393
FXUS65 KRIW 212313
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
513 PM MDT Mon Jul 21 2025

.KEY MESSAGES...

- A Red Flag Warning remains in effect through 8PM Tuesday
  evening across central and southern Wyoming, where relative
  humidity values below 15% and gusts over 25 mph will return
  during the day Tuesday.

- Shower and thunderstorm chances (30-50%) across northern
  Wyoming with gusty winds up to 55 mph and small hail being
  possible in a stronger storm. Virga showers and dry
  thunderstorms (around 20% chance) are possible across southern
  Wyoming with gusty 50 mph outflows.

- Thunderstorm potential continues across the northern and
  eastern portions of the area Tuesday and Wednesday followed
  by a return of hot and dry conditions late this week into the
  weekend.

&&

.UPDATE...
Issued at 257 PM MDT Mon Jul 21 2025

The forecast largely remains on tack for this afternoon with
critical fire weather conditions across southern and central Wyoming
and shower and thunderstorm chances. The only change is the addition
of virga shower and dry thunderstorm chances across southern Wyoming
this afternoon.

Current (19Z) water vapor satellite imagery shows an upper-level low
over the Pacific Northwest and an upper-level ridge over the central
CONUS, ultimately leaving the Intermountain West in southwesterly
flow. An embedded disturbance is beginning to trek across the region
along with a slight uptick in moisture aloft. This provides
support aloft for shower and thunderstorm development, along
and near a front across northern Wyoming and non-frontal focused
convection across southern Wyoming. The frontal boundary is now
(19Z) roughly located in a Cody to Thermopolis to southern
Johnson County line. For thunderstorms north of the front, where
dewpoints are in the 40s to low 50s, the primary threats with
any strong storms are gusty winds around 55 mph and small hail.

South of the front, dewpoints are largely in the 20s and 30s. These
lower dewpoints combined with warming daytime temperatures are
causing relative humidity values to fall into the teens. Critical
relative humidity values (15%) are already being reached across
Sweetwater County. Values are expected to continue to drop
through the afternoon with increasing gusty southwest winds.
Thus, the Red Flag Warning remains in effect through 8PM this
evening across central and southern Wyoming. As mentioned prior,
the disturbance aloft has a slight uptick of moisture
associated with it. This will provide support for isolated
convection (around 20% chance) across southern Wyoming. Because
of the very dry low-levels with mean dewpoint depressions 40 to
50 degrees, virga showers and dry thunderstorms will be capable
of 50 mph gusty outflows.

On Tuesday, recent guidance has been consistent for another day
of minimum relative humidity below 15% and gusty winds above 25
mph for much of central and southern Wyoming. As a result the
Red Flag Warning has been extended into Tuesday for the same
locations. Otherwise the forecast remains on track with a slight
uptick in shower and thunderstorm activity on Tuesday and
Wednesday followed by hot and dry conditions Thursday into the
weekend.

&&

.DISCUSSION...
Issued at 317 AM MDT Mon Jul 21 2025

I often compare forecast reasoning to common things in these
discussions. And for the next week, it reminds me of some of the
chewing gum I chewed growing up back in the day. The gum that was
dry for most of the outside. However, the middle often had some
liquid flavor in the middle. But the flavor did not last long.
And that describes the pattern over the next week, mainly dry,
but with a couple of potentially moister days in the middle,
especially Wednesday.

The main concern today continues to be fire weather. We will have a
weak frontal boundary over Montana that will slowly sag southward
with a shortwave approaching from the west. This will provide the
mixing and enough a pressure gradient for another day of breezy
conditions. With humidity falling to under 15 percent, this means
critical fire weather across much of the central and southern
portions of the area. So, we will continue the Red Flag Warning as
is for now. As for convection, there will be more than yesterday.
Most model guidance is showing around the northern third of the area
as the most likely location. For area, around the 0.50 inch
precipitable water line was used, roughly from north of an Afton to
Dubois to Buffalo line. The main threat from any of these showers or
storms would be strong wind gusts, with some models giving as much
as a 55 degree dew point depression. The best chance of any hail
would be east of the Bighorns where moisture is a bit deeper.
Chances are still not very high though, anywhere from around 1 in 3
in the mountains to around 1 in 5 at any location in the lower
elevations. Further south, we can`t rule one out but the drier
atmosphere and less forcing will keep the chance to less than 1 out
of 10. And it will be a hot day as well. We raised temperatures a
few degrees, with some locations in central Wyoming likely reaching
the upper 90s this afternoon.

Reasoning is similar for tomorrow. There are some subtle differences
though. For one, there may be a bit more low level easterly upslope
flow tomorrow across  the northern mountains, which would lead to a
few more storms. The other difference is wind. Most guidance is
showing wind at most locations across central and southern Wyoming
anywhere from 2 to 5 knots lighter. So, although humidity will
remain at critical levels, wind may not meet Red Flag criteria. So,
we will punt to the day shift for any decision of extending the
warnings another day. Storms would likely occur in same locations.
One difference though is that the next shortwave looks to be later
in arriving, meaning a later show for convection that could extend
well into the evening hours, especially in Johnson County. Chances
generally rise from 1 in 5 across central Wyoming to as high as 1 out
of 2 across northern Johnson County and the Bighorn Range.
Temperatures may cool a few degrees, but still be above seasonal
averages.

Wednesday for now looks like the day of the most coverage of
convection. A deeper shortwave may move into the area on this day.
This means more cloud cover and somewhat cooler temperatures and
higher humidity, especially East of the Divide, easing fire weather
concerns. Precipitable water values also rise in this day, rising
anywhere from around 120 percent of normal in Fremont County to as
much as 150 percent of Johnson County. We will also have low level
upslope and weak steering flow at 700 millibars. This could lead to
some slow moving thunderstorms and locally heavy rain. The Weather
Prediction Center has areas east of the Bighorn Range in a marginal
risk of excessive rainfall, for example. With the lighter wind
fire weather looks more likely not to reach critical levels on
this day.

We will then go into a drier pattern as ridging tries to build
northward into the state. The general pattern is for warming
temperatures and decreasing chances of precipitation. As for fire
weather concerns, not a lot of certainty. Confidence is fairly high
humidity will be at critical levels in many locations. The question
is will wind speed reach criteria and there is too much uncertainty
to pinpoint the criteria this far out.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 512 PM MDT Mon Jul 21 2025

Once again, VFR conditions are generally expected for the forecast
period, with very low chances of MVFR conditions due to localized
showers and thunderstorms early this evening at KCOD/KRKS. In
addition, low clouds are possible at KCOD after 22/06Z through
22/14Z as low-level moisture pushes south from Montana. Showers and
thunderstorms will continue over northwest and northern WY through
about 22/03Z, with some development still expected over south-
central WY. KCOD, KWRL, and KRKS are the most likely to be impacted.
KJAC should see a vicinity of showers and storms in the surrounding
mountains. Overall gusty southwest winds will decrease around sunset.
Models are showing some light areas of showers moving through
western and southwest WY during the overnight hours, which may bring
light rain to KJAC as well as thicker clouds.

High-res models are showing a low-level moist north push from
Montana into the Bighorn Basin and Johnson County after midnight.
Any low clouds and north winds should retreat by late Tuesday
morning. Gusty dry southwest winds will return by 18Z Tuesday from
southwest into central WY. Showers and thunderstorms are expected
again across northern WY and the western mountains during the
afternoon and early evening, primarily impacting KJAC/KCOD/KWRL. A
back door low-level push is expected from KBYG into KCPR and the
Bighorn Basin Tuesday evening.  Light showers will linger over
northwest WY from Tuesday evening into early Wednesday morning.

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

&&

.FIRE WEATHER...
Issued at 317 AM MDT Mon Jul 21 2025

A combination of a gusty breezy and humidity falling below 15
percent will bring critical fire weather to most of central and
southern Wyoming this afternoon into early this evening.
Elevated fire weather is likely elsewhere in the locations where
fuels are not critical or wind is not strong enough to meet
criteria. Elevated fire weather will remain likely on Tuesday
but the chance of critical fire weather looks lower due to wind
being somewhat lighter than on Monday afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT Tuesday for WYZ277>280-283-288-
289.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...McDonald
FIRE WEATHER...Hattings