477
FXUS65 KRIW 140342
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
942 PM MDT Wed Aug 13 2025

.KEY MESSAGES...

- Critical fire weather conditions continue Thursday for much of
  western and southern Wyoming. Red Flag Warnings have been
  extended and expanded, and are in effect until 8 PM Thursday.

- High-based showers and a few thunderstorms will produce
  frequent outflow wind gusts of 35 to 45 mph across the
  northern-third of the forecast area through Wednesday evening
  and into the early morning hours Thursday.

- Increasing moisture brings daily rain chances through the
  weekend, with the highest chances over southwest Wyoming.

&&

.UPDATE...
Issued at 1132 AM MDT Wed Aug 13 2025

Current (17Z) radar and satellite imagery shows an area of showers
and thunderstorms moving into the Upper Wind River Basin. This
convection is at the nose of a 500mb wind and vorticity maximum that
is associated with the upper-level high pressure axis. This
convective area will shift east through the afternoon, as the upper-
level axis shifts east, with additional convective initiation across
higher elevations through the afternoon and early evening. Showers
and thunderstorms are still expected to be scattered to widespread
so exact timing and location remains uncertain. Thunderstorms
will be capable of frequent lightning and gusty winds around 40-50
mph.

Additionally, 17Z surface observations show higher dewpoints (upper
40s) across Johnson County, northern Natrona County, and the eastern
Bighorn Basin. These dewpoints are expected to drop a few degrees
with daytime heating and mixing. However, with additional moisture
to work with, mean CAPE across the aforementioned areas is forecast
to be around 500 J/kg. With this afternoon instability along with
the upper-level wind maximum overhead, there will be support for
an isolated strong to severe thunderstorm with gusts up to 60 mph.
Thus, these locations are in a SPC Marginal Risk for severe weather
this afternoon.

Otherwise the forecast remains on track with critical fire weather
conditions and Red Flag Warnings across much of the forecast area.
17Z observations already show relative humidity values below 20%
across southern and central Wyoming. Relative humidity values will
continue to drop with increasing winds across the area as daytime
mixing increases. Nearby showers or thunderstorms could also create
gusty outflows. Limited moisture across much of the area will result
in low chances of wetting rain. With any thunderstorm there is
a threat of new fire starts and erratic fire behavior.

&&

.DISCUSSION...
Issued at 1230 AM MDT Wed Aug 13 2025

The focus for today is on fire weather, which consists of low
relative humidities (RH), gusty winds, and shower/thunderstorm
chances. Starting with RH. The axis of high pressure moves over
today, and with it, the warmest and driest air compared to the past
few days. Afternoon minimum RHs will generally be around or under 25
percent for all locations (both higher and lower elevations) today.
Most low elevations will be under 15 percent. Driest locations will
be in central and southern Wyoming, with RHs down into the single
digits. High temperatures today will be in the upper 80s to mid-90s,
which are a bit above normal for mid-August.

Next element is gusty winds. A shortwave moving through northern
Wyoming, in combination with an approaching low system tightening
the pressure gradient, afternoon, synoptic winds increase. Gusts 25
mph or greater should occur across much of central and southern
Wyoming. In addition, the higher elevations, namely the western
mountains, will see gusts over 35 mph. The least windy spots will be
the Bighorn Basin and northern Johnson County. The other wind factor
comes from thunderstorms chances.

Showers and thunderstorms are possible (at least 15 percent) across
the entire area today. Hi-res models show convection developing over
the far western mountains as early as about 9am MDT, though would be
more likely with typical diurnal trends starting 11am to 1pm.
Chances than become very difficult to pinpoint. Each model has
differing locations and timings of showers and storms through the
afternoon and evening. Have blended them altogether for the
forecast, and "broad-brushed" to give the entire area at least a 15
percent chance. Not everywhere will get a shower or storm, but given
the widespread coverage models depict, there is at least a chance
everywhere. The main hazard with the showers/storms is wind in the
form of localized outflow gusts. Given dew points, gusts over 40 mph
are very possible, with the HRRR producing some gusts over 50 mph.

Combining the previous paragraphs` weather elements, elevated to
critical fire weather is in place today. Red Flag Warnings are in
effect for much of the area, where the synoptic winds, low RHs, and
thunderstorms chances best align. RH recovery is fairly poor
Wednesday night, especially across central and southern portions of
the area, and with winds increasing again Thursday, Red Flags
continue for some locations (fire zones 279, 280, 289, 300) into
Thursday.

On Thursday, moisture increase for the area as the high shifts east
and advects moisture from the south. RHs begin to improve a little,
especially across western Wyoming where moisture gets to first. This
also brings the chances of showers and thunderstorms, primarily for
southwest Wyoming, though there are still some chances into central
and northern Wyoming. The monsoonal moisture push keeps daily rain
chances into the weekend. Temperatures remain very similar through
the rest of the week as well. The upper-level pattern remains messy
through the weekend, with long range models showing the ridge
building back into the start of next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 941 PM MDT Wed Aug 13 2025

Widespread showers and isolated thunderstorms producing gusty winds
of 30 to 40kts have continued through much of the evening. These
will continue to decrease in coverage to start the period, with
gusty outflow winds still lingering at KCOD and KWRL for the first
0.5 to 2.5 hours of the period. Light winds and FEW to SCT high
clouds will then prevail at all terminals overnight. Widespread
showers and isolated thunderstorms return Thursday afternoon by
around 20Z to 22Z. Gusty outflow winds of 30 to 40kts will again be
the primary concern with any showers and storms. Shower and
thunderstorm coverage will start to wane around 02Z for most
terminals, but similarly to tonight, lingering activity could
continue through the end of the period. KJAC and KCOD will see the
least likely chances (20 to 30%) for showers and storms, with most
likely chances (30 to 50%) at KCPR, and across southwest WY
terminals (KBPI, KPNA, and KRKS).

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 1230 AM MDT Wed Aug 13 2025

Winds increase late morning today, with critical RHs in the
afternoon. Therefore, Red Flag Warnings are in effect across
portions of southwest and central WY today. Across Fire Zones 277,
278, 279, 280, 283, 289, and 300, RHs as low as 8 to 12 percent,
coupled with west winds 10 to 15 mph gusting 25 to 30 mph in the
afternoon, will result in critical fire weather conditions. Notably
for zones 279, 280, 289, and 300, similar conditions will return
Thursday afternoon, so Red Flag Warnings for those zones are in
effect through 8 PM Thursday. Conditions are expected to improve
Wednesday night with winds decreasing to mostly light and variable,
however, overnight recoveries will be fairly poor, with RHs only
recovering to around 30 to 45 percent.

Across mountain zones in western and northern WY, including zones
414, 415, and 416 along with 286 and 288, RHs will be much more
marginal, generally 14 to 16 percent in the valleys and around 20
percent across higher elevations. Despite this, winds gusting 30 to
40 mph across higher terrain and the thunderstorm threat (20 to 40
percent chances) across those regions this afternoon necessitated
the issuance of Red Flag Warnings for those zones. In particular,
there is increased concern for new fire starts and erratic fire
behavior with any new starts, given the thunderstorm chances. Though
there should be some rain with any thunderstorms, precipitation will
not be enough to extinguish any lightning starts that may occur, as
wetting rain chances remain near zero. Thursday, the thunderstorm
threat shifts to southwest WY, but storms should have more notable
moisture with them on Thursday as monsoonal flow begins to become
more dominant.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT Thursday for WYZ140-277>280-
286-288-289-300-414>416.

&&

$$

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