783
FXUS65 KRIW 281806
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1206 PM MDT Sat Jun 28 2025

.KEY MESSAGES...

- Elevated to critical fire conditions continue this afternoon
  across much of the area. A Red Flag Warning is in effect for
  portions of Natrona County/Zone 280 from noon to 9PM today.

- Isolated to scattered thunderstorms are possible (from
  Sweetwater County to Johnson County) this afternoon and Sunday
  afternoon. This afternoon, any showers or thunderstorms will
  be capable of dry microbursts producing 40-60 mph winds.

- Much of next week will feature above normal temperatures and a
  possible increase of convection, especially Wednesday and
  beyond.

&&

.UPDATE...
Issued at 1205 PM MDT Sat Jun 28 2025

The forecast largely remains on track this afternoon and evening
with fire weather concerns and convection chances. A weak shortwave
disturbance, enhanced by lee-side troughing, will provide support
aloft for isolated to scattered (20-40% chance) high based
convection from Sweetwater County to Johnson County and western
Bighorn County this afternoon and evening. With deep mixing, mean
SBCAPE values range from 500 J/kg across northern, central, and
southern Wyoming to 1200 J/kg across Johnson County. Combined with
an uptick in midlevel moisture, this will be sufficient for
convective initiation off of higher elevations across the
aforementioned locations and trek to the east. Showers and
thunderstorms will be high based because of the deep mixing and
large dewpoint depressions (40-60F). These very dry low-levels will
support any showers or thunderstorms producing dry microbursts with
40-60 mph gusts, especially across the Wind Corridor. Convection
chances diminish around sunset with a few lingering thunderstorms
possible across Johnson County past midnight.

The deep-mixing contributing to the gusty winds and very dry low
levels across much of the area continues to support elevated to
critical fire weather conditions across the Wind Corridor and
portions of the Wind River and Green River Basins. A Red Flag
Warning remains in effect for Zone 280 until 9PM this evening.

&&

.DISCUSSION...
Issued at 248 AM MDT Sat Jun 28 2025

Compared to yesterday, we have some things that are similar and some
things that are not. As for similarities, we will continue to have
above normal temperatures, with most locations having high
temperatures from 5 to as much as 12 degrees above normal. There
will also continue to be a gusty breeze in many locations, although
a little less than yesterday in northern portions of the area. And,
we will continue to have low humidity across a good portion of
the forecast area, although a bit higher than yesterday. And
that leads to the question of the day. Do we need another Red
Flag Warning for Natrona County? Humidity will still be
critical, dropping down to close to 10 percent at times. Wind
will also be gusty, but is borderline, with most areas having
around 1 in 2 chance of gusts past 25 mph. But we do have a
difference. That is an approaching shortwave and weak cold
front that has a bit more moisture than the wave of yesterday.
Most of the moisture is of the mid level variety, with
soundings in most locations showing a very substantial inverted
V signature. This includes at Casper, with dewpoint depressions
approaching 60 degrees. As a result, any storms or even showers
that form could cause dry microbursts with wind gusts around 60
mph. And with the possibility of lightning, this was the factor
that tipped the balance to issuing the Red Flag Warning for
this afternoon and evening. Much of northern Wyoming could have
a shower or thunderstorm as well, but the chance of one at any
particular location is only 1 out of 5. This also looks like a
later show, with a few storms lingering in Johnson County after
midnight before everything ends between 1 and 4 am.

Fire concerns should be less tomorrow across most of the northern
two-thirds of the area, with somewhat cooler temperatures and
higher humidity moving in as flow turns north to northwesterly.
Elevated to critical fire weather will continue across southern
Wyoming though, with gusty wind continuing and humidity again
falling below 15 percent. Another shortwave will bring a chance
of a few showers and thunderstorms. There is more model
disagreement in placement this day though. For the most part, we
kept a 1 in 4 chance across portions of Johnson and Natrona
Counties while keeping the rest of the area largely dry.

Ridging then gradually builds over the area through the middle of
the week, bringing a return to above normal temperatures but
nothing out of the ordinary for late June / early July. Monday
looks like a mainly dry and nice day with lighter wind and any
isolated convection (1 out of 6 chance) limited to in and near
the mountains. Coverage of storms may increase Tuesday and
especially Wednesday as deeper moisture moves in from the south,
with precipitable water values rising to as much as 150 percent
above normal by Wednesday afternoon and a weak shortwave
bringing some additional destabilization of the atmosphere. The
end of the week looks to be the same with above normal
temperatures and a scattering of showers and thunderstorms, with
an average of a 1 in 4 to 1 in 3 chance each afternoon and
evening. As per usual this time of year, the best chance will be
in and near the mountains.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1131 AM MDT Sat Jun 28 2025

VFR conditions are expected through the period. A shortwave passing
over Wyoming will kick off a few showers or thunderstorms this
afternoon, most likely at KCPR where a TEMPO group is now in place.
The main concern with this activity will be strong outflow wind
gusts given the dry boundary layer. Large temperature/dew point
depressions, as well as consistent CAM guidance, supports brief wind
gusts of 40+ knots at KCPR during this period. KRKS also has a brief
window for impacts as convection builds off the Uinta mountains this
afternoon, but confidence here is lower and have kept in the
PROB30 group. Any convection in the area will end by late
evening, leaving mostly clear or scattered skies with calm wind
overnight through Sunday morning.

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 1205 PM MDT Sat Jun 28 2025

Low relative humidity values ranging from 8-15% combined with wind
gusts as high as 30 mph continue to create elevated to critical fire
weather conditions across the Wind River and Green River Basins, the
eastern Bighorn Basin, and the Wind Corridor. A Red Flag Warning is
in effect for portions of Natrona County / Zone 280, where fuels are
dried. Conditions begin to improve shortly before sunset. On Sunday,
Fire weather concerns should ease for central Wyoming, but continue
across southern Wyoming as humidity remains below 15 percent and a
gusty wind continues.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening for WYZ280.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Myers
FIRE WEATHER...Gerhardt