457
FXUS65 KRIW 241714
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1114 AM MDT Sat May 24 2025

.KEY MESSAGES...

- A weather system across the central Rockies is bringing
  showers and thunderstorms today, especially to areas along and
  east of the continental divide. A few strong storms are
  possible around Natrona County. Some areas from the eastern
  Wind Rivers to the central mountains and into Johnson and
  Natrona Counties could see moderate precipitation totals over
  half and inch.

- Chances of showers and thunderstorms continue into Sunday
  along with cool temperatures and an easterly wind.

- Temperatures warm by 5-8 degrees on Monday, Memorial Day,
  though mountain showers and thunderstorms are expected in the
  afternoon and early evening, with some precipitation also
  stretching from South Pass into eastern Sweetwater County.

- An active weather pattern continues through all of next week,
  especially with isolated showers and thunderstorms in the
  afternoon hours. A somewhat stronger storm system could impact
  the state next Friday and Saturday.

&&

.DISCUSSION...
Issued at 226 AM MDT Sat May 24 2025

In western and central Wyoming, Memorial Day weekend is usually the
wettest of the major summer holidays, falling in the wettest time of
the year climatologically in this part of the country. And this
weekend is not exception. Things are largely quiet right now with
only a couple of spotty showers. However, this will not last. The
players in this pattern are an upper level low spinning over the
southwest and a shortwave that will be moving through the area
today. This will result in a good coverage of showers and
thunderstorms across the area, especially East of the Divide where
most locations have at least a 1 in 3 chance of a shower or
thunderstorm. Activity will increase in northern Wyoming later this
morning and then shift southward through the day, with the greatest
chance this evening across central Wyoming. As for the chance of
stronger thunderstorms, there are a few factors in favor of it.
There will be some decent directional and speed shear across
portions of the area. The area will also have some additional lift
from the right rear quadrant of the jet. One limiting factor may be
cloud cover, which would limit surface instability. CAPE maximizes
in Natrona County, where some areas may see as much as 1000 J/Kg
this afternoon with lifted indices as low as minus 4. So, if we were
to get stronger thunderstorms, the best chance would be here. The
Storm Prediction Center also agrees, having a marginal risk here.
And, given the decent shear, there is even a 2 percent chance of a
tornado in the mentioned areas. The greatest coverage of these
storms would be in Johnson and Natrona Counties this afternoon, with
some weak easterly upslope flow. Some guidance also indicates and
area of convection over the Wind River Basin this evening, shifting
eastward until around midnight. As a result, we increased POPs this
evening. Areas along East of the Bighorn Range have at least a 1 in
2 chance of a quarter inch of rain or more through 6 am Sunday.

Cyclonic curvature will remain over Wyoming on Sunday, bringing
another day where most areas will have a chance of a shower or
thunderstorm. Dynamics are not as impressive this day and stability
parameters are not as impressive either at this time. As a result,
there is a lesser chance of stronger storms. Probabilistic guidance
also shows a lower chance of over a quarter of an inch of rain at
most locations, with at most a 1 in 3 chance. Temperatures will
remain below normal through these days.

Coverage of convection should decrease and temperatures should
increase as we head into Monday as ridging tries to build over the
area. There will still be a few around but most areas should remain
dry. The next potential weather maker will be low dropping in the
from the north for the middle of next week. Guidance still has some
differences on placement of it and the resultant coverage of
convection. We are confident chances will increase, but details
about timing and placement of showers remains up in the air.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1112 AM MDT Sat May 24 2025

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

At KBPI and KPNA, some isolated showers and thunderstorms are
possible (20%-40%) this afternoon. Peak chances occur between about
20z and 23z/Sat, with decreasing chances through 06z/Sun. Better
chances occur at KRKS (50%-60%). Low chances at KJAC (15%) this
afternoon, so have kept mention out of TAF.

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

Showers and thunderstorm will be around this afternoon, through
about midnight (06z/Sun). Models and guidance have large
disagreements currently in where and how widespread showers/storms
occur. As an overview, showers/storms will be around this afternoon,
with a 30% to 70% chance to impact a site; the focus is over central
Wyoming, so KCPR, KLND, and KRIW have the highest chances. For this
afternoon, have kept PROB30 groups to indicate storm chances
due to lack of confidence due to model disagreement mentioned
earlier. Showers/storms look most widespread after 22z/Sat, so
have prevailing and TEMPO groups to cover the evening (chances
after 23z are generally 60% to 90% for KCPR/KLND/KRIW).
Northwest winds should help upsloping at KCPR, KLND, and
possibly KRIW. Main hazards over central Wyoming with storms are
outflow winds and hail. Heaviest rain occurs after 22z/Sat, so
more likely for MVFR to brief IFR conditions with rain after
that time. Overall, low confidence in exact timing of storms,
but high confidence storms will be around.

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.

&&

$$

DISCUSSION...Hattings
AVIATION...Wittmann