549
FXUS65 KRIW 182221
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
421 PM MDT Fri Sep 18 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected across the state
  through at least 9PM, with a few strong to severe
  thunderstorms possible across west and southwest Wyoming. The
  primary threats will be frequent lightning, heavy rain, small
  hail, and breezy winds.

- Heavy rain and localized flooding will be possible in any
  shower or thunderstorm through Sunday afternoon, especially
  across northern Wyoming. Thus, a Flash Flood Watch is in
  effect for portions of northern Wyoming, including Yellowstone
  National Park, through Sunday afternoon.

- This wet pattern will continue Saturday and Sunday, with the
  greatest coverage and chances across northern and central
  Wyoming.

- Drier conditions return Monday with temperatures slightly
  cooler than normal. The next chance for precipitation looks to
  return Tuesday night into Wednesday, as a storm system moves
  over the Cowboy State.

&&

.UPDATE...
Issued at 144 PM MDT Fri Sep 18 2026

The forecast remains on track with an active weather pattern through
Sunday as an upper level low, currently over northern Nevada and
southern Oregon, shifts east into and across Wyoming. As of 1PM,
showers and thunderstorms are spreading northeast across Wyoming as
a shortwave rounds the parent upper low. Strong to severe
thunderstorms remain possible this afternoon and evening, mainly
across western Wyoming, with height rises aloft, BLCAPE values of
700 to 1000 J/kg, and 30 to 45 kts of 0-6km shear. The threats in
any strong to severe thunderstorm will be hail and gusty winds
up to 50 mph. The severe thunderstorm threat should diminish by
9PM as the shortwave moves out of the region. More of note,
abundant moisture continues to be present across the state (the
RIW 18Z launch indicates PWATs of 0.86"). Thus, any shower or
thunderstorm will be capable of heavy rain that causes localized
flash flooding, especially in poor drainage and urban areas and
steep terrain. Although shower and thunderstorm coverage is
expected to decrease overnight, there will likely be at least a
few showers across the area.

Model guidance continues to be in agreement that the upper level low
will progress east across Wyoming on Saturday and Sunday. The
position of the low will focus the most numerous shower and
thunderstorm activity across northern Wyoming. The Flash Flood Watch
remains in effect for higher elevations of northern Wyoming and
Johnson County. An axis of rain is forecast to position itself over
northwest Wyoming, including Yellowstone National Park. Given the
steep terrain and low drainage areas across Yellowstone and current
(1PM) 24 hour rainfall amounts ranging from 0.12" and 1.00", the
Flash Flood Watch has been extended to include Yellowstone National
Park. Through Sunday, many locations could see 0.75" to 1.75" of
rain, with a majority of this rain falling quickly in thunderstorms.
If a thunderstorm has higher rainfall rates, these values may be
underdone. Additionally, if the upper low deviates from its
forecasted path these values may be unrepresentative. For example,
if the low tracks further to the south, anticipate higher totals.
All in all a rainy and stormy weekend is in store.

&&

.DISCUSSION...
Issued at 240 AM MDT Fri Sep 18 2026

It looks like quite an active few days across the area as my
nemesis, the monsoon, with an assist from an upper level low,
will continue to bring rounds of showers and thunderstorms for
the next few days. We continue to have convection tonight,
especially in western Wyoming where some storms have been quite
strong. They should slowly weaken as we head into the night.
However, many of the storms have produced very heavy rain back
in Idaho and some localized flooding will be possible before
sunrise, especially north of Jackson and into Yellowstone Park.

As for today, some showers and an isolated thunderstorm will be
around in the morning. The main action looks to be this afternoon
and tonight as we get some daytime heating to destabilize the
atmosphere and a shortwave around the parent upper level low
brings some additional lift in the afternoon and evening. The
result will be a good coverage of showers and thunderstorms
across the area. As for timing, the most numerous storms should
be west of the Divide and across southern Wyoming in the
afternoon, then moving east of the Divide later in the
afternoon and into the evening. There is fair model agreement in
the most numerous storms to be in the northern counties east of
the Divide this evening, with lessening chances further south.
This lines up well with the Flood Watch, which we will continue
as is. Precipitable water values remain abnormally high,
generally 150 to as much as 200 percent of normal, especially
east of the Divide. So, as would be expected, any storm could
produce heavy rain and localized flooding. But there is another
aspect. There could be some strong to possible severe
thunderstorms. There is close to 1000 J/kg of mid level CAPE as
the trough approaches the western portions of the state. There
is obviously plenty of moisture. Directional shear will also be
increasing as the upper level low moves closer. The main threat
other than heavy rain would be hail, especially with fairly low
wet bulb zero levels. Showers and storms should continue into
the night, although coverage should gradually decrease.

The upper level low continues to drift eastward on Saturday, moving
into Wyoming later in the day. On this day, the most numerous
showers and storms will shift to northern Wyoming. Some drier air
may try to wrap into the southern portions of the area, possibly
shutting off convection for a while. We have maintained at
least a 1 in 5 chance for all areas though. A better chance of
showers likely returns Saturday night as low pressure moves
across the area. Again, despite decent steering flow, the amount
of moisture will be sufficient for locally heavy rain and
possible flooding.

As we head into Sunday, conditions may begin to improve as the low
moves east into the Dakotas. There will still be some showers around
from the wrap around moisture though. The main threat on this day
shifts to northeastern portions of the area, with decreasing chances
to the south and west, with mainly dry conditions expected across
southwestern Wyoming. With flow turning northwest, it should be a
cooler day as well. Showers should then end for all areas by Sunday
evening.

There continues to be more uncertainty regarding next week. Some
guidance brings another system up from the southwest with another
chance of showers on Monday or Tuesday, others keep it south of the
area, primarily impacting Utah and Colorado. Given the concerns in
the short term, we didn`t have time to take a good look this far
out. One thing we are more confident is that flow will turn
south to southwesterly again, and bring another period of above
normal temperatures for the middle and end of next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 415 PM MDT Fri Sep 18 2026

Multiple rounds of widespread showers and thunderstorms will
move through the region over the next 24 hours. Although VFR
conditions will prevail, any thunderstorms in the vicinity of
airports will be a threat, and if a storm passes directly over
an airfield, brief MVFR to IFR conditions are possible as the
heavy rain passes over. Winds today will generally be light,
but could gust into the 25 to 35 kt range near any storm
downdraft/outflow. These winds would be erratic, and gusty VRB
winds groups are prevalent in TEMPO and PROB30 groups at
individual TAF sites.

Showers and thunderstorms will be widespread west of the Divide
to start the TAF period, impacting KJAC, KBPI/KPNA and KRKS. A
line of showers and thunderstorms should be exiting Johnson and
Natrona counties, but could still be impacting KCPR. Showers and
thunderstorms are expected to wane across thew area through 06Z.

Low clouds (and IFR to MVFR conditions) look to push into the
Bighorn Basin overnight into Saturday morning, including for
KCOD. These low clouds will likely persist until around
17Z, after which time thinning of low clouds is expected, while
surface mixing help to lift cloud decks slightly. Showers and
thunderstorms will begin to develop again after 18Z, mainly
impacting terminals east of the Divide and KRKS. KCOD, KWRL and
KCPR have the best chances to be impacted.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Sunday afternoon for WYZ001>003-007>011.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...VandenBoogart