768
FXUS65 KRIW 312011
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
211 PM MDT Thu Jul 31 2025

.KEY MESSAGES...

- Scattered thunderstorms east of the Divide this afternoon and
  evening, some of which could be severe with heavy rain, hail,
  and gusty outflow winds.

- Elevated fire weather conditions this afternoon and evening
  for southwestern Wyoming.

- Scattered showers and thunderstorms continue Friday through
  the weekend.

- Monday into the middle of next week will be warmer, about 5
  degrees above seasonal normals. Drier during this timeframe as
  well, with low chances (less than 20 percent) for
  precipitation.

&&

.UPDATE...
Issued at 1100 AM MDT Thu Jul 31 2025

The current forecast remains on track. Another afternoon and
evening of thunderstorms will begin between 1 PM and 3 PM MT,
with storms reaching their greatest coverage & intensity in the
5 PM to 8 PM MT timeframe. Convective analysis shows widespread
areas of 1000 to 1500 J/kg of mixed layer CAPE for basins east
of the Divide. This is paired with 0-6 km Bulk Shear in the 25
to 35 kt range, enough to support some strong storms today.
High-resolution forecast models key in on the southern Bighorn
Basin into the Power River Basin for the strongest and most
widespread thunderstorms this evening, and this lines up well
with the latest 1630Z Storm Prediction Center Convective
Outlook, which put the region under a Slight Risk for hail and
damaging wind gusts. Flash flooding remains an outside threat
(less than 10%) mainly due to short duration intense rainfall
under the strongest storm cores. Steering winds of about 20 kt
should keep storms progressing northeast over time, reducing
residence time over any given location.

Elevated fire weather conditions develop over southwest Wyoming
this afternoon, with RH under 15 percent and winds gusting 20
to 25 mph. This also is the region least likely to see any
showers or storms. This is typical climatologically for late
July; nevertheless, any fire could burn efficiently in these
conditions.

&&

.DISCUSSION...
Issued at 258 AM MDT Thu Jul 31 2025

Much of central and northern WY saw beneficial moisture
Wednesday. Many locations had the first measurable rainfall in
well over two weeks. The active and unsettled weather pattern
looks to continue across much of the Cowboy State once again
today. Temperatures are forecast to warm up slightly more than
Wednesday with values ranging from the low to mid 80s. A plume
of well above normal moisture will persist across the region.
Dewpoints are expected to range from the low to upper 50s east
of the Divide. East of the Divide will have the best chances for
showers and thunderstorms today with lesser chances elsewhere.

Another shortwave will move across the region this afternoon and
evening. This should aid in the initial development of showers
and thunderstorms over portions of western and southern WY.
Convection will gradually spread to the north and east through
the afternoon. Storms may begin to strengthen as they cross the
Divide into the more favorable environment over central and
eastern WY. As mentioned earlier a plume of well above normal
moisture persists over the region. This moisture combined with
temperatures in the 80s will create sufficient instability with
CAPE values ranging from 1500 to 2500 J/kg. Locally higher
amounts are possible in parts of Johnson and Natrona Counties
where dewpoints may near the low 60s. Other components of the
favorable environment will be bulk shear of 20 to 40 knots
throughout the atmosphere. Low-level flow will remain from the
east/southeast with mid-level flow from the west/southwest.
Warmer temperatures aloft will aid in creating lapse rates of 8C
or more. All of these components will create the potential for
strong to severe storms developing with the best chances focused
east of the Divide, especially over Natorna and Johnson
Counties. The main hazards as a result of any strong to severe
storms will be large hail, strong gusty outflow winds, and heavy
downpours. There will be the potential for localized flash
flooding especially in poor drainage locations. Showers and
thunderstorms should begin to dissipate by the late evening
hours with a few isolated showers lingering into the early
morning on Friday.

The other concern today will be elevated to near critical fire
weather conditions across southern WY. The aforementioned plume
of above normal moisture will struggle to seep into southern
WY. This will lead to Min RH values remaining rather dry with
much of the area seeing percentages below 15 percent. Winds will
increase in the afternoon with periodic gusts of 20 to 25 mph.
Winds are not expected to be prolonged which may limit the
overall fire weather concerns. Due to this reason no highlights
will be issued at this time.

A trough will continue to try and make its way across the western
CONUS. This will keep ushering weak shortwaves into the region
leading to daily chances for showers and thunderstorms through the
weekend. Coverage of these showers and thunderstorms will continue
to decrease becoming more restricted to central/northern WY and
eventually mainly northern WY. Moisture will remain across
these areas as well, which should limit any fire weather
concerns for the time being. Southern WY looks to miss out on
this moisture leading to the continued chances for elevated to
near critical fire weather conditions through the weekend. Min
RH values over this area remain at or below 15 percent for this
period. These dry conditions will combine with daily afternoon
increases in winds of 15 to 25 mph. If slightly higher winds are
forecasted their may be the need for some future fire weather
related highlights. Temperatures remain around seasonable values
through the weekend. Longer range models are starting to show
signals of warmer above normal temperatures returning for the
start of next week. These warmer temperatures will likely also
lead to lesser chances for precipitation and increased fire
weather concerns.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1159 AM MDT Thu Jul 31 2025

Another day of afternoon convection is expected for the beginning of
the period. Showers and storms will develop along and west of the
Divide early this afternoon, moving northeastward through the
evening. The best coverage of storms will be east of the Divide,
where all terminals there have a TEMPO or PROB30 group, as well as
KJAC. Strong outflow wind gusts, brief heavy rain, and isolated
large hail will be possible with this activity today. Convection
will begin to wane after sunset, mostly ending by 06Z Friday.
Decreasing clouds and lighter winds are then expected through Friday
morning.

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...Dziewaltowski
AVIATION...Myers