Severe Thunderstorm Warning 21, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
517 PM MDT Sat Jun 20 2026

.KEY MESSAGES...

- Widespread showers and thunderstorms today, most numerous
  across northern Wyoming. Strong gusty wind over 50 mph is the
  main hazard with any storm, but some small hail is also
  possible, most likely around Johnson and Natrona Counties.

- Shower and thunderstorm chances continue for Monday and
  Tuesday, mainly across northern Wyoming.

- Elevated fire weather conditions remain into next week, given
  the low humidities, breezy winds, and lack of rain chances.

&&

.UPDATE...
Issued at 1037 AM MDT Sat Jun 20 2026

Not seeing any notable changes regarding the storm potential for
today compared to the discussion below. The latest HRRR runs in
particular continues to have a widespread push of 50 to 70 mph
associated with storms this afternoon, from the Wind River Basin
eastwards to Natrona and southern Johnson Counties. The HRRR can
sometimes overdo these types of events, so caution is taken. The
timing of this is between 2pm and 7pm (moving west to east in that
time). Other storms will also have the capability to bring isolated
gusts of similar magnitude. The HRRR also brings another push of
storms and strong winds to Lincoln County and the Green River Basin
between 6pm and 9pm, so another area to watch. This is all far from
certain, but gives an idea of potential wind hazards today.

&&

.DISCUSSION...
Issued at 233 AM MDT Sat Jun 20 2026

It is always rather interesting when coming back from a vacation to
see what you walk into. There are some things to talk about this
morning, as we look to have an active few days across western and
central Wyoming.

Even at the early morning hour this morning, a subtle shortwave is
moving into western Wyoming and bringing a few showers and even an
isolated lightning strike or two. There have been a few wind gusts
with this, but the maximum has been around 40 mph or so. A few
showers will continue this morning, but the main event still looks
to be this afternoon. Most guidance continues to favor the areas
East of the Divide as well as western Wyoming, where the deeper
moisture will be found, with a maximum chance of around 3 out of 5
across the northern border as well as Johnson and Natrona Counties,
with tapering chances further south as you move southward into the
drier air. The highest chance will be during peak heating, with a
gradual decrease through the evening. However, a few showers will
linger through the night. As for the chance of any stronger storms,
most of the area is under a Marginal Risk of severe thunderstorms.
There will be decent forcing and dynamics as a shortwave will
approach the area and there will be a but of upper level support
from the left front quadrant of a jet streak moving into southern
Wyoming this afternoon. The limiting factor, as it usually is in
this part of the country, is moisture. We have several threats, we
will tackle each one of them individually.

Severe wind gusts...Not surprisingly, this is the main threat from
the storms, as it usually is in western and central Wyoming. Model
soundings show well defined inverted V soundings across much of the
the area, with an emphasis on the southwestern half of the area.
There are steep lapse rates as well, approaching 9 degrees celsius
per kilometer in some locations. In addition, shower and
thunderstorm bases look to remain fairly high, generally more than a
mile above the surface. Dew point depressions are not the highest I
have seen, but are still decent. The largest are in southwestern
Wyoming, where some approach 60 degrees. Depressions drop to around
40 degrees in central Wyoming and to around 30 degrees in northern
Wyoming where the deepest moisture will be found. However, any
shower or thunderstorm could have a strong wind gust, even any
cumulus cloud that could collapse. The dreaded "Little Green Blobs"
are definitely possible this afternoon, especially in the southern
half of our area.

Hail...There is a very small area of a marginal risk of large hail,
largely along an east of a Glenrock to Midwest into eastern Johnson
County. This does make sense since moisture will be deepest in this
area, along with the lowest cloud bases. This is also the location
of the best instability, with CAPE values approaching 1000 J/Kg and
lifted indices as low as minus 4. Johnson County looks to have the
best chance, as most guidance shows some weak upslope flow into the
Bighorns this afternoon, which should aid in convective initiation.
The best chance of large hail remains across eastern Wyoming, but
there is around a 1 in 10 chance of some in our area, mainly along
and east of Interstate 25.

Flooding...The chance of this is low but not zero, and mainly in the
northern half of the state. As for climatology, the predicted
precipitable water value of 0.63 at 00Z this afternoon puts moisture
in the top 20 percent of historical values for this value and around
125 percent of the daily mean (0.50 inches). The most likely
locations will be, again across Johnson County, where some areas
have over an inch or precipitable water, as well as the Big Horn
Basin. However, there will be some steering flow today, so most of
the storms will have some movement, mitigating the threat somewhat.
In addition, it has been dry, so the soil does have a decent amount
of capacity. The Weather Prediction Center does have eastern Johnson
County under a marginal risk of excessive rainfall, and this looks
reasonable given the pattern.

Dry Lightning and Fire Weather...This will largely be a threat
across the southern half of the area, where cloud bases will be
higher. This will obviously bring a threat of some fire starts,
given the continuing drought conditions across the area. Elevated to
near critical fire weather is likely across southern Wyoming where
winds gusting over 30 mph and humidity falling to around 15 percent
may bring erratic fire behavior if any fires can start.

And one bonus I will throw in, tornadoes. The threat is very low,
less than 2 percent across the entire area as the best
instability and shear will be across eastern Wyoming. Like many
of the other threats, the main area of concern is Johnson
County, where the most shear will be found and deeper moisture
will allow for lower cloud bases and allow for the possibility
of any rotation to reach the ground. This threat is by far the
lowest of all the threats though.

Looking further out, things look to stay active over the next couple
of days. Another shortwave will move across the area Sunday with
another chance of convection. Instability values are around 25
percent less with this wave and there is much less jet support. So,
the threat of strong to severe thunderstorms will likely be less.
And like today, the main threat will be East of the Continental
Divide.

Monday has the potential to be a bit more interesting. Another
shortwave will be crossing the area. The main difference here is
that there will be more jet support, with an 100 knot jet streak
moving along the Wyoming / Montana border. The limiting factor here
is moisture, with precipitable water values falling to as low as
0.20 inches across western Wyoming. If the dry air wins out, the
main threat would be fire weather with a gusty wind and lower
humidity with the jet close by. However, if moisture could linger,
we could have a situation with storms firing along a pseudo dry
line. There is still a lot of uncertainty but we will have to watch
this.

More settled conditions should move in for Tuesday as a building
ridge brings upper level convergence. Uncertainty increases from
midweek on as some guidance has ridging over the area and only
isolated convection while others have the area near the top of the
ridge with shortwaves topping the ridge and bringing a few chances
of convection. Temperatures look to remain close to normal, with the
biggest temperature anomalies across southern Wyoming where drier
air and less cloud cover should allow for warmer temperatures.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 510 PM MDT Sat Jun 20 2026

Main round of thunderstorms is moving into eastern WY currently,
with improving conditions at most sites. Another round of
showers and thunderstorms is bubbling up along the WY/ID border
and will spread smaller thunderstorms into TAF sites west of the
divide and at KCOD during the next 3 hours or so. Once daytime
heating decreases, these storms should be quickly dissipating
finished for the night. A few showers may redevelop Sunday
morning for KCOD/KWRL/KRIW/KLND, but will again be very isolated
and any impacts would be brief. A low (10-20%) chance for a
shower will remain for most of the day east of the divide.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...Straub

Preliminary Local Storm Report
National Weather Service Riverton WY
441 PM MDT Sat Jun 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0425 PM     Tstm Wnd Gst     1 SE Sussex             43.69N 106.27W
06/20/2026  M67 MPH          Johnson            WY   Mesonet

            Public weather sensor measured gust.

&&

LV

Preliminary Local Storm Report
National Weather Service Riverton WY
442 PM MDT Sat Jun 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0432 PM     Tstm Wnd Gst     1 SE Sussex             43.69N 106.27W
06/20/2026  M65 MPH          Johnson            WY   Public

            Public weather sensor measured gust.

&&

LV

Preliminary Local Storm Report
National Weather Service Riverton WY
520 PM MDT Sat Jun 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0430 PM     Tstm Wnd Gst     13 WNW Midwest          43.49N 106.51W
06/20/2026  M57 MPH          Johnson            WY   Mesonet

            Mesonet station PC549 Tisdale Mtn (PC).

0452 PM     Tstm Wnd Gst     14 SE Kaycee            43.54N 106.48W
06/20/2026  M57 MPH          Johnson            WY   Mesonet

            Mesonet station WY63 I-25 Tisdale Divide
            (DOT).

&&

Special Weather Statement
National Weather Service Riverton WY
523 PM MDT Sat Jun 20 2026

WYZ023-024-027-210030-
Star Valley-South Lincoln County-Salt River and Wyoming Ranges-
523 PM MDT Sat Jun 20 2026

...STRONG THUNDERSTORMS WILL IMPACT SOUTHWESTERN SUBLETTE AND LINCOLN
COUNTIES THROUGH 630 PM MDT...

At 522 PM MDT, Doppler radar was tracking a cluster of strong
thunderstorms 10 miles east of Montpelier, moving east at 25 mph.

HAZARD...Wind gusts of 50 to 55 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

This storm will remain over mainly rural areas of southwestern
Sublette and Lincoln Counties, including the following locations...
Fairview, Afton Municipal Airport, Lincoln County Fairgrounds,
Kemmerer Municipal Airport, Auburn, Bedford, Sacajawea Campground,
and Cokeville Municipal Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4294 11105 4306 11038 4292 11038 4289 11043
      4285 11043 4284 11038 4254 11038 4254 11040
      4158 11037 4158 11104
TIME...MOT...LOC 2322Z 265DEG 20KT 4237 11112

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...55 MPH

Rowe

Preliminary Local Storm Report
National Weather Service Riverton WY
546 PM MDT Sat Jun 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0500 PM     Tstm Wnd Gst     15 NNE Mayoworth        44.02N 106.64W
06/20/2026  M59 MPH          Johnson            WY   Mesonet

            Purdy East PC sensor. Sustained 37 mph wind
            with 59 mph gust.

&&

LV

Special Weather Statement
National Weather Service Riverton WY
559 PM MDT Sat Jun 20 2026

WYZ023>028-210045-
Star Valley-Upper Green River Basin-South Lincoln County-Upper Green
River Basin Foothills-Rock Springs and Green River-Salt River and
Wyoming Ranges-
559 PM MDT Sat Jun 20 2026

...STRONG THUNDERSTORMS WILL IMPACT SOUTHWESTERN SUBLETTE...LINCOLN
AND NORTHWESTERN SWEETWATER COUNTIES THROUGH 645 PM MDT...

At 559 PM MDT, Doppler radar was tracking a cluster of strong
thunderstorms 12 miles north of Calpet, which is 12 miles southwest
of Big Piney, moving east at 35 mph.

HAZARD...Wind gusts of 50 to 55 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

This strong thunderstorm will be near...
  Big Piney around 615 PM MDT.

Other locations in the path of this storm include Marbleton.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

A Severe Thunderstorm Watch remains in effect until 900 PM MDT for
south central Wyoming.

&&

LAT...LON 4302 11104 4307 11026 4290 10997 4242 10976
      4156 10994 4158 11068 4255 11062 4271 11079
      4284 11105
TIME...MOT...LOC 2359Z 258DEG 29KT 4245 11033

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...55 MPH

Rowe