702
FXUS65 KCYS 291713
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1113 AM MDT Tue Sep 29 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 242 AM MDT Tue Sep 29 2026

Upper-level troughing will continue to dominate the region
throughout much of the upcoming week as multiple systems push across
the CONUS. Today, the upper-level trough will finally pinch off a
closed low across the southwestern CONUS by the afternoon hours,
leading to a very strong jet from the Pacific Ocean northeastward
through portions of the Midwest. Southwesterly flow aloft will
slowly move around to due southerly by early Wednesday morning as
the low continues to circulate across the Four Corners region. A
large swath of 500mb vorticity will develop across the Intermountain
West during the day today with the turbulent upper-levels. This
vorticity swath promotes modest 500mb cyclonic vorticity advection
across all of the CWA from northwest to southeast this afternoon,
leading to modest synoptic forcing. Despite the turbulent upper-
levels, 700mb winds will be weaker throughout the day, switching
from southwest to northeast by early Wednesday morning. The 700mb
shortwave will weaker as the upper-level trough pinches off the
closed low, favoring the relatively quite 700mb flow. Weak to modest
700mb warm air advection is expected as temperatures warm from the 3
to 5C range this morning into the 6 to 8C this afternoon. Overall,
precipitation chances will be lower this afternoon compared to
previous days due to the erratic atmosphere. The best chances for
precipitation will be confined to the southern portions of the CWA
with PoPs around 15 to 25 percent, decreasing into the evening hours
as low-level winds flip around to the north and the best forcing
moves out of the region. Temperatures will remain cool again today
with overcast skies expected again. Surface temperatures will only
warm into the low-60s to mid-70s this afternoon, with the mountain
remaining around 40 to 50F. The overcast skies will also prevent any
significant destabilization of the atmosphere this afternoon and
will keep any thunderstorms at bay.

The turbulent atmosphere will continue for Wednesday as the closed
low over the Four Corners regions starts to absorb Hurricane Polo as
it ejects eastward into the Southern Plains. As the upper-level low
pushes eastward, much of the CWA will be under little to no
significant steering flow at 250mb as it remains in the quiet region
due north of the progressing low. This low is currently anticipated
to continue its eastward progression and become more narrow (west to
east). As a result, the 500mb vorticity maxima look to just to the
southeast of the CWA, potentially just passing overhead in Cheyenne
County Wednesday afternoon. As the low starts to eject northward
early Thursday morning, it is currently progged to remain just off
the southeast of the CWA. The 700mb flow will be a touch more
exciting early Wednesday morning throughout the early afternoon
hours as two 700mb lows attempt to develop before the northeastern
low of western Kansas becomes to dominant low and absorbs the other.
The dominant low will intensify quickly Wednesday afternoon, but
struggles to eject very far to the northeast. This low will still be
over western Kansas by Wednesday evening into the overnight hours.
Easterly to northeasterly 700mb flow is expected across the CWA,
favoring the potential for upslope development along and east of the
Laramie Range. 700mb temperatures will remain quite warm in the 5 to
7C range through the afternoon hours before cooling into the 3 to 5C
range across southeastern portions of the CWA as the 700mb low
advances. If 700mb temperatures were a few degrees cooler, below 0C,
this could be quite the winter event with precipitable water above
the 90th percentile thanks to additional tropical moisture. However,
warm temperatures win and precipitation is expected to only be rain
and thunderstorms. The bigger question will be how far north and
west the precipitation makes it. As of now, the currently projected
path of this system would keep the precipitation confined to a line
from Cheyenne to Alliance and points southeast. If the track were to
shift more northwestward, precipitation chances would extend
northwest past the Cheyenne to Alliance line. Despite this, breezy
north to northeast winds are expected Wednesday as the surface low
increases pressure gradients across the region along with increased
700mb height gradients. Cloudy skies will persist Wednesday before
slowly clearing as Thursday morning approaches.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 242 AM MDT Tue Sep 29 2026

Much quieter weather returns for the second half of the week as
250mb flow turns northwesterly to westerly behind the departing
trough. Northwesterly flow on Thursday will keep temperatures cool
across the CWA with highs currently forecast to be in the 60s
everywhere. Little to not precipitation is anticipated on Thursday
as subsident flow remains in place. A warming trend will start
Friday and continue through the weekend as a ridge attempts to build
across the western CONUS. Temperatures will warm into the upper-60s
to mid-70s with no precipitation expected as the moisture plume from
the Pacific Ocean remains cutoff. Upper-level ridging will continue
to build into the weekend as a trough digs southward just off the
coast of California. This system could be the next precipitation
producing to impact the region, but for the remainder of the long
term forecast period it remains well off to the west. Temperatures
on Saturday will warm into the upper-60s to mid-70s with
temperatures on Sunday following suit, in the upper-60s to mid-70s.
Temperatures start to warm more Monday onwards as an upper-level
ridge builds ahead of the incoming trough off the coast of
California. Highs on Monday are currently forecast to be in the mid-
70s to low-80s, with slightly warmer temperatures possible on
Tuesday, depending on the system`s track.

The system off the coast of California could start to impact the
region as early as Monday morning, with long range models currently
suggesting a brisk eastward movement through the weekend. The GFS
and ECMWF are in fairly good agreement on this system with the 00Z
runs, though the GFS positions the trough slightly further north
than the ECMWF. Models disagreement starts to increase into Tuesday
morning, with the GFS suggesting a more progressive trough while the
ECMWF slows the trough down and keeps it much further west. Long
range models suggest precipitation chances starting to return to
start the work week, with low-end PoPs over western portions of the
CWA as soon as Monday afternoon.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1106 AM MDT Tue Sep 29 2026

VFR conditions will dominate at all terminals this afternoon.
Expect generally light and variable winds less than 8kt with sky
conditions generally remaining clear to mostly sunny. Expect a
mostly clear overnight at all terminals with light winds
continuing. On Wednesday morning, expect lower ceilings to move
in at SNY after sunrise with drops into MVFR and perhaps IFR
category by 18z. Isolated showers will also affect SNY as early
as 16z on Wednesday.

&&

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

&&

$$

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