/MOS guidance has
expectedly backed off on suggestions of switching to northerly flow
overnight, thus have continued to leave prevailing south-
southwesterly flow through the 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...
None.

&&

$$

UPDATE...McDonald
DISCUSSION...Dziewaltowski
AVIATION...Hensley