Severe Thunderstorm Warning 2, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
1245 PM MDT Wed May 13 2026

.KEY MESSAGES...

- Record high temperatures are forecast this afternoon,
  especially east of the Continental Divide where highs in the
  upper 80s to mid 90s will be common.

- Mostly dry showers and thunderstorms are forecast (40%-60%
  chance) after 2PM/3PM, especially across the western half of
  Wyoming. There is potential for strong to severe wind gusts
  (55 to 60+ mph) in any shower or thunderstorm, regardless if
  any rain makes it to the ground. Storms with strong wind gusts
  will also move into into central and north-central WY from
  5-8PM.

- Elevated to near critical fire weather conditions expected
  today through Saturday with widespread minimum relative
  humidity values below 15 to 20 percent, gusty 25 to 50 mph
  winds, and above normal temperatures.

- A pattern change with cooler temperatures and increased
  precipitation potential is looking more favorable beginning
  late Saturday/early Sunday.

&&

.UPDATE...
Issued at 1245 PM MDT Wed May 13 2026

With the strong shortwave trough approaching western WY today,
the NWS Storm Prediction Center (SPC) has portions of a Slight
Risk of severe thunderstorms over western WY. A Marginal Risk
extends eastward to include north-central, central, and south-
central WY as well. The previous forecast, as outlined in the
discussion below, is still generally on track, though have
increased the chances of storms and precipitation this afternoon
and evening. Have also indicated the areas and times where
severe storms are more likely. Very strong winds gusts of 50 to
70 mph are the primary threat today due to the reasons discussed
below and by SPC in their convective outlooks and mesoscale
discussion (for MT but applies to WY).

As of 1230 PM, convective development is beginning over
southwest Sweetwater County as well as the west-central WY
mountains. Continued boundary layer heating and increasing mid-
level moisture along with slight height falls will see storm
development in the next 1-2 hours as the high-resolution models
are indicating. The 18Z RIW sounding is showing high potential
for an inverted V as temperatures climb to the upper 80s and
drier surface air moves in from the southwest. As convection
moves from west and southwest WY to the northeast, expecting
light rain, lightning, and strong gusty outflow winds of 50 to
70 mph.

&&

.DISCUSSION...
Issued at 145 AM MDT Wed May 13 2026

07Z satellite water vapor imagery shows an upper-level ridge over
the Intermountain West, with it's axis over the western Wyoming
border. An upper-level low is also present on water vapor imagery
just off the Oregon coast. The ridge will shift east through midday
today as the upper low ejects towards the northwestern
Montana/southern Canadian border. By the early afternoon, strong
southwest flow will be present over Wyoming with an embedded
shortwave and a slight uptick in mid-level moisture. Through the
afternoon, the surface pressure gradient will tighten ahead of an
approaching cold front. All this movement in the upper atmosphere
and with the approaching cold front will bring heat, wind, elevated
to near critical fire weather concerns, and shower and thunderstorm
chances to much of Wyoming today.

There is high confidence that this will be the hottest day of 2026
thus far for most, if not all, locations east of the Continental
Divide. As the upper ridge shifts east today, 700mb temperatures
will be in the 12 to 15 Celsius degree range and this will translate
to surface temperatures near 90 degrees Fahrenheit. Wind will be
present today, thanks to the approaching cold front, and aid in
temperatures warming even more due to downsloping off the mountains.
Locations such as Lovell, Greybull, and Worland are expected to be
the hottest with highs near 95 degrees. Those west of the Divide
will not see this hot of temperatures with highs ranging from the
mid 70s to low 80s, partly due to increased cloud cover as mid-level
moisture increases in the southwest flow. It is important to note,
that if cloud cover is a bit more widespread east of the Divide,
temperatures may only peak in the upper 80s. Regardless if high
temperatures are in the 80s or 90s, today's temperatures are greater
than 20 degrees above normal for the middle of May.

Despite the uptick in mid-level moisture this afternoon, air near
the surface will remain rather dry. Relative humidity values are
forecast to bottom out in the 8 to 12 percent range this afternoon
for most areas and in the 15 to 20 percent range across far west
Wyoming. This low humidity, widespread gusty 25 to 40 mph southwest
winds, and much above normal temperatures leads to another day of
elevated to near critical fire weather conditions. Because fuels
have been classified as not critical, no fire weather headlines will
be issued. Fire headlines or not, burning is not advised today.

By this afternoon (2PM/3PM MDT), a potent shortwave, and the
associated cold front, will begin moving over Wyoming. This support
aloft will be enough for shower and thunderstorm development ahead
of and along the cold front this afternoon and evening. The
atmosphere will also be increasingly unstable ahead of the front,
especially across western Wyoming, where there will be anywhere from
600 to 800 J/kg of CAPE and 30 to 40 kts of 0-6 km bulk shear. This
is enough signal for some strong to severe thunderstorms capable of
at least 60 mph winds. This severe thunderstorm threat is maximized
over western Wyoming due to where the "greatest" moisture resides
(15% to 30% chance of 0.10" or more) during peak heating. The severe
thunderstorm threat is highlighted well in the SPC Day 1 Slight
Risk. Model soundings are also showing widespread strong inverted-V
soundings with dewpoint depressions of 50 to 60 degree Fahrenheit and
cloud bases above 10,000 feet west and east of the Divide. This
suggests that any shower or thunderstorm could produce little to no
liquid and produce gusty 60+ mph downbursts and outflows. This is a
prime example of a "Green Blob" afternoon across most of the area.

The cold front will have trekked east across the state by early
Thursday morning. The cold front will have brought temperatures
somewhat closer to seasonal norms with highs in the 60s and 70s. On
Thursday a tight post-frontal pressure gradient will be present with
a 60kt to 80kt jet overhead. This will translate to a windy day
across the entire area with widespread 35 to 50 mph wind gusts on
Thursday afternoon. This leads to another day of elevated to near
critical fire weather conditions with relative humidity values in 9
to 15 percent range. The windiest locations will be over northern
Wyoming, specially across the Absarokas and Cody Foothills, where
700mb winds of 35 kt to 50 kts look to develop. For wind prone areas
along the Cody Foothills, there is currently less than 20 percent
chance of wind gusts of 60 mph. However, higher elevations and wind
prone areas of the eastern Absarokas have a 70 percent chance of
seeing gusts of at least 60 mph. Chances of exceeding 70 mph across
the eastern Absarokas drop to less than 25 percent.

Strong zonal flow looks to continue on Friday with an upper-level
jet still overhead. An embedded shortwave looks to also trek across
the area on Friday afternoon, aiding in windy conditions. Widespread
winds of 30 to 45 mph can be expected Friday afternoon. High
temperatures are forecast to in the 70s, thanks to 700mb
temperatures around 5 Celsius. Dry air will still remain across the
area with widespread minimum relative humidity values between 10 and
15 percent. This will lead to yet another day of elevated to near
critical fire weather conditions across the entire area.

Ensemble guidance has been consistently signally a pattern change as
early as Saturday afternoon. There is a reasonable amount of
agreement among ensemble guidance that a longwave trough, and
associated cold front, will slowly traverse across the Intermountain
West in the Saturday/Sunday timeframe. The core of this trough looks
to originate from near the Alaskan Panhandle. This means that this
system will have more of a "cold punch" and more moisture. By Sunday
and Monday of next week, high temperatures look to be 10 to 15
degrees below normal with highs in the 50s. The potential moisture
associated with this system will be welcomed considering how dry the
year has been so far. There are many uncertainties with how this
system will pan out, including specific timing, moisture content,
and precipitation type. These questions will hopefully come to
fruition over the next several days.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1207 PM MDT Wed May 13 2026

Scattered showers and thunderstorms are expected to develop early
this afternoon as a trough and associated cold front approach. With
a dry surface layer, the main concern with this activity will be
strong outflow wind gusting 40 to 60 knots through the afternoon,
often out of virga showers with little visible rain reaching the
ground. This will begin across western and southern Wyoming by 20Z,
moving northeastward through the evening. By 03Z convection should
mostly be east of the Divide, with coverage waning everywhere after
06Z. PROB30 groups with -TSRA were persisted given the lower and
scattered precipitation chances. Otherwise, VFR flight
conditions should prevail, with any ceiling or visibility drops
associated with the convection expected to be short in duration.

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

&&

.FIRE WEATHER...
Issued at 145 AM MDT Wed May 13 2026

Temperatures today are forecast to be 20 to 25 degrees above normal
with many locations potentially seeing record highs. It will also be
very dry with widespread minimum relative humidity values in the 8
to 15 percent range. Mostly dry showers and thunderstorms are
forecast after 2PM/3PM MDT, especially across the western half of
Wyoming. Any shower or thunderstorm will be capable of gusty 55 to
60+ mph winds. Winds will also be gusty outside of any shower or
thunderstorm activity through the afternoon with frequent gusts of
25 to 40 mph.

Temperatures cool slightly on Thursday and Friday with less than a
10 percent chance of any showers or thunderstorms. However, there is
relatively high confidence in continued elevated to near critical
fire weather conditions due to minimum relative humidity below 15
percent and gusty 30 to 50 mph winds.

&&

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

&&

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

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