004
FXUS65 KCYS 191121
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
521 AM MDT Sat Sep 19 2026

.KEY MESSAGES...

- Active with scattered to numerous showers and thunderstorms
  into the weekend. Strong to marginally severe thunderstorms
  may return to the area as a potent Pacific storm system moves
  into Wyoming Saturday and Saturday night.

- Much cooler temperatures expected to return on Sunday with cloudy
  skies and wetting rainfall expected for most locations.

- Active weather with cooler temperatures expected for much of
  the upcoming week.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 225 AM MDT Sat Sep 19 2026

Residual showers continue to slowly move eastward out of the CWA
early this morning. Cloud cover remains across the region, keeping
overnight temperatures fairly mild, in the 50s and 60s for most
locations. The upper-level system responsible for the precipitation
today remains positioned over Idaho, with stout, southeasterly flow
across the CWA. This system will slowly slide easterly this morning
into the afternoon hours, keeping synoptic forcing in place with the
dynamic rising motion downstream of the trough axis. Ample 500mb
vorticity maxima will eject out ahead of the incoming trough,
further supporting ongoing forcing. 700mb flow remains benign today,
with weak, southeasterly flow slowly becoming westerly into the
overnight hours. As the upper-level system pushes closer, cold air
advection will develop at 700mb with temperatures cooling from
around 7 to 9C down to 3 to 6C by early Sunday morning. Weak ridging
starts to develop across Montana, northward into Canada, leading to
a 700mb high developing and a surface high pressure system
associated with it. The high pressure over Canada will strengthen
throughout the day Saturday, turning surface flow easterly to
slightly northeasterly as far south as Colorado and as far west as
the Laramie Range. As a result, the surface flow will be
perpendicular to the incoming cold front associated with the
synoptic system, as well as upslope along the Laramie Range and
Cheyenne Ridge. The collision of moist, easterly, upslope flow and
the incoming cold front from the west will result in widespread
precipitation across the region as constant, moist, rising motion
persists through the day. With precipitable water values between
0.75 and 1.00 inches, the 90+th percentile for this time of year,
widespread, wetting rainfall is anticipated this afternoon. The
Weather Prediction Center highlights much of the CWA east of the
Laramie Range in a Marginal Risk for Excessive Rainfall this
afternoon. Precipitation looks to continue into the evening and
overnight hours, but surface flow starts to turn westerly along and
just east of the Laramie Range late tonight, resulting in drier flow
and decreasing precipitation across southeast Wyoming.

The biggest uncertainty for today will be the severe weather
potential will the potent collision between easterly flow from the
high pressure system to the east and the incoming cold front from
the west. The Storm Prediction Center has highlighted most areas
along and east of the Laramie Range in a Marginal Risk (Level 1 of
5) of severe weather this afternoon, with the primary hazard being
large hail and damaging winds. With this collision, a storm or two
could become severe, especially with modest CAPE across the region.
However, cloud cover will likely continue through much of the day
due to the moist atmosphere and continued ascent from the incoming
trough. These clouds look to limit daytime warming across the
region, with most locations in the upper-60s to mid-70s. That being
said, forecast sounding from the RAP suggest a convective
temperature around 76F across the western Panhandle. Many locations
may struggle to reach that temperature today, ultimately limiting
convective potential and decreasing the severe weather threat. This
will be especially true if fog is able to develop across portions of
the region this morning, as dewpoint depressions are starting to
approach 0 as of 08Z. The dynamic setup today may be able to
initiate a few stronger storms with the collision of boundaries, but
widespread severe weather will be very hard to get, instead only an
isolated severe storm or two. If a storm can get going this
afternoon, the overall atmospheric profile is favorable for large
hail and damaging winds, with the majority of the CAPE in the hail
growth zone and boundary collisions potentially leading to severe
gusts. Despite the severe potential being highly conditional,
widespread wetting rainfall over 0.10in are expected this afternoon
with multiple rounds of showers and storms throughout the morning,
afternoon, evening, and overnight hours, likely continuing into the
early morning hours Sunday.

Moving into Sunday, the upper-level trough will still be slowly
progressing eastward across central Wyoming, continuing to support
synoptic ascent from the dynamic lifting mechanisms downstream of
the trough axis. This trough will slowly, mosey along and finally
move out of the CWA late Monday night into very early Tuesday
morning. Northwesterly flow behind the trough will continue to
funnel cooler temperatures into the region. 500mb flow will remain
active with multiple vorticity maxima traversing across the region
with the slow-moving, upper-level trough. This will further support
some ascent across the region, favoring continued synoptic forcing
for ascent throughout the day Sunday. The diffuse 700mb low will
continue to circulate overhead before slowly meandering
northeastward and strengthening briefly Sunday evening before
weakening once more by the early morning hours Monday.The brief
intensification between 00Z and 06Z Monday will result in a brief
period of gusty winds across the region, though well below high wind
warning criteria. 700mb temperatures will also bottom out across the
region Sunday afternoon, dropping to 1 to 4C for most areas. With
continued synoptic support across the region and a weak low
attempting to develop across southern Colorado, much of the region
will see another round of widespread showers and storms. With
precipitable water values once again around 0.75 to 1.00in, the
90+th percentile, another round of wetting rain is expected. WPC
once again highlights the region east of the Laramie Range in a
Marginal Risk of Excessive Rainfall for Sunday. Sunday will also be
much cooler, with temperatures in the 60s everywhere with mostly
cloudy skies.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 225 AM MDT Sat Sep 19 2026

No major changes. Please see previous Long Term discussion...

Forecast for next week looks to remain largely the same from day to
day under weak pressure gradients aloft with a few chances for some
scattered precipitation, but otherwise a cool start to the weak with
a slow and steady warming trend through the remainder of. Upper
level pattern will consist of weak ridging trying to build in on
Monday, but this will be overtaken by a weak low lifting up from
California and slowly progressing across the region through the end
of the forecast period. At the surface, Monday will see our forecast
region in a post-frontal environment, with a weak boundary then
sitting across the WY/NE border area and then lifting east-northeast
as a warm front into the later days of the week. Under this pattern
Monday will be the coolest day of the forecast period as widespread
highs generally sit in the 60`s to right around 70 in the warmest
spots. Increasing warmth then expected as warmer air from the
southwest moves across the region, but don`t expect a crazy warmup
as we increase only 1-3 degrees each day, keeping the region mostly
in the 70`s through the week until Thursday when a few more 80`s may
finally begin popping up. A brief blip of moisture may produce
increased chances of showers and storms on Tuesday as the California
low lifts up, but without a stout forcing mechanism don`t expect
much outside of general showers and thunderstorms and coverage
isolated to scattered at best. Otherwise, a few isolated showers or
storms possible each day as well with nothing impactful expected.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 518 AM MDT Sat Sep 19 2026

A complex forecast this morning as low clouds and fog has moved
into KBFF and KAIA with little to no model consensus on the time
of the fog dissipating. Mid-level clouds will continue this
morning after fog and low clouds dissipate, with a couple rounds
of showers and thunderstorms expected throughout the day today.
Timed the rounds of showers and storms, with some uncertainty in
initial development near KLAR and KCYS. IFR conditions may
return late in the TAF period, depending on how much
precipitation fall this afternoon.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.

&&

$$

SHORT TERM...AM
LONG TERM...AM/CG
AVIATION...AM