247
FXUS65 KCYS 242338
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
538 PM MDT Sun Aug 24 2025

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms will occur this
  afternoon through this evening, producing gusty winds to 40 mph
  and periods of moderate to heavy rain.

- Patchy fog is expected to develop late tonight into tomorrow
  morning across portions I-80 from Laramie to Sidney.

- Cool and wet pattern expected for the entirety of the forecast
  period, with daily chances for precipitation and highs 5 to 15
  degrees below average.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 323 PM MDT Sun Aug 24 2025

Not a whole lot of change in the upper levels from yesterday for the
short term period as the two main players continue to influence and
dominate our weather pattern here locally, a stout upper level low
slowly meandering east across southern Canada while ridging pushes
north into our CWA from high pressure over the Southwest. The
interaction of these two will keep northwest flow aloft flowing into
our CWA, keeping temperatures below normal along with a wetter
pattern across most of the CWA through this upcoming work
week.

Current visible satellite imagery is showing some clearing across
our western zones, which will destabilize the atmosphere and help
initiate thunderstorm activity for this afternoon with hi-res
models showing the highest threat from Rawlins to Laramie to
Cheyenne. Due to weaker severe parameters, any thunderstorm activity
should remain below the severe threshold with lightning, heavy
rainfall, and gusty winds being the main hazards. Due to the
possibility of heavy rainfall not only today but through the short
term period, flooding concerns will continue for portions of
the CWA. As such, WPC has most of our CWA, especially across the
southern zones, under a Marginal Risk of flooding. Also, with
the rainfall that we`ve received today, coupled with higher
dewpoints and upslope flow, patchy fog is expected to develop
late tonight into tomorrow morning across portions I-80 from
Laramie to Sidney.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 323 PM MDT Sun Aug 24 2025

Cool and unsettled weather will continue through most of the week
ahead. Picking up on Tuesday, the synoptic weather pattern is
expected to feature a dirty ridge to our southwest, with a broad and
unseasonably strong upper level trough to our northeast near the
Great Lakes region. Dry air circulating around this trough will
infiltrate into our northern and eastern zones on Tuesday, likely
leading to a day of lower thunderstorm coverage compared to the days
preceding and following. However, moisture will remain above average
along the I-80 corridor from Rawlins to Cheyenne and points
southwestward. These areas can then expect widely scattered showers
and thunderstorms once again Tuesday afternoon. Models are in fairly
good agreement showing the moisture surging northeastward overnight,
which may bring a few showers or storms back into the High Plains in
the late evening or overnight hours. Activity is expected to
increase into Wednesday as the axis of the southwest ridge begins to
skirt eastward over our area. Meanwhile, a shortwave trough will
move over the top of the ridge and provide an enhancement to local
forcing for ascent. The two of these features will pull in abundant
monsoon moisture over the area, pushing precipitable water over the
97.5 percentile for the majority of the area per NAEFS means. Nearly
the entire column will be saturated, with dewpoints expected to
reach the mid 50s Wednesday morning even between Laramie and
Rawlins. Scattered showers and isolated thunderstorms can be
expected all through the day, even in the morning hours. Some of
this could even be classified as stratiform precipitation, but
instability creeping up again in the afternoon should flip activity
back to a convective mode. Drier air starts to work back into the
area from the west on Thursday, but precipitable water is expected
to remain near the climatological 90th percentile. Look for
additional scattered shower and thunderstorm activity on Thursday.
Forecast sounds for both Wednesday and Thursday generally appear to
be more conducive for locally heavy rainfall rather than severe
thunderstorm activity, with relatively skinny CAPE profiles and
modest vertical wind shear. Still, we can hardly ever rule out
strong winds and large hail especially at a lead time of several
days. The forecast reflects the cool temperatures and fairly good
precipitation chances throughout the period, but PoPs were nudged
down somewhat from the NBM once again. Confidence in storm coverage
at this lead time is not high enough to allow "definite" wording
into the forecast. As a result, PoPs were capped at "likely" for
most population centers.

Friday into the weekend will begin a very gradual warming and drying
trend, breaking out of the cool and unsettled weather pattern.
However, there is quite a bit of discrepancy between models for this
period. For example, the GEFS system has most ensemble members
showing a disorganized ridge setting back up over the Rockies,
leading to a faster warm up and much more limited thunderstorm
potential. Meanwhile, the ECMWF ensemble system keeps a disorganized
trough over the northern Rockies, and shows the southwest ridge
failing to amplify nearly as much. This would mean that a fairly
active thunderstorm pattern continuing into Friday and Saturday at
least. It`s difficult to pick a favorite at this time, so the
forecast was left unchanged from a blend of both scenarios.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 531 PM MDT Sun Aug 24 2025

Northwest flow aloft prevails.

Wyoming TAFS...For Rawlins and Laramie, scattered to broken
clouds from 5000 to 10000 feet will occur, with showers in the
vicinity until 06Z to 08Z, and areas of fog at Laramie from
08Z to 15Z, reducing visibilities to 6 miles.

For Cheyenne, ceilings will range from 2500 to 3500 feet until
03Z, with occasional showers and fog from 03Z to 15Z, reducing
visibilities to 1/2 to 3 miles and ceilings under 1000 feet,
then ceilings will be near 5000 feet after 15Z Monday.

Nebraska TAFS...For Chadron, scattered clouds near 6000 feet
will occur.

For Alliance, Scottsbluff and Sidney, ceilings will range from
1700 to 6000 feet until 05Z, then areas of fog will occur until
16Z, reducing visibilities to 1/2 to 6 miles and ceilings under
1000 feet, then ceilings will improve to 1500 feet from 15Z to
19Z, then ceilings will be near 4000 feet after 19Z Monday.

&&

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

&&

$$

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