981
FXUS65 KRIW 171857
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1257 PM MDT Tue Jun 17 2025

.KEY MESSAGES...

- Showers and thunderstorms will continue to develop along and
  east of the Continental Divide through the afternoon. Gusty
  outflow winds of 40 to 55 mph and small hail will be the main
  concerns.

- Hot and dry conditions will be the main story through the rest
  of the week, with critical fire weather conditions expected
  through Saturday, particularly across southwest WY.

- A cold front will move through Sunday, cooling temperatures
  into the 60s and 70s and providing relief from the fire
  weather concerns.

&&

.UPDATE...
Issued at 1256 PM MDT Tue Jun 17 2025

Showers and storms are actively popping up along and east of the
Divide as of this writing (1 PM MDT). The main concern with these
showers and storms will be gusty outflow winds along with small
hail. Though no Marginal risk is in place across the region,
instability is certainly in place (500 to 1000J/kg ML CAPE and LIs
around -3), though less so than previous days, some stronger to
severe storms are possible. Severe hail across Johnson and Natrona
Counties would be the main severe weather concern, along with
possible severe outflow gusts. With cooler temperatures today,
dewpoint depressions are much lower than previous days, but the wind
threat is still there, especially farther west along the Divide, or
with any convection that develops across Sweetwater County, where
dewpoint depressions are much higher (closer to 50F). Convective
threat looks to end pretty quickly after 6 PM, with clear skies and
a slightly cooler night expected.

For the rest of the week, critical fire weather conditions will
remain the primary weather concern. Across southwest WY the critical
fire weather concern is greatest with southwest winds gusting 20 to
30 mph along with RHs less than 10% each afternoon through the end
of the week.

&&

.DISCUSSION...
Issued at 103 AM MDT Tue Jun 17 2025

Convection ended across much of the CWA earlier Monday night around
10pm. Very weak showers will continue to move over southern portions
from UT/CO this morning. This activity will get pushed eastward out
of the forecast area between 7am and 10am, as a shortwave trough
approaches the Cowboy State from ID. This will result in mostly dry
conditions across the area for a few hours. The flow aloft will turn
more northwesterly by this afternoon as a leeside low develops over
northeast CO/the NE Panhandle and the aforementioned
trough/associated cold front move across the area. Temperatures
today will be 5 to 10 degrees cooler as a result. Showers and
thunderstorms will then redevelop across northern portions between
12pm and 2pm, with other areas east of the Divide seeing similar
development through the rest of the afternoon as these storms move
to the southeast. Strong gusty winds, small hail and brief heavy
rain will be the main threats. Any strong thunderstorms are expected
in Natrona County and the Wind River Basin, where the residual
moisture from Monday continues to linger. However, instability will
not be as strong in these locations as has been the case the last
few days. With that being said, a severe thunderstorm cannot be
ruled out. MLCAPE will be 500-1000 J/kg, lifted indices as low as
minus 6, sfc dewpoints near 50F, and bulk shear values up to 35 kt
will be in place. Thus, large hail, wind gusts over 60 mph and
localized flash flooding could occur. Convection will end across the
area by midnight as high pressure builds into the region.

A strong ridge will then build over the Northern and Central Rockies
Wednesday, beginning a warming trend and drier conditions
through the rest of the week. Wednesday will be the quietest
day, with seasonal temperatures and mostly clear skies. Elevated
to critical fire weather conditions will develop by Thursday,
as temperatures rise into the 90s across much of the area and
relative humidity values drop below 15 percent over central and
southern portions of the CWA. Southwest winds, with gusts of 25
to 35 mph, will be mainly confined over southern portions.
Similar conditions return Friday, with of 35 mph being more
widespread across the south. By Saturday, these conditions
spread into the Bighorn Basin and southern Johnson County.
Overall winds will increase again, with gusts of 35 to 45 mph
occurring across the south and central portions. This will be
due to a digging longwave trough with a closed low over the
PACNW and a strong (596dm) high center over the Ohio Valley.
This will, in turn, strengthen the pressure gradient over the
Cowboy State and lead to the higher winds. Far western portions
will have some relief from the hot temperatures as a cold front
associated with the closed low will move over the area late
Friday night/early Saturday morning, resulting in high
temperatures in the 70s. The front will move across the area
Sunday, with precipitation chances being confined to mostly
northern portions of the CWA. Temperatures will be much cooler,
dropping 10 to 15 degrees. Needless to say, fire weather
conditions will not be as critical. The below normal
temperatures look to continue into Monday as the longwave trough
remains in place across the Intermountain West. This would be a
nice reprieve for this time of year, when we can see record
highs and heat waves in mid to late June.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1149 AM MDT Tue Jun 17 2025

VFR conditions are expected at terminals through the period. Weather
concerns today are mainly limited to terminals east of the
Continental Divide as another round of afternoon convection kicks
up. Chances are most favorable at KCOD where confidence is high
enough for TEMPO impacts. Elsewhere east of the Divide, PROB30
groups have been retained given lower confidence on direct terminal
impacts. Gusty outflow winds will be the primary concern with storms
as they move from northwest to southeast across the area. Otherwise,
a breezy afternoon is expected west of the Divide. Convection will
end and wind will decrease quickly after sunset. There will be a
brief period for a potential lower cloud deck at KCPR around this
time as northeasterly winds set in. Westerly wind will return there
by 06Z, with mostly clear skies then expected everywhere
through Wednesday 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...Hensley
DISCUSSION...LaVoie
AVIATION...Myers