Damaging Wind in Wyoming, June 21, 2026

Area Forecast Discussion
National Weather Service Riverton WY
1248 PM MDT Sun Jun 21 2026

.KEY MESSAGES...

- Showers and thunderstorms are possible (15 to 40 percent
  chances) this afternoon, primarily east of the Continental
  Divide.

- Showers and thunderstorms again possible Monday. A few could
  be strong, especially around Johnson nd Natrona Counties.
  Large hail and gusty outflow winds are the primary hazards.

- A warming trend occurs through rest of the week, with daily chances
  of showers and thunderstorms for some locations.

&&

.UPDATE...
Issued at 1247 PM MDT Sun Jun 21 2026

No notable changes made to the key messages for today's forecast
with this update. Have included the latest hi-res guidance for
placement of precipitation. Generally 15 to 40 percent chances occur
this afternoon and evening for locations around and east of the
Divide. Rain chances linger overnight for the Bighorn Basin and
Johnson County. Currently, radar showers a mix of showers, and a few
thunderstorms, developing over the mountains. Some are moving off
into the surrounding basins, but are not lasting long.

&&

.DISCUSSION...
Issued at 224 AM MDT Sun Jun 21 2026

We are now entering my favorite season the year, summer, which
arrives at the exact time I am sending this discussion. I was
feeling rather nostalgic before I came into work and put on the
song the many fellow Gen Xers that are Hip Hop fans is the
perfect summer song. And of course I am referring to the classic
"Summertime" by DJ Jazzy Jeff and the Fresh Prince, now 35
years old (I'm getting old). And the main weather concern over
the next few days will be the typical summertime weather
concern, thunderstorms, with a bit of fire weather mixed in.

It was quite active yesterday, with many reports of high wind gusts
and even a landspout tornado. One thing we didn't see a lot of,
unfortunately, was rain, which we need badly. Things are fairly
quiet this morning, although we still have a few slight showers
across the northern third of the area. Today will see more showers
and thunderstorms as another weak shortwave moves through the
area. However, they should not be as numerous or as strong as
yesterday. For one, instability parameters are less, with CAPE
anywhere from 20 to 30 percent less than yesterday. Also, there
is less shear and most jet energy will remain north and south of
the Cowboy State. We could see a couple of stronger storms, but
not nearly as widespread as yesterday. Precipitable water
values East of the Divide remain above normal though,
approaching the 90th percentile in some locations (nearly 0.75
inches, around 150 percent of normal). There is still steering
flow though, and this should mitigate the chance of flooding.
Most storms should be East of the Divide today, with less than
20 percent coverage in the west and south. And like tonight, a
few showers could linger into the night.

I still have concerns about Monday. Another shortwave will be moving
in from the west. However, there are a couple of other factors that
could make for some stronger storms today. For one, an almost 100
knot jet will be passing close to the Wyoming-Montana border in the
afternoon, enhancing upper level forcing and upper level divergence.
There will also be a push of rather dry air moving in from the west,
with precipitable water values in western Wyoming falling to as low
as 0.25 inches. At the same time, weak low pressure passing to the
south will bring easterly flow into eastern portions of the area,
enhancing upslope flow over Natrona and especially Johnson Counties.
Instability parameters look rather impressive as well, with much of
northern Wyoming having over 500 J/Kg of CAPE in the afternoon with
as much as 1500 J/Kg in Johnson County. Lifted indices fall as
low as minus 5 as well. With Storm Relative Helicity rising to
over 200, we could see rotating storms. How far west storms can
get still looks dependent on how far east the dry air can get,
as the pseudo dryline could be the place where storms will
fire, as well as off the Bighorns with low level upslope flow.
The Storm Prediction Center already has a slight risk into
eastern Johnson County, with a marginal generally along and east
of Interstate 25. This pattern generally favors large hail, but
all hazards are possible. This is still far from a certainty
but needs to be watched. If the drier solution wins out, fire
weather will be the concern. At this time, relative humidity
should remain above critical levels in areas where fuels are
critical though. Lower humidity will be found across the south
though and with a gusty wind from the proximity of the jet,
elevated fire weather looks like at least a 2 out of 3
possibility Monday afternoon, especially in central and
southern Wyoming.

Coverage of storms should decrease somewhat on Tuesday as ridging
moves across the area and the deeper moisture gets pushed further
east out of the area. The best chance of storms this day would again
be in the eastern counties. The models then diverge somewhat. Most
show a warming trend starting Wednesday with many areas possibly
seeing 90s by Friday. A few shortwaves will move through the area as
well with chances of showers and storms, but guidance has different
timing of the waves so confidence on timing remains low.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1118 AM MDT Sun Jun 21 2026

Conditions remain VFR through midday Monday with the possible
exception of brief MVFR with a thunderstorm east of the
Continental Divide. Showers and thunderstorms erupt over the
Absaroka, Bighorn, and Wind River Mountains between
18Z-20Z/Sunday before moving east-southeast into the lower
elevations through the afternoon. The convective wind hazard
will be much less Sunday than it was Saturday. Potential outflow
gusts of 25-35kts occur with stronger convection. Gusty
westerly surface wind over the southwest terminals fades around
02Z/Monday before returning between 16Z-18Z/Monday. Light
showers linger across the northwest quadrant of Wyoming
overnight.

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...Wittmann
DISCUSSION...Hattings
AVIATION...CNJ