439
FXUS65 KRIW 182145
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
345 PM MDT Fri Jul 18 2025

.KEY MESSAGES...

- Afternoon and evening shower and thunderstorm chances (40-70%)
  spread southwest to northeast across the area this afternoon.
  A few stronger thunderstorms could be capable of gusty winds
  up to 60 mph and isolated small hail.

- Shower and thunderstorm potential on Saturday with a focus
  across southwestern Wyoming.

- Hot with limited shower and thunderstorm chances Sunday into
  next week, thus elevated fire weather conditions are
  anticipated much of next week.

&&

.UPDATE...
Issued at 1243 PM MDT Fri Jul 18 2025

A shortwave moving through the region will bring increased moisture
and instability today for the region. SPC continues with a Marginal
risk of severe weather across nearly the entire state today, with
strong winds being the primary threat. Weak convection has already
begun to develop across southwest WY late this morning, as expected.
Dewpoint depressions are expected to increase to around 40 to 50
degrees by late afternoon, particularly across southwest WY and
portions of central WY, which will enhance the gusty outflows and
dry microburst concern this afternoon. Gusts 40 to 50 mph should be
common with isolated gusts 60+ mph.

East of the Divide, instability will gradually increase with capping
across central WY quickly eroding away as temps warm this afternoon.
CAPE >1500J/kg with 20 to 25kts of SFC-3km shear across central WY
will allow the potential for some stronger storms. Will be a much
later show for areas east of the Divide with convection not
spreading across the Divide until around 3 to 4 pm this afternoon.
Isolated super-cellular type development is not out of the question
across portions of central WY, particularly east of the Bighorns,
and thus, there is a isolated hail threat across Johnson County.
Small hail will be possible across much of the region, but the
threat for larger (severe) hail should generally be confined to
Johnson County and points east.

The forecast remains on track for Saturday and beyond, with
scattered showers and thunderstorms across southwest WY Saturday
afternoon still expected during the afternoon. Beyond that, the
forecast remains dry with elevated to critical fire weather being
the primary concern.

&&

.DISCUSSION...
Issued at 244 AM MDT Fri Jul 18 2025

Current (08Z) satellite water vapor imagery shows west-southwest
flow aloft as an upper-level high builds over the south-central
CONUS. Embedded within this west-southwesterly flow is a developing
disturbance over eastern Nevada along with a fetch of increased
Pacific moisture aloft. This disturbance is forecast to trek to the
northeast through the day today and provide support aloft for shower
and thunderstorm chances (40-70%) this afternoon and evening.

Before the disturbance makes its way into southwest Wyoming, dry air
at the surface combined with 700-mb temperatures between 12-15C
mixing to the surface will allow for near seasonal temperatures
(upper 80s to low 90s) east of the Divide today. Temperatures west
of the Divide will likely be near to slightly below normal (low 80s)
as morning cloud cover will hinder sufficient daytime heating. High
temperatures could be a few degrees cooler than what is currently
forecast east of the Divide if cloud cover ahead of the disturbance
moves in earlier in the morning. Nonetheless, the dry air and breezy
winds across eastern Sweetwater County could create elevated to
critical fire weather conditions for a few hours ahead of any
precipitation. However, given the small spatial and limited temporal
range of potential critical fire weather conditions, no highlights
are expected at this time.

The aforementioned upper-level disturbance is forecast to reach
southwest Wyoming around 18Z (noon local) with convection initially
firing off the Uinta Mountains and trekking north. By early
afternoon, increased low-level moisture convergence, owing to
divergence aloft, will help focus convection over west and southwest
Wyoming. Shower and thunderstorms will shift east of the Divide
through the afternoon and into the early evening as the disturbance
treks across Wyoming. Because the low levels will likely remain
relatively dry with peak dewpoint depressions between 40-50 degrees
and inverted-V soundings, especially across the central Wyoming and
the Wind Corridor, showers or thunderstorms could produce up to 50
mph gusts with any collapsing showers or thunderstorms.
Additionally, areas east of the Divide, primarily the Bighorn Basin
and Johnson County, would have the highest probably of seeing a
strong to severe thunderstorm during the afternoon and early
evening. This is due to these locations staying more cloud free for
longer allowing for daytime heating to destabilize the atmosphere
more. East of the Divide, mean SBCAPE values range from 600-1000
J/kg and 0-6km shear values are between 30-40kts. This supports the
primary threats being gusty winds up to 60 mph and small hail.
Thunderstorm chances decrease shortly after sunset, however, a few
lingering showers could continue past midnight.

This pattern largely remains in place again on Saturday with west-
southwest flow aloft and an embedded disturbance trekking across the
area. The main difference on Saturday is that moisture will be more
limited and east of the forecast area, thanks to today`s
disturbance. So, convective initiation will mostly occur off the
higher elevations of southern and western Wyoming and trek northeast
with less coverage compared to today.

Ensemble guidance is in consensus that dry zonal flow will take hold
over the region Sunday and Monday as a broad trough circulates over
the Pacific Northwest. Embedded disturbances trek across the region
both days, however, with limited moisture, shower and thunderstorm
chances will likely be focused across higher elevations of northern
Wyoming. The trough finally begins to push east as an associated
upper-level low digs into the Great Basin by Tuesday. An associated
upper jet then becomes centered over Montana. This leaves the
forecast area in deep southwesterly flow and within the right
entrance region of the upper-level jet. Despite being in a favorable
location for showers and thunderstorms, the greatest moisture
currently looks to be confined over Montana, thanks to a building
high over the southern CONUS, so only isolated convection is
forecast across northern Wyoming at this time. With the deep
southwesterly flow due to a building high, especially across the
Wind Corridor, and dry air, elevated to critical fire weather
conditions are looking favorable much of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 345 PM MDT Fri Jul 18 2025

VFR conditions expected at all TAF sites for the entirety of the
period. Storm activity will be diminishing through the evening
hours still expecting at CPR/RIW/LND through 01Z, COD/RKS
through 02Z, WRL through 06Z, and JAC through 03Z. The storms
through JAC will affect BPI/PNA between 03-06Z before
diminishing after sunset across the Winds. Main threats
initiallywill be gusty outflows through 03Z before weakening
even with the later storms through 06Z at the aforementioned
locations. Otherwise, ceilings lifting to the upper levels
overnight remaining so through much of Saturday. Some more
storm activity expected Saturday afternoon but much less in
coverage. RKS the most likely to see any further south, with a
PROB30 group but no mention anywhere else. Very low confidence
in any making its way far enough north to affect BPI/PNA/CPR
remaining dry for all other locations.

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 543 AM MDT Fri Jul 18 2025

Elevated to critical fire weather conditions are possible ahead of
any showers or thunderstorms across portions eastern Sweetwater and
southeast Fremont Counties (this includes portions of Fire Zones 279
and 289). Minimum relative humidity values could fall just under 15%
with occasional gusts above 25 mph between 1200L-1600L. However,
given the limited spacial and temporal extent of potential critical
fire weather conditions, no highlights are expected at this time.
Early afternoon dry showers or thunderstorms could produce 40-50
gusty outflows across the aforementioned locations.

&&

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

&&

$$

UPDATE...Hensley
DISCUSSION...Gerhardt
AVIATION...Lowe
FIRE WEATHER...Gerhardt