820
FXUS65 KCYS 201115
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
515 AM MDT Sun Sep 20 2026

.KEY MESSAGES...

- Much cooler temperatures expected on Sunday and Monday behind
  a strong cold front.

- Strong winds will be possible Tuesday night into Wednesday
  morning across the western Nebraska Panhandle with a stout
  low-level jet expected to develop.

- Daily precipitation chances continue for much of the week with
  ample monsoonal moisture in place.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 214 AM MDT Sun Sep 20 2026

Upper-level troughing will continue across the region today with
cooler air filtering in as it continues to slowly move off to the
east. Dynamic forcing for ascent will still be present today across
the region, due to dynamic lifting mechanisms downstream of the
trough axis. The trough should be east of the region by early Monday
morning, as recent model runs have sped up the propagation of the
system. 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.
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 09Z Monday will
result in a brief period of gusty winds across the region, though
well below high wind warning criteria. 700mb temperatures will
bottom out this afternoon with most areas dropping into the 1 to 4C
range. These cooler 700mb temperatures will correlate to surface
temperatures between about 60 and 72F everywhere. Easterly flow this
afternoon will further promote precipitation development, due to the
moist, upslope flow. In combination with continued synoptic support
and the weak low attempting to develop in southern Colorado, much of
the region is expected to see widespread showers and storms and
mostly cloudy skies. Precipitable Water values remain around 0.50 to
1.00 inches, placing portions of the region in the 90th percentile
per the NAEFS Mean Precipitable Water. Decent rainfall accumulation
will be possible today, with shower and storm chances decreasing
after midnight tonight.

On Monday, models are trying to come to a consensus on whether or
not a weak ridge develops on the backside of the trough. While weak,
this ridge would help temperatures warm up quicker for Monday than
would otherwise be expected on the backside of a strong cold front.
Additionally, precipitation chances would be more limited Monday.
Temperatures on Monday are currently forecast to be in the mid-60s
to low-70s. Limited precipitation is expected, but much of the
surface flow across the CONUS will be influenced by a strong,
Canadian high pressure system north of Minnesota. This high will
turn winds across the CWA to the southeast, favoring the potential
for some additional upslope development throughout the day Monday.
Confidence in this is low at this time, though certainly possible.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 214 AM MDT Sun Sep 20 2026

Tuesday onwards will see much of the western CONUS back under the
influence of an upper-level ridge, with southwesterly upper-level
flow expected across the CWA. This southwesterly flow will promote a
fetch of Pacific moisture continuing to advect into the region
through at least Thursday. As a result, despite the upper-level
ridge in place, daily precipitation chances will continue,
especially with multiple lobes of 500mb vorticity advecting across
the region each afternoon. After a cooler day Tuesday with highs in
the low- to mid-70s, temperatures will climb back into the 70s and
80s across the region under the influence of upper-level ridging.
Saturday is currently forecast to be the warmest day this week, but
this may change over the next 6 days. Other than daily precipitation
chances, strong winds will be possible over the Panhandle Tuesday
night into Wednesday morning as a stout low-level jet develops
across the region. The GFS currently suggests a 45 to 50kt low-level
jet across the Panhandle late Tuesday night into early Wednesday
morning, favoring gusty winds across the Panhandle. GFS Omega field
remain weak, so significant downward acceleration leading to high
winds is currently not anticipated. However, this setup will need to
be monitored in the event this jet strengthens or downward Omega
values start to increase.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 512 AM MDT Sun Sep 20 2026

Isolated showers and low clouds are ongoing across portions of
the area this morning as the upper-level low circulates
overhead. The Nebraska Panhandle should remain socked in with
lower-level clouds for much of the day with increased moisture
in the area. On and off showers are expected at most locations
through the morning hours, with chances for thunderstorms in
the afternoon, mainly across western Nebraska. Winds pick up
this afternoon as the system continues to slide off to the east.

&&

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

&&

$$

SHORT TERM...AM
LONG TERM...AM
AVIATION...AM