Cheyenne, Wyoming 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

661
FXUS65 KCYS 220522
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1022 PM MST Sat Dec 21 2024

.KEY MESSAGES...

- A High Wind Warning is in effect for the wind prone areas of
  southeast Wyoming beginning late Saturday evening and
  continuing through Sunday afternoon.

- Mountain snow likely, beginning Sunday afternoon.

- Active week ahead with several systems pushing across the
  region.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 228 PM MST Sat Dec 21 2024

Latest upper air analysis depicts the ridge slowly sliding eastward
over the last day or so, with the next Pacific System building
across the Western Seaboard. Conditions throughout the morning and
early afternoon have been relatively pleasant under the influence of
the upper level ridge, with the ridge axis centered across central
Wyoming as temperatures continue to climb into the mid to high 50s
across southeast Wyoming and even into the low 60s for portions
of western Nebraska. Winds have not been too bad today, with
wind gusts around 20 mph for most regions and upwards to 30 mph
across KBRX. Should begin to see changes this evening and into
the overnight hours with the next progressive upper level
shortwave digs across Montana and into the Dakotas late Sunday
night. Primary forecast concern will focus on the high wind
event progged tonight into tomorrow. Winds are expected to be
elevated to high wind potentials on the underside of the
shortwave that is attempting to close off into a cutoff low as
evident with the strong 700mb to 500mb jet maxes. Local in-house
model guidance is in agreement with this solution with around
60% probabilities for high wind criteria around the normal wind
prone areas. Felt confident enough to upgrade current watches to
warnings with the model to model consistency being generally
the same over the course of the last few days. Main uncertainty
exists to expand headlines further east and west for the areas
along and east of the I- 25 corridor, in addition to KRWL and
KLAR. Latest in-house model guidance continues to have lower
probabilities across those areas, with upwards to 20% likelihood
of strong winds and keep most areas elevated. Nevertheless,
latest HiRes models do indicate a low level jet maximizing wind
speed at 700mb and extending from Rawlins WY to Sidney NE. Went
ahead and increased the winds across the areas outside of the
current warnings to around 40 mph and will need to monitor these
zones into tomorrow.

Mostly dry forecast expected across the CWA for the late weekend
with the primary Pacific System being a bit too far to the north to
cause any real precipitation impacts and the better moisture
advection remaining to the north and south of the CWA. Only
precipitation concerns for Sunday focus on bands of PWATs
inching northward into the southern counties in southeast
Wyoming and western Nebraska. However, the strong westerly flow
will likely aid in the downsloping for most areas and continue
to dry out the lower levels. This will be relatively shortlived,
with return of mountain snow Sunday night into early Monday
with the fast moving Pacific Shortwave tracking east, which will
allow additional shortwave disturbances to dig southward into
southern Wyoming. Models are in relatively good agreement,
placing most precipitation along the mountain zones across the
Sierra Madres/Snowy Range and some higher terrains into Carbon
county. Concerns will continue throughout the night if
precipitation will reach the lower elevations and into the I-25
corridor with some decent PWATs spreading eastward into western
Nebraska. Although the downsloping flow should keep the lower
levels dry.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 228 PM MST Sat Dec 21 2024

Active long term pattern expected across the region as progressive
flow develops aloft. An upper-level shortwave trough pushes towards
the region as a shortwave ridge ejects out to the east. Strong 500mb
cyclonic vorticity advection will propagate the shortwave trough
quickly overhead and out of the region as a ridge builds to the west
by the end of the day Monday. With this fast moving shortwave
trough, 700mb height gradients will tighten in response resulting in
a 40 to 45kt jet by Monday evening. Surface winds become elevated
once more in response to the 700mb jet, leading to yet another
breezy day for much of southeast Wyoming, while western Nebraska
remains fairly calm. GFS Omega fields further support elevated
winds, especially just downstream of the Laramie Range and at
Arlington. In house guidance shows about 60% probability of high
winds at Arlington for Monday night, but would like to see Craig to
Casper gradients stronger, as well as a stronger 700mb jet across
the region. Therefore, decided not to consider headlines for the
typical windy locations, but this system will continue to be
monitored for any changing trends. Downslope flow along the Laramie
Range will allow for much warmer than average temperatures across
the region, with highs east of the Laramie Range in the low- to mid-
50s with highs west of the Laramie range in the mid-40s. In addition
to the strong surface winds Monday, mountain snow fall is favored
across the Snowy and Sierra Madre ranges, with several inches of
accumulation possible. Moist, southwesterly winds will favor upslope
flow along the Sierra Madres and with PWats in the 90th percentile
for this time of year, significant snowfall is becoming more likely
for the mountains.

Behind the departing shortwave trough Monday evening, a high-
amplitude ridge pushes into the region with a second, stronger,
upper-level trough right behind it. Under this ridge, temperatures
will remain above average, but without the additional assistance of
downslope flow and 700mb temperatures in the -2C to +1C range,
daytime highs are expected to be a few degrees cooler than Monday.
Highs in the low-40s to low-50s are likely across the region. Skies
will be mostly sunny during the daytime hours. Clouds move in for
the evening and overnight hours as the regions start to become under
the influence of the approaching upper-level trough. The neutrally
tilted, upper-level trough looks to be fairly potent as this time,
as both the ECMWF and GFS suggest the development of a closed low
just south of the CWA. Usually, this setup would promise some decent
snow for much of the CWA. However, the 700mb low continues to
trend further south in each model run, with the surface low
positioned over southeastern Colorado rather than the preferred
northeastern portion. With this surface low placement, northerly
flow develops and remains across the region for much of this event.
Therefore, snowfall totals outside the mountains seem fairly low, if
anything at all. Cannot rule out snow completely for the CWA at this
time, but if model trends continue the total snow accumulation will
rapidly approach zero for all locations outside of the higher
terrain.

Model disagreement begins to ramp up for Thursday, with the GFS
suggesting the upper-level closed low to sling-shot around the low
and moving northerly from eastern Colorado to the North Dakota/South
Dakota border. The ECMWF on the other hand propagates the low much
further south the off into the Midwest on Thursday and into Friday.
Despite this, both the ECMWF and GFS suggest a second upper-level
trough moving across the region Thursday into Friday. The exact
potency of the trough is hard to discern with the significant
differences in the long range models. With the general evolution of
the second upper-level trough, another chance for snow looks
possible across the CWA.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1019 PM MST Sat Dec 21 2024

The main aviation concerns tonight into Sunday AM are due to
wind gusts and LLWS. Expect LLWS to develop over the next few
hours at all terminals, with occasional gusts making it to the
surface at Wyoming terminals. Gusty W to NW winds can be
expected Sunday mid morning through mid afternoon before
weakening for the late afternoon. A weak storm system may bring
lowering CIGs and possibly a few snow showers to the vicinity of
KRWL Sunday late afternoon and evening, but confidence is too
low to add to the TAF at this time.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 5 PM MST Sunday for WYZ106-116-117.
     High Wind Warning until 8 PM MST Sunday for WYZ110.
NE...None.

&&

$$

SHORT TERM...MRD
LONG TERM...AM
AVIATION...MN