921
FXUS65 KCYS 131929
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
129 PM MDT Thu Aug 13 2026

.KEY MESSAGES...

- Numerous to widespread showers and thunderstorms Thursday will
  have the potential to produce torrential rainfall and
  localized flash flooding. Isolated strong to severe storms are
  possible.

- Unsettled weather continues through at least Sunday, bringing
  cooler than average temperatures and daily chances for showers
  and thunderstorms.

- Isolated strong to severe storms and localized flash flooding
  will remain possible through Sunday.

- Drier weather and temperatures close to seasonal averages will
  return early next week.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 130 PM MDT Thu Aug 13 2026

The forecast remains largely on track as concern for torrential
rainfall and localized flash flooding continues. Moisture is about
as abundant as it gets across our area today. Mesoanalysis indicates
precipitable water reaching 1.2" to 1.3" along the I-25 corridor and
as high as 1.5" in our eastern zones. Even Carbon County is largely
over 1.0" at this time. Per the NAEFS, this is showing up with a
return interval of between 2 and 10 years, meaning that precipitable
water gets this high once every 2 to 10 years depending on the exact
location. In addition, GOES water vapor imagery can help identify a
few synoptic features helping to drive today`s weather. A more
notable upper level shortwave is moving into western Wyoming at this
time, while a very subtle vort-max is moving out ahead of it,
currently located more or less over the I-25 corridor. The leading
feature was driving the showers with a few rumbles of thunder that
rolled through southeast Wyoming this morning. In between these two
synoptic features, we are starting to see some clearing skies which
should aid in destabilization over the next few hours. All together,
these observations have improved confidence that numerous to
widespread showers and thunderstorms will indeed develop this
afternoon, and the main uncertainty remaining is how far east this
activity will last.

Conditions remain marginally supportive for a few strong to severe
thunderstorms, mainly along and south of the North Platte River
where instability is most potent at this time. We have MLCAPE around
500 to 1000 J/kg and 0 to 6KM shear around 30 knots. Lapse rates are
fairly paltry, as is typical in a very moist environment like today.
So, severe weather really isn`t the primary hazard today strong
outflow winds and small to marginally severe hail will be possible.
The main focus remains on torrential rainfall and localized flash
flooding. Storm motions are fairly slow today, and the strong
forcing will also support potential for training thunderstorms,
allowing multiple rounds of heavy rainfall in the same area. We are
already seeing some heavy showers anchoring to the Laramie Range
early this afternoon. The existing Flood Watch area remains the area
with the best instability, but we will also need to keep an eye on
Converse, Platte, Goshen, and possibly portions of Albany county as
high resolution models remain fairly aggressive on rainfall totals
in this area. The weaker instability in those areas reduces
confidence. As storms move east, they will move into weaker
instability. PoPs were trended downward in the northern panhandle as
high-resolution models are in better agreement showing storms
dissipating before reaching there, but the southern panhandle still
looks like a better bet for heavy rainfall. Showers and
thunderstorms will continue all through the evening, and possibly
well into the overnight hours. Isentropic lift will continue all the
way until the axis of the aforementioned primary upper level
shortwave passes through the area around daybreak Friday. While
confidence was too low to make any changes to the Flood Watch now,
we may have to consider extending to daybreak, at least in the
Nebraska panhandle.

Low clouds and fog will be around most of the High Plains Friday
morning, but expect winds to turn westerly from west to east during
the morning hours and clear things out more quickly than today. We
should get a bit more sunshine Friday morning, allowing temperatures
to climb close to seasonal averages, several degrees warmer than
today. Westerly flow will spread across the area, but this will not
actually be that much drier. Precipitable water remains around the
90th percentile of climatology tomorrow despite a "drier" airmass
moving in. Overall storm coverage should be lower tomorrow with
weaker forcing expected. The area favored for storms also looks more
localized, mainly along the I-80 corridor from Laramie to Sidney,
and possibly extending north by 50 to 100 miles or so. While forcing
is unimpressive elsewhere, models show a small region of positive
theta-e advection creeping in from the south, which could help
initiation, and the I-80 corridor looks to have the best instability
of the forecast area. So overall, expect lower coverage but similar
hazards to today. Parameters remain marginally supportive for a few
strong to severe storms, and storms will once again be capable of
producing torrential rainfall and localized flash flooding.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 1150 AM MDT Thu Aug 13 2026

Saturday...Moist westerly flow aloft and a weak shortwave trough
aloft near peak heating will combine with deep atmospheric moisture
supply to produce scattered to numerous showers and scattered
thunderstorms in the afternoon and evening, with the greatest areal
coverage east of Interstate 25. Aiding in the lift will be low level
southeast winds adding the mechanical lift in addition to diurnal
heating. PWAT, Precipitable water values look decent for beneficial
rainfall. 700 mb temperatures yield highs in the 70s to lower 80s.

Sunday...Ridging becomes a bit more pronounced over Wyoming,
producing warmer temperatures aloft which will tend to limit areal
coverage of showers and thunderstorms, compared to Saturday.
Nevertheless though, the monsoonal moisture will stretch from
central Colorado through far eastern Wyoming and western Nebraska,
where scattered to numerous showers and thunderstorms will occur in
the afternoon and evening. Slightly warmer, but comfortable,
temperatures for this time in August.

Monday...A drier pattern ensues and is on tap as the flow aloft
turns west northwest and the precipitable water values decrease, due
to drying at low and mid levels and downslope southwest winds near
the surface. Thus, a much drier day when compared to the weekend. A
warming trend begins with more sunshine and warmer mid level
temperatures.

Tuesday...Brisk westerly flow aloft prevails, and with a decent
amount of low and mid level moisture availability, expect isolated
to widely scattered late day showers and thunderstorms, mainly east
of Interstate 25 and the Laramie Range. Similar maximum temperatures
to Monday based on 700 mb temperatures.

Wednesday...The synoptic pattern looks more like September and early
autumn with a deep shortwave trough aloft sweeping across western
Montana and Idaho, with west southwest flow aloft over our counties.
Right now, we have scattered late day showers and thunderstorms in
our gridded forecasts, though with a decent increase in precipitable
water values, the moist flow aloft, and low level upslope, along
with pattern recognition, believe our POPS and convective coverage
will likely need to be increased in the next few days.

Thursday...The high amplitude and progressive shortwave trough aloft
moves eastward from western Montana, to the Dakotas and western
Nebraska by evening, sending a decent Pacific origin cold front
across our counties. There may be enough moisture at low and mid
levels for isolated to widely scattered convective coverage, though
the biggest change will likely be a return to cooler temperatures
and windy downslope west winds, which will likely need to be
adjusted in the next few days, if model consistency remains for day
7. If you are looking for an early taste of fall weather, this may
be your day.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1146 AM MDT Thu Aug 13 2026

Low clouds, fog, and numerous showers and thunderstorms will
create several aviation hazards today. Expect two main rounds of
showers and thunderstorms today. The first has already begun and
will continue through the early evening. CYS has the highest
probability for impacts, followed by LAR and SNY with this
round. Could see a few making it to BFF and RWL, but confidence
is lower there. The primary hazard will be very heavy rainfall
producing IFR to LIFR VIS, but there is also the possibility for
frequent lightning, gusty winds, and small hail. Most likely,
this round will struggle in the central and northern panhandle,
so the forecast backed off for IFR conditions in BFF, AIA, and
CDR.

A second round of showers and thunderstorms will kick up around
sundown and move east through the overnight period. This round
is more likely to make it into the Nebraska panhandle, so PROB30
groups were maintained for this potential. The hazards will be
the same as the first round.

Low clouds and fog will still be a concern throughout the
period, but overall confidence is very low as models have had a
poor handle on this so far today. Be alert for possible AMDs
through the afternoon and evening as confidence increases in
timing and location of storms, low clouds, and fog. Expect
conditions to trend towards VFR between 14z and 17z Friday
morning as drier air moves into the area and winds turn more
westerly.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Flood Watch until 10 PM MDT this evening for WYZ117>119.
NE...Flood Watch until 10 PM MDT this evening for NEZ003-019>021-
     054-055.

&&

$$

SHORT TERM...MN
LONG TERM...RUBIN
AVIATION...MN