798
FXUS65 KRIW 151120
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
520 AM MDT Tue Jul 15 2025

.KEY MESSAGES...

- Widespread shower and thunderstorm chances (60-80%) east of
  the Divide this afternoon and evening. Strong to severe
  thunderstorms will be capable of 60 mph winds, 1 inch hail,
  and periods of heavy rainfall.

- Isolated to scattered virga showers/dry thunderstorms capable
  of 40-60 mph gusty outflows are possible west of the Divide
  this afternoon and evening.

- A Red Flag Warning has been issued for portions of Sweetwater
  County due to gusty winds, low relative humidity values, and
  the potential for dry thunderstorms producing gusty and
  erratic outflows.

- Additional chances for rain and cooler on Wednesday before a
  warm up Thursday into the weekend with limited rain chances.

&&

.DISCUSSION...
Issued at 249 AM MDT Tue Jul 15 2025

Current (08Z) water vapor imagery shows an upper-level low
circulating near the British Columbia/Idaho border. At the surface,
a cold front extends from the upper low`s surface reflection and
across Montana. These features will help drive today`s active
weather.

The upper-level low will gradually shift into the region through the
day today. The surface cold front pushes south and situates itself
roughly along the Divide by mid-morning. On the west side of the
Divide, dry air and breezy westerly winds will create critical
fire weather conditions this afternoon across portions of
Sweetwater County where a Red Flag Warning has been issued.
Whereas behind the front northerly winds will usher moisture to
locations east of the Divide. Despite locations east of the
Divide being behind the front, temperatures will stay warm due
to 700mb temperatures between 10C- 12C and morning sunshine.

By early afternoon, a shortwave ahead of the approaching upper-level
low/trough combined with an associated 40-50kt supporting jet over
northwest Wyoming will provide sufficient support aloft for
widespread to numerous showers and thunderstorms, especially east of
the Divide. Mean SBCAPE values range from 1000-1500 J/kg, lifted
indices between -4C to -6C, and mean 0-6km shear between 30-50kts
all helps create an unstable atmosphere east of the Divide during
the afternoon and evening. These favorable upper-level dynamics and
unstable low-levels will support strong to severe thunderstorms
capable of at least 60 mph gusty winds and hail up to an inch. Thus,
a Slight Risk (level 2/5) has been introduced for an area
encompassing Fremont, Natrona, Johnson, Washakie, and Hot Springs
Counties. However all other locations east of the Divide remain in a
Marginal Risk (Level 1/5) for gusty winds around 60mph and isolated
severe hail. Additionally, mean PWATs east of the Divide range from
1.00" to 1.15" which is close to the daily maximum PWAT. These
abnormally high PWATs and deep warm cloud depths will favor heavy
rainfall; however, only isolated instances of flash flooding
potential exist given decent flow aloft to move thunderstorms.

West of the Divide is not out of the clear for convection this
afternoon either. Given the dry low-levels and inverted-V soundings
with dewpoint depressions between 40-60 degrees, mostly virga
showers and dry thunderstorms could create gusty 40-60 mph outflows,
especially across Sweetwater County.

The strong to severe threat diminishes through the evening as the
upper-level trough treks across the region late tonight. However,
there remains potential, mainly east of the Divide, for heavy rain
due to support aloft from increased PVA associated with the trough,
tall-skinny soundings, and abnormally high PWATs. Isolated to
scattered showers and thunderstorms exist again on Wednesday with
temperatures 20 to 25 degrees cooler than what we saw on Monday.

Ensemble model guidance is in consensus that zonal flow aloft will
be in place Thursday through the weekend. Warm and dry flow warms
temperatures back up into the 80s to 90s with limited chances for
showers and thunderstorms.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 505 AM MDT Tue Jul 15 2025

IFR/MVFR ceilings have developed at KCOD since 0930Z behind the
cold front and could last through 16Z. The front continues to
progress southward and expected to reach the Divide by 16Z.
Gusty northerly winds have developed over the Bighorn Basin and
Johnson County this morning behind the front, with gusts of 25
to 35 kt occurring in some locations. A weather system will
bring widespread shower and thunderstorm chances to the area
beginning this afternoon. Impacts are likely (70%) from these
showers and storms at all terminals, but confidence remains low
on the exact timing of impacts, thus PROB30 groups remain for
now. With the increased moisture values it is conceivable vis
may drop to MVFR/IFR with the strongest storms. Damaging wind
gusts will be possible with storms across the area and large
hail possible with storms east of The Divide. MVFR ceilings will
return to northern portions this evening, as storms exit to the
east. This trend will spread across areas east of the Divide
through 06Z, with widespread IFR/MVFR ceilings. VLIFR/LIFR
ceilings will be possible over far northern portions, including
KCOD and KBYG after 07Z.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued at 347 AM MDT Tue Jul 15 2025

A Red Flag Warning has been issued for Zone 279 between 12PM-8PM due
to west winds up to 20 mph and gusts up to 35 mph, relative humidity
values as low as 10%, and the potential for dry thunderstorms
producing gusty and erratic outflows up to 60 mph. Dry thunderstorm
potential increases after 1PM with higher confidence in dry
thunderstorms after 5PM. Although thunderstorms could continue past
8PM, relative humidity values are expected to increase as a cold
front pushes through with increased moisture.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 8 PM MDT this evening for
WYZ279.

&&

$$

DISCUSSION...Gerhardt
AVIATION...LaVoie/Rowe
FIRE WEATHER...Gerhardt