218
FXUS65 KRIW 220852
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
252 AM MDT Mon Jun 22 2026

.KEY MESSAGES...

- A weather system will bring some showers and thunderstorms to
  northern Wyoming today. Some storms could be strong to severe
  with large hail and strong wind, with the highest chance in
  Johnson County.

- Low humidity and a gusty wind will bring elevated fire weather
  to much of central and southern Wyoming this afternoon.

- Chances of showers and thunderstorms continue through much of
  the week, with another possibility of strong to possible
  severe thunderstorms on Wednesday.

&&

.DISCUSSION...
Issued at 243 AM MDT Mon Jun 22 2026

It is now June 22nd, and it is a sad day for summer lovers like
myself. It is now when days begin getting shorter. But enough of
this, we do have things to talk about today, as we will continue to
have the chance of convection, and possibly a few stronger ones.

We have been talking about today for a couple of days now, and we
continue to have some uncertainty. The main uncertainty is how far
west will the threat of stronger storms move. The slight risk area
has moved little at this point, covering mainly far eastern Johnson
County. The Marginal Risk as shifted a bit further west, right now
mainly covering an area East of a Powell to Greybull to Kaycee to
Casper line. This makes sense. A cold front will be moving through
the area, and areas further west will likely see more stable and
air move in behind it. The other factor will be moisture.
Precipitable water values will drop behind the front, leading to
little to no convection across the western and southern two
thirds of the area. Instability parameters look impressive
across northern areas East of the Divide though, with CAPE
values as high as 1500 J/Kg and lifted indices to minus 4. The
uncertainty is that some of the models, both of the synoptic and
high resolution varieties, have little coverage in our area. In
addition, the jet core, which was previously predicted to be
over the Wyoming-Montana border, has shifted further north. For
now, we have elected to largely go with the Storm Prediction
Outlook, putting convection in this afternoon across the Bighorn
Basin to northern Natrona County and northward. We have kept
chances generally less than 1 out of 4 in any particular
location though given the uncertainty. Further south and west,
the atmosphere may be too dry. A high based shower storm or
virga shower can`t be ruled out, but with a chance of less than
1 out of 8 we kept them out of the forecast for now.

Now for the threats. For areas further to the east, the main threat
looks to be large hail. Lapse rates remain rather impressive,
approaching 9 C/Km in the late morning and early afternoon. Shear
values of over 50 knots over the 0-6 km layer are sufficient for
this as well. The main threat for this would be over Johnson County,
where deeper moisture will be present and low level upslope
southeasterly flow should aid in convective initiation, with
diminishing chances further west and south as you move into the
drier air. As for strong wind gusts, this is possible with any
shower or thunderstorm. This includes areas further west and
south. Many model soundings further east show inverted Vs with
steep lapse rates. Any convection here would be higher based
with little rain. Dew point depressions do approach 50 degrees
in places like Riverton so in the unlikely chance that
convection can form, there could be a downburst when the clouds
collapse. And, as for tornadoes, this is by far the least
likely. There is a 2 percent area in Eastern Johnson County
where cloud bases are lowest and shear is the highest. Chances
are not zero, but very small. And there is one more thing.
Further south, humidity will fall into the teens and with a
gusty wind elevated fire weather is likely. This will be in
areas where fuels are not critical, so no Red Flag Warnings.
However, any fire starts could spread rapidly.

Most strong convection should end by sunset, but like tonight, a few
showers and storms will likely linger through the night. Tuesday at
this point looks like a cooler day behind the cold front. There will
still be a few showers and storms around, but coverage will be less
as shortwave ridging and some upper level convergence moves over the
Cowboy State. Chances of convection then increase again as another
shortwave passes over the area. This time, a jet streak will be
passing by and this could increase the chance of strong to severe
thunderstorms again. The Storm Prediction Center already has a
Marginal Risk over Natrona County, where northeasterly upslope flow
is predicted and where the maximum instability and moisture is
present. This is another day we are going to have to watch
closely. The active pattern looks to continue into next weekend
with more chances for convection, although timing of the
individual shortwaves remains uncertain. With the area near the
top of a ridge, temperatures should average close to normal
with no heat waves expected through next weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 945 PM MDT Sun Jun 21 2026

Prevailing VFR conditions through the TAF period. Drier air will move
into the area tonight, which will keep convection more limited
Monday afternoon. Most convection will be limited to I-25 and east.
As such, no mentions of precipitation at any terminals. A cold front
will drop down from the north Monday morning reaching KCOD and KWRL
during the afternoon hours. The cold front will continue sliding
south reaching KCPR, KRIW, KLND, and KRKS Monday evening.

Wind will become breezy from the west Monday afternoon for terminals
ahead of the cold front shifting northerly behind the front. Breezy
northerly wind will linger behind the front through the end of the
TAF period. Occasional mountain obscuration will occur behind the
front Monday evening east of the Continental Divide.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.&&

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

&&

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

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe