302
FXUS65 KRIW 160539
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1139 PM MDT Sun Jun 15 2025

.KEY MESSAGES...

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

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

- Cooler Tuesday, then hot temperatures return 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 dewpoints in the 50s whereas south
of the boundary, dewpoints 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, dewpoint
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 /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1135 PM MDT Sun Jun 15 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 around 10Z-11Z/Monday. This shallow
upslope flow leads to IFR/MVFR ceilings at KCPR between 11Z-
15Z/Monday before scouring 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, but for now have left PROB30 groups out of these three
terminals. Regardless of location, gusty outflow wind will be
the primary hazard even from weak showers across southwest
Wyoming. Gusty west-southwest surface wind 12-25kts again
increases across the southwest terminals between 17Z- 20Z/Monday
and persists 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...Gerhardt
DISCUSSION...Wittmann
AVIATION...CNJ/LaVoie