256
FXUS65 KRIW 132250
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
450 PM MDT Fri Jun 13 2025

.KEY MESSAGES...

- Scattered thunderstorms east of the Continental Divide until 9
  PM MDT Friday, mainly over Johnson County, northeast Natrona
  County, and the eastern Bighorn Basin.

- Isolated strong to severe thunderstorms possible until 8 or 9
  PM over Johnson County, where large hail is the primary
  hazard.

- Elevated fire weather conditions early Friday evening across
  southern portions of the area. These conditions continue into
  the weekend.

- Overall hot and dry this weekend, with isolated shower/storm
  chances each afternoon, mainly across the north.

&&

.UPDATE...
Issued at 1055 AM MDT Fri Jun 13 2025

Current forecast thinking regarding the severe weather threat
for this afternoon remains largely unchanged from the prior
early AM discussion.

High-resolution forecast models continue to show widespread
600-900 J/kg of mixed layer CAPE for basins east of the
Continental Divide this afternoon. Johnson County continues as
the more favorable location for strong storms in our forecast
area (with southern and central MT having the greatest threat
overall today). ML CAPE for Johnson County is on the order of
1000 to 1500 J/kg, which is plenty to support robust updrafts.
The lacking ingredient this afternoon will be shear, with only
about 25 to 30 kt of 0-6km bulk shear. But even this amount of
shear should be enough to support a few broadly rotating
supercells that are longer-lived than the garden-variety
afternoon storms common for this time of year. Gusty outflow
winds of 60 to 70 mph and medium to large hail directly under
storm cores remain the greatest threats today.

Strong storms should first develop between 2 and 3 PM MDT over
the Bighorn Basin, then track east-northeastward. Storm
initiation is likely to be closer to 3 or 4 pm MDT for Buffalo,
WY and Johnson County. The thunderstorm and severe weather
threat looks to end by 7 PM for the Bighorn and Wind River
Basins, and by 8 or 9 PM for Johnson and Natrona County.

&&

.DISCUSSION...
Issued at 157 AM MDT Fri Jun 13 2025

The focus today is on the severe weather potential, namely over and
around Johnson County. Will start with the big picture.
Synoptically, the area will be under southwest flow aloft, with a
subtle shortwave moving through from the southwest through the day.
At the surface, southwest flow over the southwestern portion of the
area will meet south to southeasterly winds over central Wyoming.
Where this boundary occurs, a dryline-like feature will occur and
will be the main influence on where the best severe potential
occurs. Currently, models show this occurring somewhere along the
Bighorn Range south to around Casper. The previously mentioned
subtle shortwave should make it around Johnson County early-to-mid
afternoon. As it moves through, there will likely be isolated storm
initiation across the area. Though hi-res models disagree on the
exact details, they are in good consensus with the overall story for
today. By noon, there will likely be a few storms popping up over
the higher terrain, focusing on the Wind River, southern Absaroka,
and Bighorn Ranges. These storms should track towards the northeast,
with some making into the surrounding lower elevations. The stronger
storms could produce strong wind gusts and hail. Storms should begin
to decrease in the evening, with the last activity around Johnson
County exiting the area by around 9pm MDT.

Focusing on the severe threat in and around Johnson County. At the
surface, southerly flow brings Gulf moisture up into Montana,
including over Johnson County. Dewpoints are forecast to be in
the mid-50s to around 60F degrees. The NAM is indicating
precipitable water (PWAT) values around 150 to 200 percent of
normal. HRRR model soundings are indicating PWAT values of
around 0.85 to 0.95 inches (around max values for mid-June),
which agrees with the NAM. So, some storms could have heavy rain
with them. With southwest flow aloft, veering winds will
contribute to shear, meaning storms can organize and be longer
lived. Surface-based CAPE values are also forecasted to be in
the 1000 to 1500 J/kg range, though the HRRR is showing pockets
of near 2000 J/kg as well. Combined with low-level lapse rates
around 9C/km, storms will have the potential for severe winds
(58 mph+) and severe hail (1 in+). These are the main threats,
though with the shear, some tornado risk is also present. Of
note, the main portion of the jet stream remains north into
Montana, and will be a relatively limiting factor in severe
potential (compared to areas north into Montana). The most
favorable period for Johnson County is between 1pm and 8pm MDT.
The Storm Prediction Center has a slight risk (level 2 of 5) for
around Johnson County today as a result of these factors.

Elevated fire weather conditions are also likely today. The
southwest flow will be bringing in drier air, first to southwest
Wyoming. Afternoon minimum relative humidity values are about 10 to
15 percent across much of the southern areas. Winds also increase
for the afternoon, with widespread gusts of at least 20 to 25 mph.
These conditions are favorable for elevated fire weather conditions.
Similar conditions persist through the coming weekend.

High pressure centered south of Wyoming dominates the weekend, with
hot and dry conditions expected overall. Shortwaves moving through
the flow may bring isolated (15% to 30%) showers and storms during
the afternoons Saturday and Sunday, but will mainly be over northern
portions of the area, favored over the higher terrain. An isolated
strong to severe storm is possible around Johnson County. 700mb
temperatures start to decrease Monday as the ridge flattens. Though
still warm, temperatures are a little cooler than the weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 450 PM MDT Fri Jun 13 2025

Most of the convection early Friday evening will be confined to
areas north of a line from Colter Bay to Thermopolis to just
north of Casper. There is a 20-30 percent chance of a
thunderstorm around KWRL until 01Z/Saturday, otherwise terminals
should remain free of thunderstorms. The strongest storms occur
around KBYG and Johnson County through 02Z/Saturday, with hail
the primary hazard. All terminals to be VFR after 06Z/Saturday
as drier air moves in from the southwest. There is a small
chance that a moist boundary layer in Montana pushes to the
vicinity of KCOD early Saturday morning. The result would be the
potential for 1000 to 2000 ft MVFR cloud cover, but confidence
is low on this occurring. As for Saturday, any late day convection
would be over the Bighorn Basin (KCOD and KWRL) and possibly
southern Sweetwater County off the Uinta Range. Westerly 10-20kt
surface wind expected at the remaining terminals beginning at
19Z/Saturday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...VandenBoogart
DISCUSSION...Wittmann
AVIATION...CNJ