675
FXUS65 KCYS 201109
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
409 AM MST Thu Nov 20 2025

.KEY MESSAGES...

- Model disagreement and variability remains high for the up-
  coming system tonight into early Friday morning. Lighter snow
  accumulations possible along I-80, while higher amounts
  possible in the mountains.

- First real shot of cold air could arrive as early as Tuesday,
  with high temperatures possibly in the 20s and 30s Tuesday,
  Wednesday, and Thursday.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 224 AM MST Thu Nov 20 2025

Mostly cloudy skies ongoing across the CWA early this morning with
mild temperatures at most locations. Light winds continue across the
area leading to a fairly mild night. Some low clouds and potentially
light rain/snow showers are drifting across the higher terrain, but
significant precipitation is not expected until late this evening
into early Friday morning.

Quiet weather potentially comes to an end late tonight into early
tomorrow morning, though model disagreement is very high with high
disagreement also present in the run-to-run for most models. The
mostly calm flow in between systems in the upper-levels comes to an
end today as a trough attempts to push northeastward before becoming
largely absorbed into the upper-level flow. A deepening upper-level
low over California will move north to south with the low aiding in
the dissipation/absorption of the trough over the Intermountain
West. Upper-level flow will be largely calm once the trough
dissipates, with minimal directing flow. While all of this is
ongoing in the upper-levels, a 700mb low develops over southeastern
Colorado and into western Kansas as the upper-level trough
dissipates. This low is largely where run-to-run and model-to-model
agreement tanks. Generally, most models have the 700mb low over
southeastern Colorado and western Kansas, though some models create
a stronger low, resulting in easterly flow at 700mb over the CWA,
while others keep the low weaker and barely impact the CWA. Overall,
the 700mb low appears to far south to cause significant precipitation
across southeast Wyoming and western Nebraska, however, it is just
far enough north to lead to a narrow band of increased precipitation
chances right along the I-80 corridor where easterly 700mb and
surface flow develops by 06Z Friday. Should this low shift slightly
further south, little to no precipitation will be expected along the
I-80 corridor. If it shifts further north, the less likely solution
based on general model guidance, more of the CWA will see
precipitation chances. Therefore, the best precipitation chances
with this system look to be right along I-80, and barely much
further north than that.

The next question about this system becomes whether it will be rain,
rain/snow or snow across the I-80 corridor. This is also where
models diverge significantly from one another. The NAM suggests that
700mb temperatures will drop quickly with this system and be more
moist, leading to a longer period with easterly upslope flow and
higher snow totals. With the colder 700mb temperatures in the NAM,
precipitation would quickly switch from rain to rain/snow then
finally snow. The 06Z NAM gives the Cheyenne area about 7inches of
snow, which is lower than the 00Z NAM which gave Cheyenne almost
10inches. The NAM looks to be the "best case" scenario for higher
snow totals, with a colder and wetter setup. The GFS is one of the
lowest models, with a cold system that is weaker, drier, and
slightly further east than the NAM. For Cheyenne, the GFS has no
snow at all, with barely any precipitation overall. The HRRR appears
to be the most realistic at this time, with about half an inch for
Cheyenne and a slower rain/snow to snow transition and a shorter
period of pure snow. Therefore, kept the snow totals for Cheyenne
right about an inch to account for the uncertainty in exact track,
but increasing likelihood for some smaller amount of snow. At this
time, the NAM looks to be the least likely scenario with the 7 to 10
inches of snow. Overall, the lower elevations will likely see an
inch of less of snow, while the higher elevations could see anywhere
between about 3 and 5 inches. With low confidence in snow
accumulations, decided not to issue any winter headlines with this
forecast package. Depending on future model runs, a last minute
Winter Weather Advisory may be needed for the I-80 Summit.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 332 PM MST Wed Nov 19 2025

This weekend a weak ridge sets up after Friday`s system continues to
push east. Temperatures look to maintain in the 50`s and 60`s east
of I-25 and 40`s to 50`s west of I-25 into the higher terrain.
Forecasted clear skies will allow radiational cooling to maximize
and drop the overnight temperatures to drop into the 20`s. Monday,
the Intermountain west will another southern system traverse the
region. The models are still a little uncertain in the placement of
the storm track but some models are showing a quicker more northern
track than others. If the more Northern solution occurs then some of
the southern counties may see some light precipitation however if
the more southern solution happens then the most likely outcome will
be cloudy skies and no precipitation. Tuesday morning, a deep trough
will begin to descend into the region with a cold front pushing
through to produce some needed precipitation for the region.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 406 AM MST Thu Nov 20 2025

Low clouds have moved into the KCDR terminal leading to LIFR
conditions, which are expected to continue for the next several
hours. Ceilings may jump between SCT004 and BKN004, but the
OVC008 layer should remain. Slight improvement expected at 16Z.
KAIA should see low ceilings within the next hour, continuing
through about 16Z. Lower-level cloud decks expected everywhere
ahead of an approaching winter system. Rain and rain/snow mix
should begin late in the 12Z TAF period, with cloudy conditions
expected throughout the day.

&&

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

&&

$$

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