107
FXUS65 KCYS 131138
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
538 AM MDT Sun Sep 13 2026

.KEY MESSAGES...

- Temperatures warm on Sunday and Monday with increasing
  precipitation chances.

- Severe weather chances return on Sunday, with Monday seeing a
  potential critical fire weather environment alongside high
  winds.

- Much cooler on Tuesday with temperatures a few degrees warmer
  through the end of next week and daily chances of showers and
  thunderstorms.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 250 AM MDT Sun Sep 13 2026

The pattern through Monday will consist of an upper level
transient ridge today that will give way to a Pacific low
moving across the US/Canada border with attending troughing
overspreading our region through Monday before departing. At the
surface, a stalled frontal boundary will lift as a warm front
today and move out of the region, with a secondary cold front
then dropping down through Monday. This should create an
eventful end to the weekend and an accompanying busy day to
start the work week as well. For Sunday, scattered shower and
thunderstorm development from surging moisture and the forcing
provided by the incoming system and frontal boundary is
expected. While the last several days model runs haven`t been as
excited for severe development, latest guidance is starting to
pick back up on the potential for scattered severe thunderstorm
development. Initially steep lapse rates with increasing
instability and an evening low level jet fueling wind shear
should allow for the potential for severe thunderstorms,
however convective inhibition may still prevent stronger storms.
Nonetheless high resolution guidance shows some destabilization
during the late afternoon into the evening, and a few storms
capable of damaging winds and large hail could be possible.
Otherwise it`ll be a warm day, with daytime highs in the upper
70`s to upper 80`s.

Moving into Monday, focus shifts off of showers and
thunderstorms and back to fire weather conditions and high
winds. Winds at 700mb are around 40-50 knots during the day due
to the passing system, and Craig to Casper pressure gradients of
50+ meters are expected to produce widespread wind gusts of
40-50 mph, with our usual wind prones potential reaching 55-60
mph for the day. While in house guidance remains generally
unconvinced with the probability of 58+ mph gusts at only around
40%, seeing enough potential in high resolution guidance that a
High Wind Watch was issued for Rawlins, Arlington, and Bordeaux
zones (the usual suspects). And as drier surface air moves into
the region and daytime relative humidities drop to the teens,
critical fire weather conditions will return as well for a good
portion of southeast Wyoming. Fire Weather Watches have also
been issued for Monday in response to this. A few showers or
storms will also be possible on the day, but these should be
mostly relegated to the I-80 corridor and southwards, with
minimal to nil stronger activity anticipated. Prior to the front
look for similar temperatures to Sunday with 70`s to 80`s
expected, but rapidly cooling thereafter leading to a much
cooler Tuesday.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 156 PM MDT Sat Sep 12 2026

Active weather is expected for much of the upcoming week with
multiple upper-level disturbances pushing through the region. Monday
features the strongest disturbance expected for the week as a stout
upper-level trough digs across Montana with a 100kt jet along the
southeastern portion. The jet will move eastward with the upper-
level low, placing the CWA within the right entrance region of the
upper-level jet for much of the day Monday. Enhanced, dynamically-
driven ascent will provide much needed forcing for showers and
thunderstorms Monday morning and afternoon. Multiple 500mb vorticity
maxima will eject across the area as the upper-level low moves
eastward, favoring addition forcing across much of the region. The
700mb low is expected to be displaced to the north across the
southern Saskatchewan Province. While this low will be displaced to
the north, it looks to be quite strong, with an associated jet
around 50 to 60kts over the Northern Plains. For Southeast Wyoming
and Western Nebraska, the low will lead to strong, westerly flow
across the CWA around 40 to 45kts Monday morning. If you have lived
in this area for a while now, you may know where this is heading...
The surface low with this system will dive southeastward from the
Black Hills towards central Nebraska, with a strong, trailing cold
front. This low progression is favorable for strong, west to east
surface pressure gradients to develop across western portions of the
area and across the Laramie Range. The surface pressure gradient
across the Laramie Range peaks around 2 to 3mb Monday morning. GFS
Omega field suggest modest to strong downward motion along the
Laramie Range and just east. With all these factors considered,
Monday morning will likely see very strong winds across much of the
region, though a High Wind Warning is currently not anticipated, but
this may change as this event moves into the short term. In-house
random forest model guidance is giving around a 30 to 40% chance for
high winds at Arlington, Rawlins, Bordeaux, Coleman, and northern
Carbon County. Therefore, high wind criteria winds cannot be
completely ruled out at this time and this setup will need to be
monitored closely as Monday morning approaches.

In addition to the potential for strong to near-high wind criteria
winds, this system will bring a strong cold front to the region,
dropping temperatures from the low-70s to upper-80s on Monday into
the upper-60s to mid-70s on Tuesday. Most locations east of the
Laramie Range will be around 10 to 12 degrees cooler on Tuesday.
Additionally, isolated to scattered showers will be possible Monday
with the front moving through, resulting in low-level forcing for
ascent. Dewpoints in the 50s and 60s east of the Laramie Range on
Monday will favor precipitation development with the front, before
dewpoints tank into the 30s and 40s behind the front. With the
frontal passage timing, precipitation will be largely confined to
the I-80s corridor across the region. The long range models
currently suggest that some CAPE may be present across the region
Monday morning and afternoon, so an isolated storm or two cannot be
ruled out especially across the southern Panhandle.

After the cold front Monday, temperatures will remain quite nice
through the remainder of the week, likely staying in the 60s and 70s
with a couple of low-80s possible Wednesday. Daily precipitation
chances are expected as the upper-level flow remains more dynamic
than it has been in the past several weeks.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 532 AM MDT Sun Sep 13 2026

VFR conditions expected through the next 12 hours or so.
Thereafter, increasing showers and thunderstorms will bring
brief periods of lowered VIS alongside generally mid level CIGs.
Overnight into tomorrow morning showers continue to be possible
across most sites with lowering CIGs for western Nebraska
terminals. Have left KBFF out of this for now based on latest
guidance, but otherwise all other NE terminals can expect the
potential for IFR conditions or lower. Strong winds, gusting
25-35 knots with locally higher and erratic gusts from nearby
thunderstorms. LLWS also expected for KBFF overnight.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Fire Weather Watch from Monday afternoon through Monday
     evening for WYZ418>422-432-433.
     High Wind Watch from Monday morning through Monday afternoon
     for WYZ106-109-110.
NE...None.

&&

$$

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