157
FXUS65 KCYS 290500
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1100 PM MDT Mon Sep 28 2026

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms will continue
  through Tuesday night. Brief locally heavy rainfall is
  possible.

- A more organized rain producer may graze the southern Nebraska
  panhandle on Wednesday, but confidence remains low in the
  exact track of the storm system.

- A cooler temperatures continue through Thursday, but warmer
  and drier weather will prevail Friday through early next week.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 245 PM MDT Mon Sep 28 2026

GOES water vapor imagery shows abundant moisture streaming in aloft
today. The approaching Pacific trough has split, with the
northern branch traversing to our north across the northern
Rockies while the southern branch is closing off over
California. Southwest flow ahead of the approaching closed low
is responsible for the influx of moisture, pushing PW values up
to between 0.7 and 1.0 inch in Wyoming, and 1 to 1.2" in
Nebraska, or around the 99th percentile of climatology. To add
to this, Hurricane Polo will make landfall in Baja California
shortly, and then contribute additional moisture to the storm
system sometime tomorrow. While we certainly have no shortage of
moisture, we are somewhat lacking in a method to get the
moisture to the ground. Thick cloud cover has inhibited the
development of much instability across the area, though a capped
500 to 1000 J/kg MLCAPE exists in the Nebraska panhandle.
Forcing has also been quite weak so far, limiting activity
through the event to mostly widely scattered light showers
across the area. Stronger forcing is expected to surge northward
into Wyoming and Nebraska shortly, which should enhance
coverage of shower activity and locally elevate rainfall rates
through the afternoon and evening hours. Once showers reach the
better instability in western Nebraska, we should see a few
isolated to scattered thunderstorms develop, which will lead to
locally heavy rainfall. Currently, wind shear is quite weak
where the instability is present, but shear will increase later
this afternoon into the evening as a jet streak aloft moves
eastward. As a result, an isolated strong to severe storm cannot
be ruled out. Most activity will wind down after about 9PM, but
expect scattered showers and possibly a few isolated storms to
continue all through the night.

Tuesday will remain cloudy for the southern portion of the area,
mainly along the I-80 corridor, but expect some sunshine to break
through as clouds decrease north of a Rawlins to Alliance line.
Temperatures will be a touch cooler, generally near seasonal
averages. While moisture aloft will remain abundant, our area will
be underneath the deformation zone in the middle of the split
trough. This means that we will have generally light and highly
disorganized winds aloft, and relatively little forcing for ascent.
Thus, expect shower coverage to decrease through much of the day
Tuesday, but isolated showers will continue. CAPE will be sufficient
to support a few rumbles of thunder as well, but expect this to be
pretty limited.

The end phase of this storm system is unfortunately, still
uncertain. Models have diverged into two distinct camps as the
closed low over the southwest absorbs the remnants of Hurricane Polo
and ejects eastward into the Plains. Most GEFS members are clustered
into a solution in which the northern branch shortwave well to our
north is a little stronger, and the closed low to the south is more
progressive and weaker. Nearly all GEFS members have the surface low
well into Kansas by mid-afternoon on Wednesday. Conversely, the
ECWMF ensemble stubbornly continues to show a slower, stronger
closed low that drifts further north as it ejects to the east. ECMWF
ensemble surface low locations are generally still in southeast
Colorado by mid-afternoon Wednesday. In this scenario, the
precipitation shield forced by the TROWAL on the northwest side of
the departing system would be far enough north to reach the
southeast portion of our area, delivering rain showers to areas
along and south of a Cheyenne to Alliance line. The GEFS scenario
would be largely dry for most of the area. The ECMWF ensemble median
precipitation total for Sidney is around 0.75" of rain, whereas the
GEFS median is only 0.1". This certainly does not help with forecast
confidence, especially considering we are only about 36 hours out
from this right now. Hi-res models are not helping either, as
individual models are also somewhat evenly split into each of these
camps. The official forecast straddles the middle between these two
scenarios, with Wednesday PoPs around 50% for Sidney and 20 to 30%
for Cheyenne, Scottsbluff, and Alliance. Regardless of how much
precipitation (if any) we get, we should see mostly cloudy skies
with breezy north to northeast winds Wednesday, followed by a
departure and gradual clearing into Thursday morning.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 245 PM MDT Mon Sep 28 2026

Thursday...In the wake of the passing trough aloft, dry northwest
flow aloft will develop, thus no showers or thunderstorms expected.
Somewhat cooler based on 700 mb temperature changes.

Friday...West northwest flow aloft strengthens, helping low level
gradients to increase enough to produce breezy to windy conditions.
Low and mid levels look too dry to support shower and thunderstorm
development.

Saturday...A fast moving positively tilted shortwave trough aloft
moves across Nebraska, sending a dry cold front across our counties.
Continued dry.

Sunday...The ridge aloft to our south over Colorado and New Mexico
will produce enough convective inhibition to make shower and
thunderstorm development quite unlikely for our forecast area.

Monday...Non-descript northwest flow aloft continues. There may be
enough low and middle level moisture to produce isolated to widely
scattered thunderstorms in the afternoon and evening, but
pinpointing their location will be iffy at best. Another warming
trend develops as 700 mb temperatures rise once again.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1100 PM MDT Mon Sep 28 2026

VFR conditions will prevail across the terminals for the
duration of this TAF period. A few exceptions are KSNY and KAIA
where BR may form and obscure VIS down to 4SM. Otherwise no
other aviation concerns for the overnight hours as winds will
remain light and variable. Additional moisture will push in from
the south, increasing chances of precipitation for KCYS, KAIA,
and KSNY. At this time, confidence is low on the placement and
timing, so kept any mention out of the TAFs for this forecast
period.

&&

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

&&

$$

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