569
FXUS65 KRIW 150851
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
251 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.

- Elevated fire weather conditions west of the Divide with
  isolated to scattered mostly virga showers/dry thunderstorms
  capable of 40-60 mph gusty 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 elevated fire
weather condiditons this afternoon. 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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 947 PM MDT Mon Jul 14 2025

VFR conditions will prevail tonight into Tuesday morning. Moisture
will increase tonight into tomorrow as a weather system moves in
from the north. This weather system will bring widespread shower and
thunderstorm chances to the area beginning Tuesday afternoon.
Impacts are likely (70%) from these showers and storms at 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 will be possible with storms east of The Divide, Tuesday
afternoon and evening. A strong cold front will push through tonight
into tomorrow from north to south, with a wind shift becoming
northerly east of The Divide. This front will result in an increase
in wind speeds tonight through Tuesday evening. low clouds will move
into northern Wyoming Tuesday evening, with IFR conditions at KCOD
by 03Z.

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 249 AM MDT Tue Jul 15 2025

Breezy west winds with gusts generally between 20-25 mph and minimum
relative humidity values below 15% will create elevated fire weather
conditions across southern Wyoming ahead of cold front today.
Wind gusts over 25 mph do not look widespread enough for critical
fire weather though. There is potential for isolated to
scattered mostly virga showers/dry thunderstorms that will be
capable of 40-60 mph outflows this afternoon.

&&

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

&&

$$

DISCUSSION...Gerhardt
AVIATION...Rowe
FIRE WEATHER...Gerhardt