495
FXUS65 KCYS 172106
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
306 PM MDT Thu Sep 17 2026

.KEY MESSAGES...

- A wet pattern is expected through the end of the week and into
  early next week, with daily chances for showers and
  thunderstorms.

- There is a Slight Chance of Excessive Rainfall and flooding
  issues on Thursday as a stronger than average late-season
  monsoon with tropical moisture moves into the region.

- Seasonably cool temperatures with highs ranging from the
  middle 60s to middle 70s through Monday. Friday will be the
  warmest day with low to middle 80s possible across the lower
  elevations.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 302 PM MDT Thu Sep 17 2026

Still expecting active weather in the short term as storms continue
today with more forecast for Friday. Scattered rain showers have
been plaguing the eastern half of the CWA for much of the day
so far, with current radar showing a few storms moving out of
the northern Nebraska panhandle and stratiform rain moving up
the Interstate 25 corridor from Colorado. Convective activity is
expected later this afternoon, however, ample cloud cover over
the region has contributed to a more stable environment than
what was previously forecast by models. The southern Nebraska
panhandle has seen the most sunshine and warmest temperatures,
leading to what is the most unstable environment in the CWA.
Mesoscale analysis confirms this, showing about 1500 J/kg of
SBCAPE and decent shear. Hi-Res guidance like the HRRR, has
begun to catch onto this, showing some of the stronger storms
developing/moving into the southern Nebraska panhandle this
evening. PWATs are still at climatological max per NAEFS, so
flash flooding will still be a concern in any storms that
develop today. The most likely area for heavy rain and flash
flooding will be for areas along and east of the Interstate 25
corridor, and the Nebraska panhandle. Given the instability from
the mesoanalysis, cannot rule out an isolated severe storm or
two with strong winds and hail. Storm chances will continue
through about midnight, before pushing northeast into South
Dakota.

Another stormy day is expected on Friday as a trough currently over
the Pacific Northwest, moves into Wyoming. The morning will likely
be precipitation free, however, once the trough swings into Wyoming
during the afternoon hours, storms will move into the CWA. Storms
will push into the area from western Wyoming and northwest Colorado,
enhanced by monsoon moisture. PWATs will still be in the 97.5th
percentile and above, so heavy rain and flash flooding still cannot
be ruled out, mainly from training storms. There will be some
instability around, however, likely not enough to get any severe
thunderstorms. Precipitation coverage looks to be fairly widespread
by the evening hours. A cold front associated with the trough will
also move across the area, with more stratiform like precipitation
expected behind it. Showers and storms will likely continue later
into the overnight hours, but should be out of the area by sunrise
Saturday.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 302 PM MDT Thu Sep 17 2026

This weekend will start out active as SE Wyoming and the Nebraska
Panhandle will remain under SW flow aloft, which will allow for
additional surges of monsoonal moisture and chances for
precipitation. A closed low associated with a rather amplified
shortwave trough will be located over Idaho and Western Wyoming,
providing a source for large-scale ascent via DCVA over our area,
which should set the stage for another round of afternoon/evening
showers and thunderstorms. PWAT ahead of a closed low will remain at
or above the 90th percentile for areas east of the I-25 corridor, so
heavy rainfall will again be a possibility with any storms. Forecast
soundings across the Nebraska Panhandle on Saturday afternoon and
evening continue to show tall, skinny CAPE profiles, with warm cloud
layers near 3 km AGL. This will result in another scenario involving
efficient warm rain processes (i.e. collision-coalescence) in this
abnormal, tropical-like air mass that has been in place over the
past few days. Flash flooding will remain a concern, especially if
storms manage to train over the same areas that received any heavy
or excessive rainfall on Thursday or Friday, which we will have more
specifics on once we see how the heavy rainfall scenario pans out
today and Friday. Luckily, cloud-layer mean flow remains in the 25-
30 knot range, so any flooding threat would be maximized with any
training convection, especially as Corfidi upshear and downshear
vectors are well above 20 knots, so any clusters/lines will be
propagating at a decent rate.

There will also be the threat for severe weather on Saturday in the
Nebraska Panhandle (though likely quite marginal), as deep-layer
shear is forecast to be around 30-35 knots, especially as we head
into Saturday evening. With rich low-level moisture with dewpoints
in the mid to upper-50s and MLCAPE ranging from 1000-1500 J/Kg,
would not be surprised to see a few supercells capable of producing
damaging winds and some marginally severe hail (although 700-500 mb
lapse rates are only ~7 degC/km). Of course, this potential will be
dependent on how convection evolves on Friday afternoon and
overnight, which will ultimately determine how much destabilization
we will see on Saturday during the day. High temperatures Saturday
will remain in the low to mid-70s east of the I-25 corridor, with
mid to upper 60s to the west.

Sunday we will continue to remain active, although most likely less
so than Saturday, as the closed low translates across our areas and
transitions into more of an open wave. However, monsoonal moisture
continues to remain trapped underneath the overall longwave ridge
pattern. Further, medium-range models are hinting at another weak
frontal passage sometime Sunday afternoon into the evening, placing
much of the southern Panhandle in a low-level upslope flow regime.
This feature, along with the open wave moving across our region, may
both serve as sources for initiation, although the phasing/placement
of the two are uncertain at this time. needless to say, heavy rain
will continue to be a threat, with PWAT remaining between the 70th
and 80th percentiles. High temperatures on Sunday will be slightly
cooler than on Saturday, with highs mainly in the upper 60s to low
70s east of the Laramie Range, and 60s to the west.

The pattern continues to remain unsettled into the work week, with a
more zonal flow regime in place downstream of a deep longwave trough
over California and Nevada. While the westerly flow should result in
a drier day on Monday relative to this weekend, this open wave will
slowly move across the Desert Southwest, returning our mid-level
flow to more southwesterly. Another influx of monsoonal moisture is
expected sometime early to mid-next week, with NAEFS guidance
suggesting PWAT anomalies nearing +2 sigma. While not as impressive
as this most recent moisture surge, can expect daily rounds of
showers and thunderstorms towards the middle of next week with high
temperatures at or slightly above seasonal values. Models have been
inconsistent regarding the timing of a building mid-level ridge over
the Central United States next week, but can expect an overall
warming trend through the middle of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1110 AM MDT Thu Sep 17 2026

A variety of hazards today across SE Wyoming and the Nebraska
Panhandle today, as numerous rounds of showers and thunderstorms are
expected this afternoon and evening. Primarily VFR conditions this
afternoon outside of any of the heavier showers and thunderstorms,
where MVFR cigs and vis will be in place. Any thunderstorms in the
vicinity of the terminals will be capable of producing gusty and
erratic winds, however the exact placement of these stronger storms
is highly uncertain, and future amendments will be necessary if one
of these storms happens to be in the proximity, with the highest
confidence being in the Nebraska Panhandle. Moderate confidence in
IFR cigs this evening and overnight for the Nebraska terminals, as
low clouds and stratus are expected to hang on given light winds and
low-level moisture. Can expect brief vis drops down to 2SM in/around
showers and thunderstorms.

&&

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

&&

$$

SHORT TERM...SF
LONG TERM...NB
AVIATION...NB