367
FXUS65 KRIW 151920
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
120 PM MDT Tue Jul 15 2025

.KEY MESSAGES...

- Widespread shower and thunderstorm chances (40-80%) east of
  the Continental Divide Tuesday afternoon and evening. Strong
  to severe thunderstorms will be capable of 60 mph winds, one
  inch diameter hail, and periods of heavy rainfall, with the
  best chance between 4 PM and 10 PM Tuesday.

- Isolated to scattered virga showers/dry thunderstorms capable
  of 40-60 mph gusty outflows are possible west of the Divide
  Tuesday afternoon and evening, mainly after 3 PM.

- A Red Flag Warning is in effect until 8 PM Tuesday for the far
  southwest due to gusty westerly wind, low relative humidity,
  and the potential for dry thunderstorms producing gusty and
  erratic outflows.

- Rain chances end from west-to-east Wednesday afternoon and
  daytime highs Wednesday will be below normal. A warm-up begins
  Thursday and continues into the weekend with limited rain
  chances.

&&

.UPDATE...
Issued at 120 PM MDT Tue Jul 15 2025

The shortwave responsible for weak morning convection has tracked to
our east, while the upstream vorticity maximum rotating across
northwest Montana can be clearly seen in water vapor imagery at
midday Tuesday. The moisture plume and associated 60-70kt jet is
nearing northwest Wyoming, while a plume of 500-300mb moisture is
moving across northern Utah and southwest Wyoming. Both plumes
will aid convective development Tuesday afternoon and evening.
SPC has a large portion of central Wyoming east of the
Continental Divide in a slight risk for severe thunderstorms,
along with portions of Lincoln County. Almost the entire
forecast area is encompassed by a marginal risk of severe
thunderstorms.

The northern moisture plume and right-entrance region of the 60-70kt
jet will ignite convection across northwest Wyoming after 2 PM
Tuesday, with a likely notable increase in coverage and intensity
between 3 PM and 6 PM. Morning cloud cover in upslope areas behind
an early morning cold front has hindered surface heating
delaying the onset of convection. SBCAPE rises to 800-1200 J/kg
during the afternoon east of the Continental Divide, where dew
points hover in the upper 40s to lower 50s. So, plenty of
instability available as daytime heating increases by mid-
afternoon. Thunderstorms move east across the central and
northern basins between 4 and 8 PM, eventually pushing east of
Interstate 25 around 9-10 PM. CAMS are all in fair agreement
with the 4 to 8 PM period providing the best chance for strong
to severe convection. Mid-level wind around 45-50kts favors the
potential for severe wind gusts (15 percent), while there is a
lower chance (5 percent) of one inch or larger hail. Proximity
of the right entrance region and channeled vorticity enables
convection to continue into early Wednesday and quite possibly
overnight across the far north. Additionally, plentiful
precipitable water values of 0.75 to 0.90 inches (around 120
percent of normal) east of the Divide enhance the potential for
brief heavy rain. Convection should move east at about 20kts,
but locations under training echoes anchored off terrain
features, or urban areas, could be susceptible to nuisance
flooding.

Across southwest Wyoming, high-based convection favors gusty outflow
wind of 40 to 60 mph. These winds are possible even from weak
convection given a dry sub-cloud layer below about 500mb. CAMS
indicate two rounds of convection across the southwest, one coming
late Tuesday afternoon (3 to 6 PM) and another between 8 PM and
midnight. In both rounds, wind will be the primary hazard. A few
showers could linger south of Interstate 80 in Sweetwater County well
into the night.

General model trends have been toward less convection Wednesday
afternoon as drier air filters into Wyoming in west-northwest flow
aloft. However, the position of a lingering moisture plume
stretched across southern Wyoming will better determine the
outcome. Scattered convection erupts by early Wednesday
afternoon over Sweetwater County, extending as far north as
Natrona County. This plume is expected to shift southeast as the
afternoon wears on. Other convection will primarily be anchored
to the central and northern mountains. Overall, Wednesday is
drier and cooler. Wednesday daytime highs are 65 to 75 east of
the Divide, and 75 to 85 in the west.

&&

.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 /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1143 AM MDT Tue Jul 15 2025

Low clouds this morning across northern WY (KCOD, KWRL, and KCPR
terminals) continue to scatter out to start the period, lingering
perhaps until 19Z at KCOD. Then, the focus shifts to widespread
showers and thunderstorms by the later part of the afternoon. Given
the atmospheric stability that exists following the AM inversions
and low clouds, CAMs have been pushing timing of the initiation of
showers and thunderstorms to later in the afternoon (22Z to 00Z)
with the most likely chances occurring between 00Z and 03Z. Around
03Z a second round of showers moves through the region. Have
included -SHRA for this second round, as earlier convection should
stabilize things and limit TS after 03Z, but isolated embedded TS
are certainly possible.

Behind the showers and storms, a secondary cold front will move
through overnight, which looks to bring low cigs to all terminals
east of the Divide. MVFR to IFR cigs are likely (70% chance), with
LIFR cigs possible at KCOD and KWRL (40% chance). Low clouds look to
scatter out between 15Z to 17Z with KCOD, KCPR, and KWRL hanging
onto the low clouds longest.

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

&&

.FIRE WEATHER...
Issued at 120 PM MDT Tue Jul 15 2025

A Red Flag Warning is in effect for Zones 277 and 279 until 8 PM
Tuesday due to west wind gusting 25 to 35 mph, relative
humidity values 8 to 13 percent, and the potential for dry
showers and thunderstorms producing gusty and erratic outflow
wind gusts 40 to 60 mph. Dry thunderstorm potential steadily
increases after 1 PM with higher confidence in dry thunderstorms
after 5 PM. Although thunderstorms could continue past 8 PM,
relative humidity values are expected to increase as a cold
front pushes through with increased moisture.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for WYZ277-279.

&&

$$

UPDATE...CNJ
DISCUSSION...Gerhardt
AVIATION...Hensley
FIRE WEATHER...Gerhardt/CNJ