560
FXUS65 KRIW 150252
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
852 PM MDT Sat Jun 14 2025

.KEY MESSAGES...

- Lingering isolated strong thunderstorms over Johnson County
  will diminish by 10 PM Saturday.

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

- Daily thunderstorm chances continue through Tuesday, mainly
  for areas along and east of the Continental Divide. A few
  storms could be strong to severe.

&&

.UPDATE...
Issued at 1229 PM MDT Sat Jun 14 2025

No major changes to the going forecast this afternoon, as basins
east of the Continental Divide look to have storms develop in
the 2 to 3 PM MDT range. The best thunderstorm fuel for our area
will again be Johnson County, where ML CAPE values push 2000
J/kg. Not too shabby for this part of Wyoming; however, there is
a lack of mesoscale dynamic support for convection. It appears
the shortwave/vorticity-maxima that could have been a good
trigger for convection may be a bit early for accessing peak
diurnal heating. High-resolution models hint at rather
suppressed afternoon development, perhaps due to the mesohigh
behind the weak shortwave. Nevertheless, daytime heating and
orographic effects should be enough to initiate a few stronger
storms later this afternoon and into the evening. Damaging wind
gusts in excess of 60 mph would be the main threat with the
strongest storms, and isolated large hail is possible directly
under storm cores. Storm exit the Bighorn and Wind River Basins
by 8 to 9 PM, and exit the Powder River Basin by 10 to 11 PM.

&&

.DISCUSSION...
Issued at 205 AM MDT Sat Jun 14 2025

Today features a similar story to yesterday, with strong storms
possible over northern and eastern portions of the area. Starting
synoptically, the area remains in southwesterly flow aloft as high
pressure remains centered to the south. A shortwave moves through
this flow during the day. The GFS/ECMWF have this wave in northern
Utah around 6am MDT, moving with the flow so that it reaches Johnson
County in the evening (roughly 6pm). This will provide much of the
lift needed for storm development today. Showers and thunderstorms
should begin to develop around noon off the Wind River, southern
Absaroka, and Bighorn Mountains. There could also be some storms
developing in the lower elevations around Johnson and northern
Natrona Counties at this time too. Hi-res models vary on the
coverage of these storms, but have a consensus of them forming
midday. As the mentioned shortwave moves through the area, storms
should develop around the Wind River and/or Bighorn Basins early
afternoon. Again, hi-res models vary a bit in coverage and exact
details, likely due to potential subtle differences in the low level
flow and interactions with the dryline-like feature from yesterday.
These storms then move into the Johnson County area around 5pm,
exiting the area to the east by around 10pm.

Storms have the potential to be strong to severe today, with peak
chances between 3pm and 11pm. Dewpoints around Johnson County
remain in the mid 50s to around 60F degrees today, lessening to
the south and west. The NAM has precipitable water (PWAT) values
in the 150 to 175 percent of normal range around Johnson County
this afternoon and evening. The HRRR has CAPE values in the
1500 to 2000 J/kg range. Shear (1-6km) values of 20 to 30 knots
are also forecast around Johnson County. Putting this together,
the same hazards of yesterday are again possible today: wind
gusts over 55 mph, hail 1 inch or larger, and an isolated
tornado risk. Like yesterday, the relatively best conditions do
look to remain east and north of the forecast area. The Storm
Prediction Center has the northern and eastern Bighorn Basin
southeast towards Casper in a Marginal risk (level 1 of 5)
today, with a Slight risk (level 2 of 5) for most of Johnson
County and the northern Bighorn Mountains.

Elevated fire weather conditions also remain today. Dry air is in
place over much of the area, especially across central to
southwestern Wyoming. Afternoon relative humidities drop to around
10 percent, with afternoon wind gusts of at least 20 to 25 mph.
These conditions continue for Sunday and Monday.

Sunday will be much like today. Hot temperatures persist, with
showers and thunderstorm development in the similar northern and
eastern areas. Some of these storms could be strong to severe. On
Monday, the trend continues, with thunderstorms once again possible,
mainly along and east of the Continental Divide as moisture makes it
further into the area. The jet stream will also be in a more
favorable position to support storms. Active weather continues
Tuesday, before lessening Wednesday as high pressure builds in. Hot
temperatures look to return for the end of the next workweek.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 522 PM MDT Sat Jun 14 2025

VFR prevails at all terminals through the overnight period and
Sunday. Ongoing convection over the Bighorn Basin moves east early
Saturday evening. This ignites a new round of stronger storms, with
heavy rain and hail, across Johnson County between 00Z-03Z/Sunday.
Isolated lingering convection over central and southwest Wyoming
will be high-based and lead to only brief rain and gusty
outflow wind of 35-45kts. Another round of convection erupts
early Sunday afternoon across many of the same areas as
Saturday. KCOD and KWRL are again the most likely terminals to
see thunderstorm impacts after 20Z/Sunday. Gusty west-southwest
surface wind 10-20kts over the western terminals diminishes
around 02Z/Sunday. A similar wind increases at these terminals
between 17Z-20Z/Sunday and continues through the end of the
forecast 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...VandenBoogart
DISCUSSION...Wittmann
AVIATION...CNJ