275
FXUS65 KCYS 042333
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
433 PM MST Wed Dec 4 2024

.KEY MESSAGES...

- Above average temperatures will continue each day through the
  end of the week with dry weather expected.

- High winds are increasingly likely for the wind prone areas
  Saturday night into Sunday.

- A strong cold front will bring gusty winds, much colder
  temperatures, and a chance for snowfall Sunday night into
  Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 240 PM MST Wed Dec 4 2024

The Stagnant pattern, that has been observed over the last several
days, continues to hang on another day with the latest upper air
analysis highlighting the upper level ridge slowly coming more
inland from the Western Seaboard to the Intermountain West. Much of
the influence for the limited movement of this ridge has been a
direct related to the upper level features observed off to the east
as the last of the series of upper level closed lows dig southward
from the Canadian provinces to the Great Lakes. Looking at the
surface, latest observations have placed a weak cool front mentioned
in the aviation discussion now across the Nebraska/South Dakota
border and starting to dig south into Northern Wyoming. Similar to
yesterday, conditions throughout the morning and early afternoon
have been relatively mild ahead of the boundary, with temperatures
in the mid 50s for most areas and around 10 degrees colder for the
far western portions of the CWA.

Main forecast concerns over the next 12 to 36 hours focus on the a
series of cool fronts expected to dig across the CWA late this
afternoon and into the early morning hours and again a second cool
front swinging through the area Thursday night into Friday. Focusing
on tonight and tomorrow morning, confidence has continued to
increase throughout the day for the boundary to stall along the
Rockies into the overnight hours, ultimately turning the winds
towards a more favorable direction for upslope flow for areas in
western Nebraska. HiRes models have begun latching onto onto this
solution with areas of patchy fog developing north of Morrill and
Scotts Bluff County and some northwestern extension into Niobrara
with forecast soundings indicating low level moisture across the
aforementioned zones. Went ahead and added patchy fog into the
forecast. However, will need to monitor for freezing fog potentials,
with forecast soundings being on the line for freezing fog.
Throughout the day tomorrow, should be another quieter day with the
daytime highs dropping a few degrees in the Panhandle due to the
stalled boundary. However, temperatures are expected to remain
relatively the same for areas along the I-25 corridor and westward
as another day of northwesterly flow aids in a warm downsloping flow
and some gusty to elevated winds along and east of the Laramie Range.

Next upper level shortwave, which is progged to undercut the main
southwesterly flow, will begin to digging to the southeast Thursday
night into Friday with the flow becoming somewhat split as a closed
low develops just to the south of the Four Corners. Little to no
precipitation is expected with the arrival and passing of the
shortwave as PWATs remain dry and having minimums around -1.5 sigmas
below climatology. Temperatures will also remain relatively the
same, with dry downsloping flow aiding in temperatures hovering
around the low to high 50s for areas along and east of the Laramie
Range and 10 to 15 degrees cooler for areas to the west. Fairly high
confidence for above average temperatures to continue through the
late week, with the NAEFS mean pushing 700mb temperatures in the
climatological 90th percentile further eastward into central and
east Wyoming late Friday afternoon.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 240 PM MST Wed Dec 4 2024

A strong Pacific trough will bring an end to the warm weather
pattern this weekend, but an impactful snow event is looking less
likely.

Warm and dry weather will continue into Saturday as a strong ridge
axis shifts over the Rockies. Downstream amplification of the ridge
ahead of the trough moving into the Pacific northwest and modest
downslope flow will support Saturday bringing likely the warmest
temperatures of the forecast period. Ensemble guidance depicts
average 700-mb temperatures reaching close to +6C, which is above
the 90th percentile of climatology and approaching the 97.5th
percentile. The warmth will be more pronounced east of the Laramie
Range, where widespread mid to upper 50s and possibly a few 60s can
be expected. The arrival of the Pacific trough will bring an abrupt
end to the warm pattern on Sunday. First, winds will increase as
height/pressure gradients build in advance of the trough. Strong
winds in the wind prone areas (Bordeaux in particular) may begin as
early as Saturday morning with considerable MSLP gradients resulting
from falling pressure over the High Plains. However, confidence is
higher for winds Saturday night into Sunday morning, with the
highest probabilities occurring with the maximum pressure gradient
just ahead of the surface cold frontal passage. In-house guidance
depicts a 50-70% chance of high winds during this period thanks to
700-mb winds in excess of 50-knots, surface gradients well above
typical criteria, and 700-mb height gradients sufficiently close.


Elsewhere, expect a morning high Sunday with most guidance showing
the front passing through between 12z and 18Z, and temperatures
dropping through the afternoon hours. Expect gusty northwest winds
behind the front that will likely continue through Monday. Monday
will be significantly colder. 700-mb temperatures plummeting to
around -13C will lead to highs struggling to reach freezing across
much of the area. This precipitation outcome with this system is
still fairly uncertain, but recent guidance has continued to trend
towards the drier scenario. Models have trended to a pseudo-split
trough scenario once again, where a southern vort-max tracks further
south towards the Four Corners while a northern vort-max remains
just to our northeast over the Dakotas. Such a scenario is an
unfavorable lifting profile for most of our area, mainly limited to
just orographics which is not very widespread in northwest flow.
Moisture also looks limited. Ensemble mean precipitable water
values are actually below average for this time of year (which is
already quite low), at less than 0.25" for the entire area. Ensemble
probabilities of 0.1" or greater of QPF are less than 20% for all
valley locations. The resulting most probable scenario would mean a
few light snow showers over the plains bringing a dusting to an inch
or so to some areas, but most accumulation limited to the mountains.
Even in the mountains, ensemble averages are only around 0.25" QPF
supporting a sub-advisory event in the most likely scenario. Current
guidance shows a 20% chance of advisory totals in the Snowy Range
and a 50% chance in the highest peaks of the Sierra Madre. Beyond
Monday, conditions will remain on the cool side. While the ridge
will try to expand again, another shortwave trough arriving around
Wednesday may bring another round of wind and light snow in addition
to replenishing the cold, but confidence is fairly low at this lead
time.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 433 PM MST Wed Dec 4 2024

The main aviation concern this evening and overnight will be the
potential for low CIGs and fog across portions of the Nebraska
panhandle. The HREF currently shows about 30 percent probabilities
for MVFR CIGs in the northern Nebraska panhandle, including KCDR.
The HRRR is a bit more excited about fog developing in the southern
panhandle, possibly affecting KSNY. Even if MVFR CIGs do not occur,
expected low stratus to move into KCDR within the next few hours.
Southeast Wyoming terminals can expect a quiet overnight.

&&

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

&&

$$

SHORT TERM...MRD
LONG TERM...MN
AVIATION...SF