171
FXUS65 KRIW 091022
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
422 AM MDT Sat May 9 2026

.KEY MESSAGES...

- A weather system will bring scattered showers and
  thunderstorms today, with gusty wind and hail possible with
  any storm.

- Warmer temperatures return Sunday. The warmest days will be
  the middle of next week, when some warmer locations may make a
  run at 90 degrees.

- Elevated to near critical fire weather is likely for the next
  several days, with Monday and Thursday looking like the most
  concerning days at this time.

&&

.DISCUSSION...
Issued at 229 AM MDT Sat May 9 2026

In some of my discussions I talk about the signs of seasons
changing. And today, I saw one of the signs that summer is
approaching. As I was driving up for my usual graveyard shift around
9:45 pm, it was the first day when there was still some light in the
western sky. It will continue to get slowly brighter as we move
toward the summer solstice on June 20th, when time of total
darkness shrinks to only around 5 hours.

And as for the weather today, two of the main weather impacts
through the forecast period are most common in the summertime,
thunderstorms and fire weather concerns. As for today, the main
concern will be thunderstorms. The trigger for this will be a
shortwave dropping southeastward out of Montana. The main impacts
from this should remain East of the Continental Divide, most model
guidance and the general thunderstorm outlook shows this. The next
question is, will any of the thunderstorms be of the stronger
variety? There are some decent parameters for stronger storms, with
around 35 knots of bulk shear to lead to longer lasting storms. We
also have decent CAPE with up to 500 J/Kg across southern Natrona
County. This lines up well with the marginal risk of severe
thunderstorms issued by the Storm Prediction Center this morning.
Model soundings are showing inverted V signatures initially,
indicating that storms would be more of the high based variety.
Wet bulb zero levels are also fairly low, meaning that
small hail will be possible of any storm that develops. A strong
wind gust will also be possible. This may also be an earlier
show, with most guidance showing storms developing by noon and
ending by around sunset this evening. Coverage looks scattered
so it is still difficult to pinpoint them, but the best chance
will be across Johnson and Natrona Counties with around a 1 in 2
chance at any location, with decreasing chances further west
where moisture is not as deep. Temperatures look to remain below
normal East of the Divide, around 5 degrees on average. West of
the Divide, expect temperatures close to seasonal normals.

It will begin to more summer like for the Sunday and much of next
week. Sunday looks nice a nice Spring day with highs in the 60s and
70s, light to moderate wind and a good deal of sunshine. We then
turn to the other concern, fire weather, for Monday. The ridge will
get flattened by an approaching shortwave. This wave has little
moisture to work with, but will bring some gusty wind, with a
greater than 2 out of 3 chance of wind gusts past 30 mph for much of
the area. Combined with temperatures 15 to 20 degrees above normal
and relative humidity falling under 15 percent across much of the
area, elevated to critical fire weather looks fairly likely for much
of the area Monday afternoon. Ridging then builds back over the area
and brings lighter wind for Tuesday before another wave approaches
for Wednesday and may bring a few showers and storms West of
the Divide. Southwest, downsloping flow will bring even warmer
temperatures. And with the theme of summer, we may see the first
appearance of a summer phenomenon, highs in the 90s. Ensemble
guidance is showing locations mainly below 4200 feet in the Big Horn
Basin with anywhere from a 1 in 3 to a 1 out of 2 chance of the
first 90 degree high on Wednesday, largely in any area following the
Big Horn River from Thermopolis though Worland, Greybull and up
toward Lovell. Many other areas will see highs well into the 80s. A
gusty breeze will also bring elevated fire weather, although wind
does not look as strong on Wednesday as on Monday. Conditions look
similar on Thursday as well, although maybe a couple of degrees
cooler. The next more potent system in the form of an upper level
low may approach for the end of the next week. However, there is low
model agreement in timing and placement for any precipitation, so
details are not possible to hash out this far out.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 414 AM MDT Sat May 9 2026

A shortwave and cold front will drop southward across the area
today and bring scattered convection to locations East of the
Divide. Most terminals East of the Divide will have a chance of
showers and thunderstorms through the morning and the
afternoon. However, the scattered nature of the showers and
thunderstorms continues to make them difficult to pinpoint. The
most likely terminal to see impacts from thunderstorms is KCPR
but with the chance remaining less than 50 percent we have
elected to maintain the PROB30 group rather than prevail
thunderstorms. We have also maintained PROB30 groups for KRIW
and KLND and added one for KWRL for thunder. Showers and
thunderstorms should end from northwest to southeast by 02Z. Any
thunderstorm could bring brief MVFR conditions after 18Z.
Otherwise, largely VFR conditions are expected.

Gusty wind is expected to develop as the front passes, with all
terminals expected to see wind gusts past 20 knots with the
possible exception of KWRL. KPNA and KRKS could see wind gusts
past 30 knots this afternoon. Wind should decrease after sunset
tonight as wind decouples.

&&

.FIRE WEATHER...
Issued at 229 AM MDT Sat May 9 2026

Locally elevated fire weather is possible across southern
Wyoming today. After a lull on Sunday, and approaching weather
system on Monday will bring a gusty wind. Combined with low
humidity and above normal temperatures, elevated to near
critical fire weather looks like for Monday. The middle of next
week could also see elevated fire weather.

&&

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

&&

$$

DISCUSSION...Hattings
AVIATION...Hattings
FIRE WEATHER...Hattings