535
FXUS65 KCYS 011730
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1030 AM MST Sun Nov 1 2026

.KEY MESSAGES...

- The latest round of localized fog is expected to gradually

- A complex but disorganized storm system will move into the
  area Sunday, bringing multiple rounds of rain and snow showers
  through Tuesday AM.

- Expect widespread rain in Carbon county today with scattered
  showers developing elsewhere.

- After a late afternoon lull, look for another round of
  scattered showers plus isolated thunderstorms Sunday evening
  into early Monday morning.

- Additional showers are expected to fill in during the day and
  evening Monday, with snow mixing in above about 5000 ft.

- Widespread windy conditions are anticipated Monday and Monday
  night.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 300 AM MST Sun Nov 1 2026

TODAY: The weather pattern will turn more active today through
Monday night as a complex storm system approaches the area. GOES
satellite imagery shows abundant mid to upper level cloud cover
streaming into the area this morning as deep moisture is pulled into
the area once again. In addition, we have yet another round of
patchy fog across much of the area this morning, but it is not quite
as widespread as the last few days. Expect fog to increase through
daybreak, and gradually dissipate by early afternoon. Looking ahead
to the primary storm system, our two main players we have been
watching for several days are entering the scene this morning. The
first is the primary elongated upper level trough now pushing into
the northern California, while the second is a moisture-laden upper
level low riding the sub-tropical jet into the southwestern CONUS
this morning. Mid to upper level moisture is expected to surge into
the area today, pushing precipitable water values to around 0.75" in
Wyoming and possibly over 1" in western Nebraska. These values are
absurdly high for early November, and more reminiscent of a late
summer monsoon surge. In fact, NAEFS mean PW exceeds the
climatological maximum over much of the area. While atmospheric
moisture is clearly abundant, the question as always is if we can
get this moisture to the ground. Short range model guidance shows a
region of positive theta-e advection pushing in from the southwest
around daybreak, first into Carbon county and then spreading
northeastward through mid afternoon. This round will be generally in
the form of scattered showers, though the Sierra Madre range and
southwest Carbon county should see more consistent rainfall with
favorable southwest orographic ascent. Snow levels will be extremely
high for this time of year when precipitation starts today. Rain is
expected to fall at least at 10 kft, if not even 11 kft, meaning
that we could see rain up to ridge-top in the Sierra Madre.
With cloudy skies expected all day, temperatures were nudged
downward towards high resolution model guidance, but we should
still reach the mid 50s for all valley areas. Forcing for ascent
should weaken a bit mid afternoon as a weak leading vort-max
passes through, decreasing coverage slightly, but expect to see
some shower activity around the area for most of the day today.

TONIGHT: By this evening, the mid-level low pressure system is
expected to be centered around western Wyoming. As the primary
trough axis approaches, we should see ascent increase again after
sunset, this time encompassing more areas east of the Laramie range
too. RAP guidance depicts decent overrunning lift combined with
frontogenesis as the surface lee cyclone begins to take over in
northeastern Colorado this evening. In addition, there is potential
for considerable elevated instability over the Nebraska panhandle
this evening. Thus, expect to see shower coverage increase after
about 7-8PM, with isolated thunderstorms anticipated as well. Due
the extremely abundant moisture, locally heavy rainfall will be
possible with this round this evening through very early Monday
morning. The primary trough axis and surface cold front are expected
to clear the area by about 5AM Monday, ending the warm phase of the
storm system.

MONDAY: At sunrise Monday, most precipitation should be confined to
the fringes of the forecast area, generally in Carbon,
Converse, and Niobrara counties. By this time, a powerful
surface cyclone will be strengthening and moving northeastward,
pulling in a tight pressure gradient on its western flank (over
our area). At the same time, we should see moisture and marginal
isentropic ascent wrapping back into the area on the back-side
as the mid-level low closes off. Expect winds to rapidly
increase Monday morning with precipitation gradually spreading
to the south and east of its daybreak position as the day
progresses. Forcing aloft is marginal and there is some bust
potential for this last round of precipitation to fail to
materialize. In order to get widespread rainfall to fill in (and
get more appreciable amounts) we will want to see a secondary
mid-level low pressure center over southern Colorado elongate
the primary system and turn winds in the 750 to 500-mb layer
from northwest (which is where they are expected to be Monday
morning) to north or northeast. This scenario is depicted by the
HRRR, while the RRFS keeps winds aloft just west of north, and
thus ends up considerably drier across the entire area Monday
into Monday evening. The official forecast favors the ECMWF
ensemble mean, which is somewhere in between the high resolution
models on each extreme. The most likely scenario fills light to
moderate precipitation in to the southeast through the day on
Monday and into the evening hours. Strong surface northwest flow
will allow some terrain features to stand out, particularly the
sub-ridges of the High Plains. The most favorable area for
notable moisture in the wrap-around phase is the US-20 corridor
from Douglas to Chadron, with a secondary favored area possible
along the southern Laramie range and the north side of the
Cheyenne ridge (Laramie to Chugwater to Chugwater. Areas where
northwest flow is downsloping (Wheatland to Scottsbluff and
Cheyenne to Kimball) look a little more marginal, and we will
likely need stronger lift aloft to overcome the surface
downslope flow.

SNOW POTENTIAL: As colder air is pulled into the system on Monday,
expect snow levels to drop quickly. Accumulating snow should begin
in the Sierra Madre and Snowy ranges early Monday morning. By mid-
morning, we should see some snow mixing into the lower elevations of
Carbon county and around the I-80 summit (once precipitation fills
back in there). During the afternoon and evening hours, we should
see snow levels continue to drop, allowing some flakes to fly as low
as about 5000 ft. It is important to note that with systems like
this, the precise snow level will probably depend somewhat on the
precipitation rates, with heavier rates flipping over to snow and
lighter rates remaining as rain. Confidence is increasing in
accumulating snow in the mountains, with probabilities for advisory
criteria amounts around 70 to 90% in the Snowy, northern Laramie,
and Sierra Madre ranges. The higher elevations of I-80
(Arlington/Elk Mountain and the I-80 summit) are sitting around 60%
chance for advisory criteria snow accumulation. All of the above
locations currently have about a 30% chance for warning criteria
amounts. Outside of those zones, confidence decreases considerably,
as which lifting scenario (see previous paragraph) will strongly
impact snow potential outside of the higher elevations too. Of the
populated areas, Laramie is most likely to see accumulating snow
(80% chance), followed by Rawlins and Saratoga (60% chance). These
probabilities decrease east of the Laramie Range, but Douglas, Lusk,
and Cheyenne all have about a 10-30% chance to see accumulating
snow, mainly late Monday afternoon and evening. Snow is not expected
to stick (if it flies at all) below about 5000 ft.

WIND POTENTIAL: Monday will be quite windy across much of the area
as a strong surface pressure gradient develops over the Plains. The
wind direction is not really aligned with most of our typical gap
wind areas, and the lack of synoptic descent (or even synoptic
ascent) along with the unusual wind direction aloft (due north)
decreases confidence. While some high resolution models (e.g. HRRR)
are very aggressive on the surface wind gusts, the other factors are
keeping confidence too low to issue any High Wind Watches at this
time. It will absolutely be windy, with high confidence in gusts 45
to 55 mph, but it looks quite marginal to exceed the critical 58 mph
threshold in most areas. For the wind prone areas, the I-80 summit
and the I-25 corridor between Wheatland and Glendo look to be the
most favorable zones, with around a 40% chance for high winds. Over
the Plains, the most likely areas to exceed criteria will be where
northwest flow is downsloping (e.g. south sides of the Pine and
Cheyenne ridges). Probabilities remain around 30 to 40% for
Cheyenne, Kimball, Sidney, and Alliance.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 300 AM MST Sun Nov 1 2026

Drier air will move in Tuesday morning, bringing an end to
precipitation, though the end time has slightly slowed compared to
previous forecasts. While not nearly as strong as Monday, breezy to
windy northwest winds are expected to continue into Tuesday with
highs rebounding to near seasonal averages. Look for plentiful
sunshine to return to the area Tuesday as well.

Mid-week and beyond, ridging will try to build back in over the
region, but a few moisture starved upper level lows will hamper a
potential warming trend. Overall, temperatures are expected to
remain fairly close to seasonal averages during this period.
Conditions look generally dry too, though a compact upper level low
passing through Wednesday may bring a few rain and/or high elevation
snow showers. Expect winds to be light Wednesday through Friday, but
a few ensemble members show stronger winds moving into the area
with an upper level low passing north of us over the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1030 AM MST Sun Nov 1 2026

Low CIGs and VIS (IFR and LIFR conditions) continues to plague
many of the terminals at this hour, including KCYS, and look for
these conditions over the next several hours to continue. As
more moisture feeds into the CWA, another push of low CIGs will
make its way into the area this evening. There will also be
multiple rounds of light showers that move through during this
forecast period with the potential for thunderstorms and locally
heavy rainfall across the Nebraska Panhandle this evening.
There will be a final push of rain showers near the tail end of
this TAF period, possibly mixing in with snow for KRWL. Shifting
gears a bit to winds, look for winds to become south to
southeast and increase today across all terminals with gusts 20
to 30 knots. Winds will ease this evening into tonight and
increase once again through Monday morning, with gusts 20 to 30
knots, possibly up to 35 knots at KAIA.

&&

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

&&

$$

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