742
FXUS65 KRIW 242257
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
457 PM MDT Sat May 24 2025

.KEY MESSAGES...

- A weather system across the central Rockies is bringing
  scattered showers and thunderstorms today, especially to areas
  along and east of the continental divide.

- A few strong storms remain possible this evening across
  central WY with hail and gusty wind the most likely impacts.

- 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 into Sunday
  morning.

- 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.

&&

.UPDATE...
Issued at 125 PM MDT Sat May 24 2025

With an upper-level low over Las Vegas pushing drier cooler air
from the southwest up and over the moist and cool boundary layer
in Wyoming, showers and thunderstorms are developing along the
air mass boundary. Storms along the continental divide seem to
be initiating along the air mass boundary, especially over the
mountains. While storms further east are more elevated in nature
with the deeper boundary layer. SPC has extended the marginal
area of severe storms a bit further to the west over most of
Fremont County today. Low-end severe hail and gusty outflow
winds of 50-60 mph are the most likely impacts from the stronger
storms over central WY from about 300pm through 900pm. As the
convection decreases around sunset, models continue to indicate
a stronger upslope component developing from the southeast Wind
Rivers along Beaver Rim toward southern Natrona County. This
pattern for about 3-6 hours overnight should produce areas with
more than a half inch precipitation before the forcing moves
eastward into east-central WY. Casper Mountain and the
surrounding northeast facing foothills have the best chance of
seeing higher rainfall amounts through Sunday morning.

The upper-level low then moves into central Utah on Sunday,
keeping the trough axis over the state, and thus the
instability, moisture, yet weak steering flow. Convection is
expected to develop over the mountains areas as a result, with
the latest models showing isolated activity over southwest into
southern WY. Weak showers should linger across Johnson and
Natrona Counties during the morning hours too. As the low moves
over WY Sunday evening, temperatures will remain on the cool
side and low level flow should remain easterly. While Memorial
Day starts off a bit cool but nice in the morning, mid- to
late-afternoon showers and thunderstorms should kick off over
the mountains along the divide and over western WY, particularly
in the early evening hours as a weak shortwave trough moves into
central Idaho.

&&

.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 /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 457 PM MDT Sat May 24 2025

Showers and thunderstorms will linger through the night, but the
strongest storm potential ends with the setting sun. Lingering
showers will mainly be east of the Divide, around KLND, KRIW, and
especially KCPR. Low clouds (MVFR to IFR) are looking most likely at
KCPR overnight, but could also occur around KLND.

Isolated showers/storms are once again possible Sunday afternoon,
with fairly widespread 20% to 30% chances. Not enough confidence at
this time to include in many of the TAFs. Wind will remain
breezy this evening, with stronger gusts possible before
convection subsides around 03-04Z.

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...Hattings
AVIATION...Rowe/Wittmann