433
FXUS65 KCYS 271153
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
550 AM MDT Sun Sep 27 2026

.KEY MESSAGES...

- Remaining warm Sunday with temperatures around 5 to 10 degrees
  above normal. There is a chance for showers and thunderstorms
  on Sunday as the next Pacific storm system moves onshore.

- A strong Pacific storm system will bring scattered showers and
  storms along with seasonably cool temperatures Monday through
  Wednesday. Slow warming trend expected later in the week.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 220 AM MDT Sun Sep 27 2026

Rather mild early this morning despite the dissipating cloud
cover, with surface readings in the 50s to low 60s across most
locations. Low level moisture is also higher than previously
expected with dewpoints already in the 50s across portions of
the western Nebraska panhandle. This should play a role in
thunderstorm development today.

All models in pretty good agreement and show the main Pacific
storm system moving onshore later today. This storm will quite a
bit of energy, and some of that energy will transition eastward
into the eastern Great Basin region and the Front Range as
a potent upper level disturbance/vort max quickly lifts
northeast across the area. Models show this disturbance entering
the area late in the afternoon and through the evening hours, so
convection may be delayed until this evening, or possibly late
this evening. SPC has a Marginal Risk of strong to severe
thunderstorms for today, but this appears to be a very
conditional risk due to the timing of the disturbance and the
time of the year. Model soundings are favorable east of the
Interstate 25 corridor, with most locations near 800 to 1400
j/kg of MLCAPE and SBCAPE. 0-6km shear is marginal, but enough
to consider some updraft rotation and tilting. However,
soundings do show some decent CIN through most of the afternoon.
Will continue to monitor the latest high res CAMs, but the HRRR,
NAMNEST, and the RRFS are not showing anything other than some
scattered rain showers over southeast Wyoming and western
Nebraska through 900 pm. Another wildcard is the Low Level Jet,
which is expected to peak around midnight across the high
plains with forecast 700mb winds around 40 knots. There is
concern that the upper level disturbance, if it continues to
trend later in the day, will interact with the Low Level Jet,
producing rare late-September nocturnal convection (likely
strong). Again, high res CAMs aren`t really showing this, but
the previous 00z HRRR was hinting towards it across the central
Panhandle. Will continue to keep an eye on this and brief the
next shift, but unsure how aggressively to message this
potential at this time. Kept POP between 20 to 40 percent across
the area for mainly widely scattered to scattered coverage,
with east central Wyoming and the northern Panhandle the most
favorable area to receive some decent rainfall. Can`t rule out a
few stronger storms across the eastern plains...with the first
chance this evening, and another chance later this evening once
the low level jet increases.

For Monday, synoptic models continue to show good run-to-run
consistency with the overall pattern, and more specifically, the
Pacific storm system moving onshore across central California
and then digging southeast into the desert southwest. Although
PWATs are forecast to be between the 95th and the 99th
percentile (0.75 to 1.25 inches) over the forecast area, there
is little to no instability present due to ample cloud cover and
cool-ish temperatures. The midlevels will still be relatively
warm with poor lapse rates. The only notable forcing is from the
100kt jet max moving into Wyoming, Colorado and the Dakotas,
with moderate to strong upper level diffluence present. Synoptic
models continue to show limited precipitation. However, to
complicate things: the last few runs of high res guidance are
much more aggressive, showing large bands of moderate to locally
heavy rainfall quickly lifting northeast across the area...more
or less colocated with the best jet dynamics. This is where a
bulk of the forecast uncertainty is present. High res CAMS also
show some strong thunderstorm activity embedded in the large
areas of moderate rainfall. Kept POP as is and increased them a
bit across western Nebraska and the mountains of southeast
Wyoming. A few of the higher peaks over 10,500 feet may see
their first snowfall of the season. There is still some concern
that this system may produce some excessive rainfall, but
confidence is not very high. Daytime temperatures will lower
into the 60s and low 70s. May need to adjust these temperatures
even lower with the lack of sunshine and possibly some low
clouds and fog for a good portion of the day.

&&

.LONG TERM /LATE MONDAY THROUGH SATURDAY/...
Issued at 115 PM MDT Sat Sep 26 2026

Unsettled weather will continue into the early part of the week
ahead, but it is widespread beneficial rainfall is looking less
likely. A split trough approaches the area on Monday, with the
northern split passing to our north, and the southern split will be
abandoned over the southwest to eventually absorb abundant tropical
moisture associated with the remnants of Hurricane Polo. While
precipitable water will be well above average Monday through early
Wednesday, pushing around the 90th to 99th percentile of
climatology, forcing for ascent looks weak at best, and instability
remains unimpressive. The official forecast maintains modest PoPs
(mostly 20 to 50%) through Monday and into Tuesday morning, as we
can expect to see at least a few showers and storms around, but odds
of significant rainfall are slim. Probabilities for total
precipitation (through Tuesday) over 0.1" are lowest between I-25
and the WY/NE state-line, running around 20 to 50% here. Western
Nebraska and the higher terrain of southern Wyoming both have higher
odds, between about 50 and 80%. That being said, expect rainfall to
be spotty, though the abundant atmospheric moisture will support the
potential for highly localized heavy rainfall. A cool front moving
through the area will knock down temperatures for Monday, with
Tuesday looking like the coolest day.

For the middle of the week, the most likely scenario puts our area
just outside of the region of precipitation associated with a soggy
closed upper level low to our south. Ensembles are still all over
the place in terms of the location of this low pressure system as it
ejects eastward into the plains Wednesday into Thursday, but only
the far northern end of the envelope of solutions (10 to 20%
probability) would bring any impact to our area. The vast majority
of ensemble members keep all rainfall with this system to our south,
but it is still worth a mention as a shift north would lead to a
vastly different outcome than currently shown in the official
forecast for Wednesday and Thursday. Expect temperatures to warm up
through the latter half of the week, with highs peaking above
seasonal averages on Friday.

Another low-confidence storm system is showing up in a handful of
ensemble members around next weekend. This will be another Pacific
trough moving from the northwest. In about 20% of ensembles, this
system is strong enough to knock temperatures enough to support a
widespread freeze Sunday morning along with a quick shot of rain and
snow showers to the area. However, the other ~80% have light
showers and a modest cool down, with overall minimal impacts.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 550 AM MDT Sun Sep 27 2026

Strong Pacific storm system will begin to push onshore and gradually
move into the Intermountain West over the next 24 to 36 hours.
Expect increasing cloudiness over southeast Wyoming and Western
Nebraska with a better chance of showers and a few thunderstorms.

HAZARDS/WEATHER TRENDS: VFR conditions will prevail at most
terminals. Low coverage and uncertain timing of thunderstorms for
today, and into late this evening, so will not add to most TAFs at
this time. Short range ensembles are also showing a larger area of
lower clouds over the Nebraska Panhandle, starting up north at first
and then eventually building southward. This is kind of a surprise
as the low level jet is expected to increase this evening through
tonight, and IFR CIGS are typically unlikely in these cases. For
now, added low-end MVFR CIGS to KCDR, but clouds to lift once the
low level jet strengthens after 03z Monday. These lower clouds may
sneak into KAIA and KSNY later on tonight.

&&

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

&&

$$

SHORT TERM...TJT
LONG TERM...MN
AVIATION...TJT