084
FXUS65 KRIW 150823
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
223 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 dew points 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 persists 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 weather conditions to
southern areas.

For Monday, low-level east to northeast flow pushes the moisture
back into the area, with increasing dew points 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 hit, and much cooler temperatures
behind it.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1030 PM MDT Sat Jun 14 2025

Scattered showers and thunderstorms over north-central Wyoming have
exited to the northeast leaving improving conditions across the
region late Saturday evening. VFR conditions prevail through
06Z/Monday at all terminals. Mid-level cloud cover and a few
virga showers cross western Wyoming late tonight as a weak
shortwave moves through the state in southwest flow aloft. This
moisture aids convective development across the Absaroka Range
and Bighorn Basin Sunday afternoon. Similar to the past few
days, convection begins across the northwest mountains between
18Z-21Z/Sunday and moves east- northeast during the afternoon
and early evening hours Sunday. Additional convection ignites
over central Wyoming, where high-based showers or storms would
lead to gusty outflow wind 35-45kts between 21Z-24Z/Sunday.
KCOD, KWRL, and KCPR are the most likely terminals to see
thunderstorm impacts after 20Z/Sunday. Gusty west-southwest
surface wind 10-20kts over the western terminals increases
between 17Z-20Z/Sunday and continues until 02Z/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...CNJ/LaVoie