986
FXUS65 KCYS 052334
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
434 PM MST Wed Feb 5 2025

.KEY MESSAGES...

- High Wind Warnings are in effect for much of southeast Wyoming
  this morning through the evening hours.

- Cold front expected on Thursday, leading to decreasing winds
  and cooler temperatures.

- A chance of rain and snow will start the weekend, with cold
  air behind this returning the region to near to below average
  temperatures to start next week.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 245 PM MST Wed Feb 5 2025

Latest upper air analysis depicts the broad upper level ridging
beginning to move eastward, with the next Pacific system forming
across the Pacific Northwest. Another breezy day across the CWA,
with widespread strong winds observed across southeast Wyoming.
Latest High Wind Warnings have performed well, with max wind speeds
throughout the morning and early afternoon hours reported 75 mph
across the I-25 corridor near KBRX and a 70 mph wind gust reported
along the I-80 corridor near Halleck Ridge. As a result, High Wind
Warnings remain in effect through 5pm today, outside northern Carbon
county, Converse county, and the Summit where High Wind Warnings
remain in effect through the evening hours. All in all, timing on
current warnings looks good with local in-house model guidance
probabilities of reaching warning criteria decreasing after 00z
today for the I-25 corridor. Only uncertainty will remain with
Arlington, with the latest in-house model guidance keeping winds
fairly elevated through 09z tonight as the progressive Pacific
System skids eastward across the Canadian border. As a result,
strong pressure gradients are possible across our I-80 corridor and
increasing the wind speeds for Arlington. Main uncertainty is if the
winds will be able to mix down to the surface or stay aloft,
resulting in more of a LLWS setup. Will hold off on any headline
extention till observations can determine if warnings need to be
extended into the evening hours.

After the wind threat today, focus on the short term forecast
will slightly shift to the ongoing wind threats early tomorrow
morning and again on Friday afternoon, combined with increased
precipitation chances tomorrow evening. Looking into the wind
threats first, latest GFS has begun to advertise a decent low
level jet setting up from KBRX in southeast Wyoming to KAIA in
the Nebraska Panhandle beginning 12z through 18z tomorrow. Still
a bit skeptical of this setup, with local in- house model
guidance dropping probabilities to near zero for high winds in
KBRX and not a lot of model agreement with the NBM v4.2. Friday
appears the better setup, with the probabilities quite a bit
higher with in-house model guidance having upwards to 70%
likelihood of strong winds across our wind prones along the I-80
corridors and much better agreement with the NBM 4.2
probabilities of exceeding 41 knots. Will still hold off on any
additional headlines until the current headlines expire. But
will be worth monitoring through the next couple of forecast
cycles to issue additional wind headlines. In addition,
precipitation chances continue to rise, with a good atmospheric
river signal beginning to take shape in the short to medium
range models, has PWATs climb to around 1.9 sigma above
climatology.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 245 PM MST Wed Feb 5 2025

Over the weekend and into next week, the area can expect a
transition out of the warm and windy pattern that has dominated the
last week or so as much colder air returns. On Friday evening,
pseudo-zonal will continue to dominate the synoptic pattern aloft as
a strong upper level low traverses eastward across the Western US.
This system will bring another round of wind and mountain snowfall,
although the lower elevation precipitation forecast is more
uncertain. Integrated water vapor transport values in the NAEFS mean
are near climatological maximums Friday night, but this is more due
to the wind speeds than the moisture. 500-mb and 200-mb winds both
look to be above the 99.5 percentile of climatology, with
precipitable water around the 90th percentile. The moisture flux
combined with fairly strong frontogenesis should produce a brief
period of moderate to heavy snowfall in the mountains and perhaps
adjacent valleys of Carbon County Friday evening and overnight.
However, dry air will move in quickly, mostly shutting off the
faucet by 12z Saturday. Meanwhile, the upper level low will support
lee cyclogenesis over central Wyoming Friday evening. The surface
low will get shunted south into east central Colorado quickly as a
strong cold front pushes through behind the system. Ensemble mean
700-mb winds are close to 50-knots over KCYS early Saturday morning,
with models also showing strong subsidence in the wake of the mid-
level trough axis. We will likely be looking at another period of
strong to high winds, at least in the wind prone areas (where
probabilities for high winds are generally 50 to 65%) and possibly
into the adjacent zones (with probabilities around 30 to 50% at this
time). The strong signal for subsidence will also work against any
potential for low-elevation precipitation, especially east of the
Laramie range where low-level winds will have a downslope component.
Frontogenesis along the 700-mb trough axis may be enough to produce
radar echoes and some light snowfall, but widespread accumulating
snow is looking less probable for the High Plains. Probabilities for
0.1" QPF are 10% or less in both the GEFS and ENS systems for areas
east of the Laramie Range and south of US-20. Expect highs back to
near to slightly below average values on Saturday with a much colder
airmass moving in behind the upper level low.

A secondary upper level shortwave is expected to follow behind on
Saturday night into Sunday. After the Friday night system, the mid-
level front will be left straddling along or just south of the WY/CO
border. As the next shortwave moves in Saturday evening, models
suggest the possibility for overrunning positive theta-e advection
to occur along this boundary. This may be just too far south to
bring renewed precipitation to our mountains, but about 60% of
ensemble members show light snow extending into the I-80 corridor
from Elk Mountain to Sidney and points southward. However, less than
10% show over 0.1" QPF once again. Isentropic lift and moisture
evaporate Sunday morning, leading to clearing skies progressively
through the day, while temperatures remain chilly.

As we head into the early part of next week, most models are showing
a strong arctic surface high pressure sliding down the eastern side
of the Rockies behind a series of upper level shortwave troughs
mainly staying to our north. Surface pressure above the
climatological 90th percentile creeps into our High Plains regions
by late Monday according to the ensemble mean. There is some
uncertainty in Monday`s high temperatures relating to the timing of
the front, but most of the area should be 15+F colder than average
come Tuesday. A few rounds of light snowfall will be possible Monday
onwards, but the timing and intensity of each round remains highly
uncertain. The vast majority of ensemble members show at least some
light snow accumulation between Monday and Wednesday, but the timing
varies widely.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 430 PM MST Wed Feb 5 2025

Strong westerly flow aloft will continue. Expect scattered
clouds near 15000 feet. Low level wind shear occurs at Laramie
and Alliance from 02Z to 15Z. Winds will gust to 38 knots at
Rawlins, to 35 knots at all other terminals until 02Z, then to
35 knots at Laramie, Cheyenne, Chadron, Alliance and Scottsbluff
after 15Z Thursday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 8 PM MST this evening for WYZ101-116-
     117.
     High Wind Warning until 11 PM MST this evening for WYZ104.
     High Wind Warning until 5 PM MST this afternoon for WYZ106-107-
     109-110-113-118-119.
NE...None.

&&

$$

SHORT TERM...MRD
LONG TERM...MN
AVIATION...RUBIN