829
FXUS65 KRIW 151823
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1223 PM MDT Sun Jun 15 2025

.KEY MESSAGES...

- Shower and thunderstorm chances (30-40%) exist this afternoon
  and early evening east of the Divide. Strong to severe
  thunderstorms are possible across the Bighorn Basin and
  greater potential in/around Johnson County with strong gusty
  winds and large hail being the main hazards.

- Elevated to critical fire weather conditions exist today and
  Monday, primarily across southern Wyoming.

- Shower and thunderstorm chances (40-60%) continue Monday and
  Tuesday afternoons, mainly along and east of the Divide.
  Strong to severe thunderstorms are possible both afternoons.

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

&&

.UPDATE...
Issued at 1222 PM MDT Sun Jun 15 2025

Current (18Z) satellite imagery shows mostly sunny skies across most
of the area. This has allowed temperatures to quickly warm into the
upper 70s and 80s areawide. With the warming temperatures and dry
southwesterly flow, relative humidity values have already dropped
below 15% across the southern half of the forecast area. Elevated to
critical fire weather conditions continue to exist this afternoon
across the southern half of the area with gusts around 30 mph
possible. Similar elevated to critical fire weather conditions exist
on Monday.

The forecast remains on track for isolated to scattered
thunderstorms east of the Divide this afternoon and evening. A
stationary boundary roughly located from the Cody Foothills, along
the Owl Creek Mountains, to the Johnson/Natrona County line. North
and east of the boundary are dew points in the 50s whereas south of
the boundary, dew points are struggling to stay above 40. North and
east of this boundary will be the greatest threat for strong to
severe thunderstorms, with shower and thunderstorms not out of
question for all locations east of the Divide. Abundant daytime
heating and moisture will result in an unstable atmosphere.
Mean SBCAPE values this afternoon are forecast to be above 1000
J/kg with values nearing 1500 J/kg in Johnson County. Thus, in
and around Johnson County, compared to the Bighorn Basin, has a
greater potential for strong to severe thunderstorms. With DCAPE
values around 1500 J/kg, dew point depressions above 50
degrees, and effective shear above 40kts, strong gusty winds and
large hail are the main hazards for all locations north and
east of the stationary boundary. However, an isolated tornado
cannot be ruled out.

The forecast also remains on track with chances (40-60%) for showers
and thunderstorms along and east of the Divide Monday afternoon and
evening as a shortwave passes through. The only change made this
forecast package is isolated to scattered (20-30% chance) showers
overnight Monday as the axis of the shortwave moves through the
region.

&&

.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 /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1136 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 through the
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 KCPR 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.

A more potent shortwave will move in beginning Monday morning, with
a weak boundary moving north to south across the region during the
early morning. Northerly light winds will thus develop for terminals
east of the divide Monday morning. This boundary will also bring
some mid-level cloud decks with it, most notably at KCPR where cigs
at or around FL040 to FL060 could develop occur for a few hours
beginning around 10Z Monday morning. Simultaneously, a wave of
moisture will push in from the southwest which will bring BKN high
clouds to much of the region by the end of the period.

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...Gerhardt
DISCUSSION...Wittmann
AVIATION...Hensley