567
FXUS65 KRIW 161900
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
100 PM MDT Mon Jun 16 2025

.KEY MESSAGES...

- Fire weather conditions are elevated again this afternoon
  from southwest into south-central WY due to gusty winds and
  low humidities.

- Showers and thunderstorms will also continue this afternoon
  and evening over the northern two-thirds of western and
  central WY. There is a 30-50 percent chance that some
  thunderstorms will be strong to severe northeast of a line
  from the South Entrance of Yellowstone to Muddy Gap, with the
  best chances over Johnson County and northern Natrona County.
  The stronger storms could produce winds over 60 mph as well
  as one inch hail or larger.

- Tuesday will see cooler conditions with northwest-north winds,
  as well as showers and thunderstorms along and east of the
  continental divide.

- High pressure returns to the area from late Wednesday through
  Saturday, with very warm temperatures on Thursday, and
  elevated fire weather conditions across southern and central
  WY (Thu-Sat).

&&

.UPDATE...
Issued at 100 PM MDT Mon Jun 16 2025

The axis of high pressure is to the east of Wyoming today, with
easterly moist return flow from the plains to the continental
divide. A couple of shortwave troughs are moving from
Idaho/Oregon toward southern MT/northern WY over the next 24
hours. The moderate southwest mid-level flow will bring slightly
unstable conditions to west-central WY over the next couple of
hours, then provide better conditions for thunderstorm
development over north-central WY as the dry and slightly cooler
air rides over the moist warming air east of the divide. Models
are still quite variable with the specific areas for showers
and thunderstorms, but consensus seems to be along the eastern
foothills of the Absarokas and Wind Rivers, then moving the
storms eastward into a more favorable environment. With these
conditions, the Storm Prediction Center (SPC) continues with a
marginal risk for severe storms north and east of a line from
Dubois to Muddy Gap. The SPC also has the slight risk for severe
storms from the far northern Bighorn Basin southeast across the
Bighorns into the NW corner of Natrona County. Johnson County
is of course included. Strong outflow winds over 60 mph along
with hail larger than one inch are possible with the storms.
With the easterly surface flow and WSW mid-level flow, storm-
relative helicity is elevated to around 150 which would provide
for a roughly 2-5% chance of a landspout tornado also
developing. At the same time, gusty west winds and humidities
around 10% will create elevated fire weather conditions across
the southwest part of the state.

As the first shortwave trough passes eastward this evening, the
moist cool return flow will strengthen east of the divide, and
even push down into parts of Sweetwater County. Models are
suggesting that overnight isolated showers are possible,
especially early Tuesday morning as another weak trough moves
through. The broader trough then moves through in the afternoon,
again resulting in conditions in WNW mid-level flow for showers
and thunderstorms during the afternoon along and east of the
divide. This pattern will continue dry gusty west winds across
southwest WY for elevated fire weather conditions.

High pressure then moves into the area starting Wednesday
afternoon, warming temperatures and drying things out. Concerns
Thursday through Saturday will be elevated fire weather
conditions. Temperatures look to be the warmest Thursday and
Friday, with mid 80s to mid- to upper-90s.

&&

.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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1125 AM MDT Mon Jun 16 2025

Terminals remain VFR through 18Z/Tuesday in southwest flow aloft.
Plenty of mid and high clouds streaming across the region late
Monday morning in this pattern. The cloud cover may delay the onset
of convection until closer to 20Z/Monday. Dew points are more
favorable for scattered showers and thunderstorms east of the
Continental Divide between 20Z/Monday and 01Z/Tuesday. High-
resolution models are inline with atmospheric dynamics, which
indicate the most robust convection occurring late Monday
afternoon around KCPR. Enough moisture is present between 500
and 400 mb to generate at least isolated high-based showers over
southwest Wyoming Monday afternoon. Gusty outflow wind remains
the primary aviation hazard regardless of location. Outflow
gusts could range from 30-40kts east of the Continental Divide,
but could easily top 40kts over southwest Wyoming given large
temperature-dew point spreads. Deeper convection wanes with the
setting sun. However, an active mid-level moisture stream will
allow for light showers or virga to persist over mainly the
south half of the region overnight. Gusty westerly surface wind
15-25kts increases over southwest Wyoming by 20Z/Monday and
continues until around 02Z/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.

&&

$$

UPDATE...McDonald
DISCUSSION...Gerhardt
AVIATION...CNJ