573
FXUS65 KRIW 151046
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
446 AM MDT Sun Jun 15 2025

.KEY MESSAGES...

- Showers and thunderstorms possible again today, mainly along
  and east of the Continental Divide. Some storms could be
  strong to severe, with strong winds and large hail being the
  main hazards. Strongest storm potential is around Johnson
  County.

- Showers and thunderstorms possible Monday and Tuesday, mainly
  for areas along and east of the Continental Divide. A few
  storms could be strong to severe.

- Low relative humidity, unseasonably warm temperatures, and
  breezy afternoon west wind will elevate fire weather
  conditions across southern Wyoming today and Monday.

- Cooler Tuesday, with hot temperatures returning Thursday
  through Saturday. A potent cold front moves through late
  Saturday.

&&

.DISCUSSION...
Issued at 223 AM MDT Sun Jun 15 2025

The weather pattern from the past couple days is slow to change.
Similar conditions regrading storms and severe potential are around
today, though there are some differences. Starting with the synoptic
setup, things remain fairly similar to the past couple days. A high
pressure system is centered in southern New Mexico, providing a dry
southwest flow into Wyoming. Low level flow is more southerly in
eastern Wyoming, bringing some Gulf moisture into eastern portions
of the area, mainly around Johnson County. This creates a dryline-
like feature, which has been situated up against the Bighorn
Mountains the past few days. This feature looks to shift a bit east
today, but will still influence the storms today. A shortwave moves
through the flow today, providing lift for storm development. It
looks to be at its strongest north, into Montana.

Evaluating the severe potential today. As mentioned in the previous
paragraph, moisture will be a bit less today, with dewpoints in
the mid-50s over northern Johnson County. These are still good
values for storms, though. As the shortwave moves through,
storms should begin initiating around noon, first over the
Absaroka and Bighorn Mountains. Storms will also begin over the
lower elevations through the early and middle afternoon, mostly
for locations east of the Continental Divide. Sweetwater County
may see some showers/storms as well. The strongest storm
potential occurs between 4pm and 9pm MDT, but overall storm
potential is between noon and 11pm. MLCAPE values in Johnson
County are generally 800 to 1500 J/kg, with 1-6km shear 20 to 30
knots, though isolated values 30 to 40 knots could occur near
the northern border. These conditions are similar to the
previous couple days, so hazards are similar as well - wind
gusts 60 mph or greater, hail 1 inch or greater, and an isolated
tornado threat. With it being a bit drier, as well as warmer,
today, the wind threat may be a bit higher than yesterday; HRRR
model soundings have DCAPE values around 1800 J/kg in the
afternoon in Johnson County. Overall, the most favorable
conditions for severe weather looks to remain north and east of
Johnson County. However, if some of the conditions move a little
south compared to model projections, the risk would increase
for Johnson County. The Storm Prediction Center has the Bighorn
Basin, Johnson County, and northern Natrona County in a Marginal
Risk (level 1 of 5) today as a result of the above. There is a
steep change to a Slight Risk (level 2 of 5) and then an
Enhanced Risk (level 3 of 5) northeast of Johnson County, for
wind and hail threats.

With the dry southwest flow continuing into the state, elevated fire
weather conditions persist for central to southwest Wyoming.
Afternoon relative humidities fall into the single digits, with
widespread values of 7 to 12 percent. Afternoon southwest winds
also increase to be at least 20 to 25 mph. Similar conditions
return for Tuesday. Looking ahead, long range models are showing
the potential for a potent cold front late Saturday. This could
bring strong southwest flow to the area, with current 700mb
winds forecasted to be 40 to 50 knots. This would bring elevated
fire to near-critical weather conditions to southern areas.

For Monday, low-level east to northeast flow pushes the moisture
back into the area, with increasing dewpoints into the Wind
River Basin. As another shortwave moves through, showers and
thunderstorms are once again possible, this time for most
locations along and east of the Divide. With some upper-level
jet support, some storms could be strong to severe, with strong
winds and large hail possible in the strongest storms. The
upper-level pattern begins to shift Tuesday as a more potent
shortwave moves through. Tuesday temperatures will be cooler as
northwest flow aloft begins to take hold, though lingering
moisture, in combination with the shortwave, will create more
shower and thunderstorm chances. High pressure then builds in
through the latter half of this week, with hot temperatures
Thursday, Friday, and into Saturday. Long range models are
projecting a potent cold front coming in late Saturday, which
would bring gusty winds ahead of it, and much cooler
temperatures behind it.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 430 AM MDT Sun Jun 15 2025

VFR conditions prevail at all terminals through the TAF period.
Mid- level cloud cover continues to cross western Wyoming to the
northeast as a weak shortwave moves through the state in
southwest flow aloft. Low level moisture will remain in place
this morning and early afternoon in the Bighorn Basin and
Johnson/Natrona counties, with a pseudo-dryline in place between
the southwest winds (drier air) and easterly winds (moist air).
This shortwave will aid convective development across the
Absaroka Mountains and Bighorn Basin this afternoon, starting
around the northern end of the Wind River Mountains and the
southern end of the Absarokas between 18Z and 21Z and move east-
northeast during the afternoon and early evening hours.
Additional convection is expected over Johnson and Natrona
counties during this time as well. Most of these storms will be
in the form of high- based showers or storms as a result of the
southwest winds, which will lead to gusty outflow wind 35-45 kt
between 21Z-00Z. KCOD, KWRL, and KCPR are the most likely
terminals to see thunderstorm impacts after 20Z. However,
confidence is too low to include into the KWRL forecast at this
time. Gusty west- southwest surface wind 10-20 kt over the
western terminals increases between 17Z-20Z and continues until
02Z. Winds will become light across all terminals by 06Z Monday.

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...Wittmann
AVIATION...LaVoie