Severe Thunderstorm Warning 39, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1145 AM MDT Thu Jun 25 2026

.KEY MESSAGES...

- Thunderstorms are expected to move from west to east across
  Wyoming through 5PM MDT. Strong to severe thunderstorms are
  possible with gusty 45 mph winds, hail, heavy rain, and
  frequent lighting. Gusts of up 60 mph are possible in severe
  thunderstorms.

- Shower and thunderstorm chances (20% to 40%) exist Friday
  afternoon across northwest and northern Wyoming as well as
  Johnson and Natrona County. Otherwise, expect warmer
  temperatures, lower humidity, and gusty winds on Friday with
  elevated fire weather conditions possible.

- A weather system will bring a cooler weekend, especially on
  Sunday. Some accumulating snow is possible in the northwestern
  mountains from Saturday night through Sunday night.

&&

.UPDATE...
Issued at 1005 AM MDT Thu Jun 25 2026

The forecast for today remains on track with showers and
thunderstorms moving from west to east across Wyoming along a cold
front. As of 10AM MDT, radar is showing a line of thunderstorms from
the western Absarokas to western Sublette County. These thunderstorms
will push east through the late morning and early afternoon, moving
across the Green River Basin between 10AM and 12PM, the Bighorn and
Wind River Basins between 12PM and 3PM and the I-25 corridor in
Natrona and Johnson Counties between 2PM and 5PM. Confidence has
increased in thunderstorms becoming strong to severe due to more
cloud breaks across the area. Hi-Res guidance is indicating 1000 to
1500 J/kg of BLCAPE, 30 to 45kts of 0 to 6km shear, and unstable mid-
level lapse rates around 7.5 C/km. This suggests that any
thunderstorms will be able to produce heavy rainfall, small
hail, gusty 30 mph to 45 mph winds, and frequent lightning with
the line. Instances of 60 mph wind gusts are possible in
thunderstorms that become severe.

Conditions are a bit different across southern and southwestern
Wyoming, where there has been abundant morning sunshine. Across the
aforementioned areas, very unstable lapse rates around 8.5 C/km,
DCAPE values around 1,000 J/kg, and nearly 1,500 J/kg of BLCAPE
are forecast this afternoon. Dew points have remained rather
high this morning with Rock Springs Airport reporting morning
dew points in the low 50s. This will help keep afternoon dew
point depressions around 40 degrees fahrenheit. Thus,
thunderstorms across southern and southwestern Wyoming will be
capable of gusty 40 to 50 mph winds, hail up to 1 inch, and
frequent lightning. All thunderstorm chances largely come to an
end by sunset as daytime heating subsides.

&&

.DISCUSSION...
Issued at 241 AM MDT Thu Jun 25 2026

There was interesting occurrence around Riverton last night. A
severe thunderstorms developed off the Wind River Range and moved
north. Just to show how tight the gradient can be on precipitation.
At our office, we were missed for the most part and only had a trace
of rain. Meanwhile, just 7 miles away, on the east side of Riverton,
one of our retired forecasters reported 1.40 inches of rain in
around 40 minutes and penny sized hail. There was a fair amount of
street flooding in town as well. We still have showers and a few
thunderstorms around as I write this around 1 am, but the trend is
down as we lose instability. There will still be some around through
the night though.

We will likely see more showers and storms today. There is some
uncertainty though. For one, especially across eastern portions of
the area, we will likely have a decent amount of cloud cover around
to start the day. This could limit instability somewhat. However,
there will some factors for it. Another shortwave will be crossing
the area. There will also still be some directional and speed shear,
although less than yesterday, generally between 30 and 40 knots
versus the 50 to 60 we had on Wednesday. Some models still have a
decent amount a CAPE through, up to 1500 J/Kg on the GFS for
instance. As for stronger storms, the Storm Prediction Center has
removed the slight risk. We now have a Marginal risk which runs
largely south and east of a Worland to Thermopolis to Riverton to
Rock Springs line. The main threat today would be strong wind, but
there is enough shear and steep enough lapse rates for some
marginally severe hail as well. And, heavy rain will be a threat
again as the atmosphere remains rather moist. Precipitable water
values are around 20 percent less than on Wednesday. However, this
is still around 150 percent of normal. There are two other
differences. One, there will be more 700 millibar flow, so storms
should be faster moving, limiting the flooding threat more. And,
unlike today, this looks like and earlier show, with almost all
storms over by sunset as the shortwave exits the area stage east.

Shortwave ridging should build over the area on Friday and bring a
quieter day. Not a quiet day by any means, but quieter.
Precipitable water fall to near of even below normal levels across
much of the area. The current forecast is more than likely the
wettest it will be, most locations, especially East of the Divide,
will likely have a dry day. Southwest flow will increase ahead an
incoming upper level, and downsloping flow should bring a warm to
hot day to most of the area. Some of the lower elevations East of
the Divide will likely see highs in the 90s on this day. With
relative humidity falling into the into the teens across southern
Wyoming, elevated fire weather looks likely in the afternoon.

The aforementioned upper level low will begin to approach the area
on Saturday. Most guidance has come into agreement in keeping the
low largely west of Wyoming, and this should keep most of the
precipitation across western Wyoming. Again, the current forecast
East if the Divide is probably the wettest it will be, as
downsloping flow will keep most of the day dry. The main concern
will be across western Wyoming, where the best precipitation will
be. And especially so from Saturday night into Sunday night. Much
cooler air will move in here, dropping 700 millibar temperatures
down to around minus 1 to minus 2, lowering snow levels to around
7500 to 8000 feet. Amounts don't look heavy, put there is a greater
than 1 in 2 chance of an inch of snow above 9000 feet. So, if you
are heading to the high country this weekend be prepared for some
wet and cold conditions.

Conditions should gradually dry out starting Monday as the upper
level low continues to lift northward. This will be followed by
southwest flow dominating early next week, bringing warm but not hot
temperatures. A couple of shortwaves may bring a few showers or
thunderstorms, but timing of the waves remains uncertain at this
time.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1145 AM MDT Thu Jun 25 2026

A line of convective storms has already developed across western
WY, moving eastward, and is now oriented from KCOD to
KPNA/KBPI. Thunderstorms are expected at all TAF sites today,
with MVFR conditions likely with lowered cloud heights. Gusty
winds of 40-60 kts are possible with the storms today, as well
as brief heavy rain and small hail. The strongest storms will
impact KWRL/KLND/KRIW around 25/19-20Z, and KCPR around
25/21-22Z. Showers will linger today behind the storms, with
breezy WNW winds, though easterly outflow winds east of the
divide should dominate in the moister environment. KCOD/KWRL are
expected to see another batch of showers and isolated
thunderstorms during the early evening hours, with stray showers
at KCOD for the rest of the night. Expecting MVFR, and perhaps
IFR, low clouds at some TAF sites east of the divide, but much
depends on precipitation intensity today. KCPR has the highest
likelihood of IFR low clouds Friday morning due to moist
easterly flow overnight.

Friday will see the general flow becomes more southwesterly,
with dry and gusty SW wind developing from SW WY to NE before
26/18Z. CAMs are indicating elevated convection developing over
quite a large area between 26/18-21Z and moving NE. Gusty
outflow winds could again be a large impact. A sharp cold front
is then expected to move into western WY Friday evening,
increasing chances for precipitation over western WY Friday
night.

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...Gerhardt
DISCUSSION...Hattings
AVIATION...McDonald

Special Weather Statement
National Weather Service Riverton WY
102 PM MDT Thu Jun 25 2026

WYZ007-017-251945-
Owl Creek and Bridger Mountains-Wind River Basin-
102 PM MDT Thu Jun 25 2026

...A STRONG THUNDERSTORM WILL IMPACT EAST CENTRAL FREMONT COUNTY
THROUGH 145 PM MDT...

At 101 PM MDT, Doppler radar was tracking a strong thunderstorm near
Boysen State Park, which is 10 miles northwest of Shoshoni, moving
east at 35 mph.

HAZARD...Wind gusts of 50 to 55 mph, nickel size hail, frequent
         lightning, and heavy rainfall.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

This strong thunderstorm will be near...
  Boysen State Park around 105 PM MDT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

If on or near Boysen Reservoir, get out of the water and move
indoors or inside a vehicle. Remember, lightning can strike out to
10 miles from the parent thunderstorm. If you can hear thunder, you
are close enough to be struck by lightning. Move to safe shelter
now! Do not be caught on the water in a thunderstorm.

&&

LAT...LON 4325 10835 4345 10836 4349 10772 4317 10772
TIME...MOT...LOC 1901Z 270DEG 33KT 4333 10828

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...55 MPH

Gerhardt

Special Weather Statement
National Weather Service Riverton WY
109 PM MDT Thu Jun 25 2026

WYZ017-019-020-252000-
Wind River Basin-Green Mountains and Rattlesnake Range-Natrona County
Lower Elevations-
109 PM MDT Thu Jun 25 2026

...A STRONG THUNDERSTORM WILL IMPACT WEST CENTRAL NATRONA AND EASTERN
FREMONT COUNTIES THROUGH 200 PM MDT...

At 108 PM MDT, Doppler radar was tracking a strong thunderstorm over
Castle Gardens, which is 30 miles north of Jeffrey City, moving east
at 20 mph.

HAZARD...Wind gusts up to 50 mph, nickel size hail, frequent
         lightning, and heavy rainfall.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

This storm will remain over mainly rural areas of west central
Natrona and eastern Fremont Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4283 10765 4291 10770 4300 10764 4306 10737
      4277 10736
TIME...MOT...LOC 1908Z 268DEG 14KT 4290 10762

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...50 MPH

Gerhardt

Special Weather Statement
National Weather Service Riverton WY
124 PM MDT Thu Jun 25 2026

WYZ011-020-252015-
Southeast Johnson County-Natrona County Lower Elevations-
124 PM MDT Thu Jun 25 2026

...A STRONG THUNDERSTORM WILL IMPACT NORTH CENTRAL NATRONA AND SOUTH
CENTRAL JOHNSON COUNTIES THROUGH 215 PM MDT...

At 124 PM MDT, Doppler radar was tracking a strong thunderstorm 14
miles west of Midwest, which is 17 miles south of Kaycee, moving east
at 10 mph.

HAZARD...Wind gusts of 50 to 55 mph and nickel size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

This storm will remain over mainly rural areas of north central
Natrona and south central Johnson Counties.

This includes Interstate 25 between mile markers 227 and 238.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4349 10677 4354 10660 4354 10628 4331 10645
TIME...MOT...LOC 1924Z 280DEG 7KT 4346 10655

MAX HAIL SIZE...0.88 IN
MAX WIND GUST...55 MPH

Rowe

Special Weather Statement
National Weather Service Riverton WY
130 PM MDT Thu Jun 25 2026

WYZ004-006-008-252015-
Southeast Bighorn Basin-North Bighorn Basin-Bighorn Mountains West-
130 PM MDT Thu Jun 25 2026

...A STRONG THUNDERSTORM WILL IMPACT SOUTHEASTERN BIG HORN AND
NORTHEASTERN WASHAKIE COUNTIES THROUGH 215 PM MDT...

At 130 PM MDT, Doppler radar was tracking a strong thunderstorm 9
miles southwest of Hyattville, which is 14 miles northwest of Ten
Sleep, moving east at 25 mph.

HAZARD...Wind gusts of 50 to 55 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

This storm will remain over mainly rural areas of southeastern Big
Horn and northeastern Washakie Counties, including the following
locations... Leigh Creek Campground and Wigwam Fish Rearing Station.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4404 10780 4421 10786 4437 10741 4405 10730
TIME...MOT...LOC 1930Z 252DEG 20KT 4414 10770

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...55 MPH

Rowe

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