437
FXUS65 KRIW 171841
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1241 PM MDT Thu Sep 17 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected throughout the day
  today, with the greatest coverage across western and southern
  Wyoming. Moderate to heavy rainfall could result in localized
  instances of flooding.

- An active, rainy pattern continues Friday through late
  Saturday. The most active day is forecast to be Friday where a
  few strong storms are possible along with instances of
  flooding.

- A Flash Flood Watch has been issued for the Absarokas, the
  Cody Foothills, the Owl Creek and Bridger Mountains, the
  Bighorn Mountains, and Johnson County from 12PM Friday through
  6PM Sunday.

- Fall-like temperatures are forecast through the weekend with
  plenty of clouds. Anticipate highs ranging from the mid 60s to
  upper 70s.

&&

.UPDATE...
Issued at 1240 PM MDT Thu Sep 17 2026

Noon radar imagery shows an area of showers and thunderstorms across
western Wyoming. This convection is supported by a shortwave
disturbance and will progress to the north and east through the
afternoon. However, with the dense morning cloud cover keeping the
atmosphere stable across central Wyoming, thunderstorm coverage is
expected to decrease. Another round of showers and thunderstorms is
forecast to develop across far southwest Wyoming late this afternoon
(3-5 PM MDT time range). A few strong thunderstorms are possible
before sunset due to increased instability (500 to 700 J/kg of
BLCAPE) and moderately unstable mid-level lapse rates. The threats
with any strong thunderstorms will be hail up to 1 inch, gusty 40
mph winds, and moderate to heavy rainfall that is capable of
isolated flooding. The strong thunderstorm threat is expected to be
relatively short-lived and come to an end by sunset. As mentioned in
the morning discussion, rain showers could linger through the
overnight hours.

The active and rainy pattern continues Friday through Sunday as an
upper level low slowly treks across Wyoming. Precipitable water
amounts with this system have consistently remained above the 98th
percentile of climatology across northern Wyoming. The track of the
low is directly across Wyoming, west to east, and puts the highest
rainfall amounts across southern and central Montana. However, the
Absarokas, the Cody Foothills, Owl Creeks, Bighorns, and Johnson
County are forecast to receive anywhere from 0.75" to 1.5" of rain
from Friday afternoon through Sunday afternoon. Thus, a Flash Flood
Watch has been issued for the aforementioned areas Friday 12PM
through Sunday at 6PM. There will be breaks between thunderstorms
and it will not rain for two days straight, but the thunderstorms
are expected to have moderate to heavy rainfall rates. The areas of
most concern are steep terrain and poor drainage areas. It is
important to note that the totals noted above represents the mean.
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 father to the south, anticipate higher totals.

&&

.DISCUSSION...
Issued at 206 AM MDT Thu Sep 17 2026

The story today and for the next few days, as it has been for
seemingly forever (actually the past two and a half months) is
monsoonal moisture. Satellite imagery shows an area of moisture
advancing northward with showers and embedded thunder moving toward
Sweetwater County as I write this around 1 am. This area of
showers will spread northward tonight. Meanwhile, most of
northern Wyoming should remain dry tonight.

This deeper moisture will overspread the entire area today, with
precipitable water levels rising to 150 to 175 percent of normal.
Just about all areas will have at least a 1 in 6 chance of a shower
or thunderstorm today. The best chances though will be across
western Wyoming where there will be more forcing, and also across
areas East of the Big Horns, with the least chance in the center,
especially the central basins. There continues to be one factor that
makes me think coverage of thunderstorms may be less though.
There will be a decent amount of cloud cover around the area,
and this could limit surface instability somewhat. And it is we
are now almost to the autumnal equinox, not the middle of
summer. So, the sun is weaker and daytime heating will be less.
This may also limit the chance of heavy rain and any flooding
potential. Steering flow is less than yesterday, but we still
have 10 to 20 knots and this should be enough to keep storms
moving somewhat. However, with the high precipitable waters
there could still be isolated minor flooding, especially west of
the Continental Divide. Most showers should end after sunset,
but a few showers will likely linger through the night. And it
will probably be more than expected, the models have been
horrible this season with nocturnal monsoonal convection.

An upper level low that has been helping to funnel the moisture into
the area will finally begin to move eastward today, drifting
toward Idaho by tomorrow. This may be the most active day for
convection of the period, especially thunderstorms. Most
guidance is showing a clearer sky in the morning, allowing for
more daytime heating and surface instability. Instability
parameters are decent for this time of year, with CAPE
approaching 700 J/Kg and lifted indices as low as minus 4. With
the approaching upper level flow, there will also be some decent
directional shear, especially in western Wyoming. The result
will be a chance for some stronger thunderstorms, again
especially in western Wyoming where there is a marginal risk of
severe weather. The main threat would be hail given the low wet
bulb zero levels. With more sun and southwest flow continuing,
this will also be a warmer day, especially East of the Divide
where warmer spots may get close to 80 for a high. Showers will
continue into the night as well. With the added instability and
a shortwave rotating around the low, if there are any flooding
problems, this would be the time for it.

The latest guidance continues to slow the progression of the low,
only slowly moving it over Wyoming by Saturday. On this day,
the best chance of showers and storms would be across northern
Wyoming. Most guidance is in decent agreement on this. The
deepest moisture would be over Montana, but most areas will
have a chance. There may also be a dry slot that moves in
portions of the area, but exactly where is still in question.
The low will slowly cross the state then on Sunday, but will
still be close enough for some wrap around moisture to bring
more showers, mainly to northern and eastern portions of the
area. This also looks like a cooler day as flow turns to the
northwest, with temperatures anywhere from 5 to 10 degrees
cooler than on Saturday. Following this, there is more
divergence in the models with some bringing another system
across the area early next week with another chance of showers
and others suppressing it to the south, keeping the area largely
rain free. All guidance shows south to southwesterly flow
returning though, so temperatures look to be back above normal
as we head into the start of autumn, which arrives on Tuesday,
September 22nd at 6:04 pm.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1045 AM MDT Thu Sep 17 2026

VFR conditions expected for all TAF sites with rain showers
pushing north currently of RIW to CPR nearing COD/WRL by 20Z.
JAC has ongoing shower activity that will extend through 00Z
becoming more limited the remainder of the TAF. Low but VFR
ceilings associated with the rain showers and relatively light
winds 10kts or less through the entirety of the period.
Otherwise, storm activity increases after 21Z for LND/RIW and
push northeastward there after through 01-02Z remaining north of
CPR. RKS should only see very isolated activity later in the
period carrying only VCSH due to lower confidence. Showers/storm
activity increase once again at the end of the period for Friday
afternoon after 18Z.

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...
Flood Watch from Friday afternoon through Sunday afternoon for
WYZ002-003-007>011.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...Lowe