920
FXUS65 KRIW 161030
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
430 AM MDT Mon Jun 16 2025

.KEY MESSAGES...

- Elevated to critical fire weather conditions exist today
  across southern Wyoming.

- Shower and thunderstorm chances (up to 70%) today with strong
  to severe thunderstorm potential, primarily along and east of
  the Divide. The main hazards will be strong gusty winds and
  large hail.

- Shower and thunderstorm chances again on Tuesday with a slight
  cool down. Hot and dry weather Wednesday through the remainder
  of the week.

&&

.DISCUSSION...
Issued at 202 AM MDT Mon Jun 16 2025

Satellite water vapor imagery shows the upper-level high centered
over New Mexico and Arizona that has been keeping the Intermountain
West in southwesterly flow. This pattern remains today before
being broken down by a potent shortwave on Tuesday. Thus, deep
southwesterly flow continues today with mean 700-mb temperatures
ranging from 12-15 C. Despite these warm 700-mb temperatures,
today`s highs will not be as warm as yesterday (Sunday) due to
increased cloud cover associated with the approaching weather
system on Tuesday.

The main stories for today are elevated to critical fire weather
conditions across southern Wyoming and convection chances along and
east of the Divide. Current observations (08Z) show dewpoint values
across southern Wyoming struggling to rise into the upper-teens,
resulting in relative humidity values less than 20 percent.
Slight improvement is still expected by sunrise this morning,
however, will not last long with the dry southwesterly flow.
Minimum relative humidity values are forecast to be around 10-15
percent across southern Wyoming today. These low values and
southwesterly winds with 30-35 mph gusts caused by a tightening
pressure gradient ahead of the Tuesday cold front will combine
to create another day of elevated to critical fire weather
conditions across southern Wyoming.

Where fire weather conditions are not as much of a concern (along
and east of the Divide), is where greater shower and thunderstorm
chances (up to 70% chance) exist. An embedded disturbance aloft will
support afternoon convection developing along and east of the Divide
with strong to severe thunderstorm potential all areas east of the
Divide. Mean SBCAPE values are forecast to be around 500-700 J/kg
for most locations east of the Divide with mean 0-6km shear 30-45
kts. Johnson County is the exception with mean SBCAPE values around
1500 J/kg and 0-6km shear values around 50 kts, thus Johnson County
has a higher chance at seeing more strong to severe storms (in a
Slight Risk from SPC). This is due to a stationary boundary being
roughly located from Casper, along the Owl Creeks, to the Cody
Foothills and areas north of the boundary able to tap into the
more abundant Gulf moisture with Johnson County being in the
prime spot for this moisture. The main hazards associated with
any strong to severe storms will be gusty winds, due to 50+
degree dewpoint depressions and a subtle upper-level jet
overhead, and large hail. An isolated tornado is also possible.
High based, dry thunderstorms that develop over high elevation
areas west of the Divide could bring gusty outflows as well.
Thunderstorm chances decrease after sunset with scattered shower
(up to 30% chance) potential late tonight into Tuesday morning.

The shortwave remains on track to traverse the region on Tuesday
along with its surface reflection (cold front). This cold front will
bring cooler temperatures areawide and shower and non-severe
thunderstorm chances. Ensemble guidance is in consensus in upper-
level ridging building in on Wednesday and slowing shifting east
through the end of the work week. This means warming temperatures
and dry weather the latter half of the week.

Looking ahead to next weekend, ensemble guidance is beginning to
align in a potent weather system late next weekend. Ahead of it, on
Friday and Saturday, strong southwesterly flow with an upper-level
jet overhead, 700-mb temperatures 17-20C (90% chance of greater than
17C), and low relative humidity values will bring more fire weather
concerns, especially across southern Wyoming.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 425 AM MDT Mon Jun 16 2025

All terminals to be VFR through the forecast period with the
exception of KCPR. A moist, northerly airmass behind a shallow
surface boundary reaches KCPR this morning. This shallow
upslope flow leads to IFR/MVFR ceilings near KCPR, however these
ceilings are a bit isolated and to the west of the terminal as
of 10Z. These clouds are expected to scour out by 16Z.
Attention then turns to afternoon convection. While sub- cloud
layers remain quite dry west of the Continental Divide (surface
dewpoints in the teens and 20s), there is enough mid- level
moisture (400-500mb layer) to generate high-based late day
convection. To the east of the Divide, moisture profiles are
more favorable for scattered convection. KCPR, KCOD, and KWRL
all have PROB30 groups for late day convection. It will be a
closer call at KRKS, KRIW, and KLND and continue to leave
PROB30 groups out of these three terminals. Strong to severe
thunderstorms will be possible east of the Divide this
afternoon, with the better chances occurring along and east of a
Powell-to-Kaycee line. Regardless of location, gusty outflow
wind will be the primary hazard even from weak showers across
southwest Wyoming. Gusty west- southwest surface wind 12-25 kt
again increases across the southwest terminals between 17Z-20Z
and persists until around 02Z. There is a chance (20-30%) for
weak showers across areas west of the Divide, moving toward the
northeast into the Wind River Basin between 06Z and 12Z
Tuesday.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence
forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gerhardt
AVIATION...LaVoie