239
FXUS65 KRIW 230908
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
308 AM MDT Mon Jun 23 2025

.KEY MESSAGES...

- After a cool start, a dry and warmer day today with only
  isolated showers and storms in the east.

- Tuesday could be active with strong to severe thunderstorms
  possible, especially in Johnson and Natrona Counties.

- More typical summer weather Thursday into the weekend with
  warm temperatures and isolated afternoon thunderstorms.

&&

.DISCUSSION...
Issued at 306 AM MDT Mon Jun 23 2025

Welcome to the start of the first work week of summer. It does not
feel like it though. I duck outside a short time ago and it was
chilly, with temperatures in the 40s at the office. Other locations
are even cooler though. Both Big Piney and Pinedale are at our below
freezing with other locations in the 30s. With this, we will leave
the freeze warnings in effect through the expiration time at 8 am,
although temperatures will warm before that since sunrise is before
6 am. Other then that, there are few concerns today. Most locations
will have a nice day that will feel more like May than late June
with high temperatures averaging 10 degrees below normal. As for
convection, most locations will have upper level convergence today
and with the lack of moisture, most places will be rain free. The
main exception may be portions of Natrona and Johnson County where a
thin area of CAPE and negative lifted indices may be enough for an
isolated late day or evening shower or storms, but the chance is
less than 1 in 5. We can`t rule one out in the Absarokas as well,
but with the chance less than 1 out of 10, we will keep the forecast
dry for now.

By far the most interesting and impactful day of the forecast would
likely be Tuesday, especially late in the day and in the evening. We
have a decent set up for some severe weather. A trough will be
approaching the area in the afternoon, increasing upper level
divergence. We also have a jet over us, although there are some
model differences in where exactly the left front / right rear
couplet will set up. We will also have a pseudo dryline setting up
near the Continental Divide, which may be the line for development.
Soundings also show fairly steep mid level lapse rates and veering
winds with a decent amount of directional and speed shear. As for
stability parameters, it depends on the model. Some show as much as
1500 J/Kg of CAPE with lifted indices as low as minus 5, others are
not as impressive. There are some limiting factors though. The main
one bring moisture. Surface dew points are only expected to be in
the 40s, and I would like to see 50s for a good outbreak of severe
weather. However, the potential is there. With the decent shear and
potential for rotating updrafts and tilted storms, the main threat
would be large hail, with areas east of Interstate 25 from north of
Casper through eastern Johnson County in a hatched area for hail. We
do not see this much in this part of the country. And many places
could see strong wind gusts, but this is the case with any
thunderstorm in Wyoming. And yes, we do have the potential for a
tornado. It appears small right now, since Lifted Condensation
Levels look fairly high, which could keep any rotation from touching
down. There is a 2 percent area across eastern portions of the area
though. As for where, the best chance would be along and east of the
Bighorns, where low level upslope flow would aid in convective
initiation, but most areas across the northern half of the state
could see one. Soundings are also showing a decent cap, so this may
turn out to be a later show, starting later in the afternoon and
continuing into the evening and possibly after midnight. Severe
weather is not a certainty, but there is a decent chance, especially
in Johnson and Natrona Counties later Tuesday afternoon and Tuesday
evening.

Thunderstorms will remain possible on Wednesday as another
shortwave moves across the area, but coverage would be more limited.
We will again have a pseudo dryline across the area, but likely
further north. There will be more moisture in far northern and
eastern areas though, which may enhance the threat of heavy rain.
Had to give details on specific threats on this day, but the
potential is there for another active day, though possibly in a
smaller area.

Starting on Thursday and continuing into the weekend, we will move
into a more summer like pattern with warm temperatures, generally
above normal but not record breaking. A couple of shortwaves will
pass by to the north and may bring isolated convection each
afternoon across northern Wyoming. But on these days, most areas
will remain rain free each day. If there is a more active day, most
guidance favors Saturday, but shortwaves are hard to time this far
out.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 952 PM MDT Sun Jun 22 2025

Drier air is moving into the area to start the period and this will
bring dry, mostly clear, and VFR, conditions to all terminals
through the TAF period. The cold front that passed through
Saturday night has become mostly stationary across central
Wyoming, which has resulted in changing wind directions at KRIW
and KCPR. This boundary has already been weakening and will
fully dissipate by Monday morning. Windspeeds will continue to
drop tonight and for most terminals will be light (below 10
knots) from Monday morning through Monday night.

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...
Freeze Warning until 8 AM MDT this morning for WYZ013-023-
025>027.

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe