885
FXUS65 KRIW 192318
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
518 PM MDT Sat Sep 19 2026

.KEY MESSAGES...

- Widespread showers and thunderstorms are expected through
  sunset today before chances become confined to areas along and
  east of the Continental Divide overnight and Sunday.

- The primary threat in any shower or thunderstorm will be heavy
  rainfall, with localized flash flooding possible, through
  Sunday. The best chances (60% to 80%) for flooding are along
  and north of a Powell to Kaycee line.

- Cooler temperatures are expected to close out the weekend. A
  warm up and overall drier weather is looking more favorable
  for the upcoming work week beginning Tuesday.

&&

.UPDATE...
Issued at 1218 PM MDT Sat Sep 19 2026

Showers and thunderstorms continue to be the forecast story as a
potent upper level low treks across the state over the next 24
hours. 18Z satellite water vapor imagery shows the upper level low
centered over eastern Idaho. As the low progresses east today,
shower and thunderstorm potential will increase across much of the
state with the most numerous thunderstorms occurring along and
east of the Continental Divide and across Yellowstone National
Park. The track of the low puts the higher moisture content
across northern Wyoming, so the showers and thunderstorms with
the greatest heavy rain, and associated flash flooding, threat
will also be in northern portions of the state. With that being
said, the potential for heavy rain and localized flooding across
west and southern Wyoming cannot be ruled out but chances are
significantly less (20%). This much lower heavy rain threat is
due to the fact that drier air has begun to shift into those
areas this morning (evident by morning sunshine!)

The sunshine mentioned above has allowed the atmosphere destabilize
with widespread BLCAPE values above 500 J/kg, and even up to 1000
J/kg. Additionally, 0-6km shear values of 35kts to 45kts and
unstable lapse rates are expected this afternoon. This means that
thunderstorms could contain small hail, with some 1 inch hailstones
not out of question. This threat will be greatest across southern
and central Wyoming roughly from 2PM to 7PM. Tonight and Sunday
morning, the shower and thunderstorm chances, and heavy rainfall
potential, are largely confined to locations along and east of the
Continental Divide. Given the rain over the past few days and
forecasted clear overnight skies, low clouds and fog development is
possible across western and southern Wyoming Sunday morning.

&&

.DISCUSSION...
Issued at 243 AM MDT Sat Sep 19 2026

The monsoon has decided to give me a brief break tonight, as the
heaviest showers are either in Montana or across Colorado. There
are still a few showers around as I write this, but everything,
at least at the time of publication is on the light side. The
main show still looks to be today though, especially this
afternoon and into tonight.

The big bowling ball, AKA the Upper Level low continues to spin and
slowly move eastward. Getting close to the Wyoming border around
sunset. This will lead to another day of showers and thunderstorms
across the area. One difference is that the most numerous will
likely shift to the East of the Divide, especially after sunset as
the low moves eastward and drier air moves in behind the low. Now
for the main concern, which we have been talking about for the past
several days, the potential for heavy rain and flooding. It will
still be present today, although the main threat will again be
across northern Wyoming. Precipitable waters one most guidance
top out at around an inch across the Big Horn Basin and Johnson
County, approaching twice of climatological normals, then
gradually decreasing as you head south and west. There two areas
in particular that are concerning, the Cody Foothills and
Absarokas as well as Johnson County and the eastern slopes of
the Bighorn Range. The main concern is if the low can take a
further south track, this would set up an upslope flow in these
areas, bringing greater precipitation efficiency. The main time
of concern would be this afternoon and evening in the Cody
Foothills, and later tonight and possibly into Sunday morning
for the Bighorns and Johnson County as the low makes it`s
closest approach. The Weather Prediction Center agrees as well,
as the Excessive Rainfall Outlook has been upgraded to slight
roughly north of a Cowley to Sussex line, with Marginal north of
a Lake Yellowstone to Casper Mountain line. The chance of
flooding is less elsewhere but not zero. As for stronger
thunderstorms, the best instability will be east of our area.
However, I could see some hail with low wet bulb zero levels at
times. Precipitation should finally end in western Wyoming later
tonight as the low moves eastward.

There is one more day to get through on Sunday. The low will be
still be moving eastward, finally crossing into South Dakota and
Nebraska Sunday night. Some wrap around moisture will still be
around in northern Wyoming though, especially along and east of the
Bighorns. The threat of heavy rain will likely linger another day
here, especially in the morning before gradually tapering off in the
afternoon. Chances of showers decrease further west, with most
of western and southwestern Wyoming being dry by noon on Sunday,
As for the flood watch, we will continue it as is this morning.
We may be able to trim some of the western zones out for Sunday
if the low moves far enough east, but we will cross that bridge
when we get there.

Ridging builds over the area on Monday, bringing a nice last full
day of astronomical summer with mild temperatures and a good deal of
sunshine for most locations. Models are still split from later
Tuesday through Wednesday with the next potential system in the
form of a trough moving in from the southwest. Some guidance,
like the European, brings it across the state bringing another
chance of showers. Other guidance keeps it to the south with
mainly dry conditions. The current forecast leans toward the
wetter solution, but this is far from certain. Mainly dry
weather then likely returns for Thursday and Friday. And, with
largely south to southwest flow continuing, temperatures will
likely average above normal through much of next week as we head
into astronomical autumn, which begins Tuesday at 6:04 pm.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 518 PM MDT Sat Sep 19 2026

Scattered rain showers and thunderstorms continue this evening, with
most ending west of the Continental Divide after 06Z. Conditions
will drop to MVFR/IFR when showers and storms impact terminals this
evening. Rain will continue east of the Continental Divide through
Sunday afternoon and mainly impacting only KCPR beyond Sunday
morning as the weather system pushes east into the Plains. Low
clouds are likely (70%) east of the Continental Divide tonight into
Sunday morning, with prevailing MVFR conditions in general. Skies
should mostly clear out west of the Continental Divide tonight,
which will allow for fog development, especially where rain fell
today. The best chance will be at KBPI and KPNA with mentions of
VCFG and SCT001 at this time. This will be re-evaluated for the 06Z
TAF issuance.

Wind will average 5-10 knots through the TAF period, with gusts up
to 20 knots at KCPR Sunday afternoon. Mountain obscuration will
improve across the west tonight as clouds clear out and deteriorate
east as low clouds develop overnight.

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...
Flood Watch through Sunday afternoon for WYZ001>003-007>011.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Rowe