352
FXUS65 KCYS 222242
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
342 PM MST Wed Jan 22 2025

.KEY MESSAGES...

- Scattered snow showers will continue through early evening.
  Some of these may be locally intense. Gusty northwest winds
  continuing through the night and into Thursday morning will
  keep blowing snow concerns for areas that have yet to develop
  a crust on the snowpack, including the I-80 summit.

- Areas of light snow will continue through early Thursday
  morning, mainly in the higher terrain and in Niobrara County.

- Another high wind event producing blowing snow is expected
  Thursday afternoon into Friday at all wind prone areas.

- Widespread snow showers expected this weekend.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 345 PM MST Wed Jan 22 2025

An upper level shortwave trough moving across the area is leading to
some active weather this afternoon. Scattered snow showers have
developed with ongoing gusty northwest winds across the entire area.
Aloft, we have a stratospheric intrusion moving overhead this
afternoon which is producing extremely cold 500-mb temperatures.
Values below -36C are below the first percentile of climatology. The
result of this is very steep lapse rates in the lower atmosphere,
with soundings showing nearly dry adiabatic lapse rates in the
lowest 3-km. The trough axis appears to be nearing the CO border at
this hour, so we have likely reached the minimum temperature
aloft, with the dynamic tropopause returning to a more normal
elevation over the next few hours. Resultant warming aloft
should rapidly reduce lapse rates through the evening, and cut
down on the intensity of snow squall activity. Still, shallow
snow showers are likely to continue into the evening hours,
particularly in the typical northwest flow confluence zones
(i.e. Chugwater to Pine Bluffs in the lee of the northern
Laramie Range, and just east of Chadron in the lee of the Black
Hills).

A secondary shortwave trough is currently visible on GOES
satellite imagery over Montana, and this is expected to dive
southward overnight tonight. This wave will be more typical,
providing broader synoptic lift with the vorticity advection
aloft and decent mid- level moisture, but without the extremely
steep lapse rates from a stratospheric intrusion. Snowfall
tonight will be mainly orographic as a result, although the
confluence zones will probably continue to produce light to
moderate snowfall. We`ll need to watch for accumulations in the
higher terrain, as well as the Pine Ridge area. A Winter Weather
Advisory has been issued for Niobrara county due to expected
snow accumulation up to 2" combined with strong winds leading to
hazardous conditions near Lusk. The mountains can expect
perhaps 2-4" of snow with this, but it should remain sub-
advisory. Much drier air will move in aloft after about 12z,
bringing a quick end to snow shower activity for Thursday as
skies start to clear.

Attention then shifts back to strong winds for Thursday into Friday.
Strong northwest flow will remain aloft through Thursday, so look
for widespread gusts of 35 to 50 MPH across the High Plains once
again. One area to watch for High Winds early will be the I-80
summit, where a stronger cross-barrier pressure gradient is expected
to return this evening. Models show pockets of 700-mb winds
exceeding 50-knots over the area between 03z and 12z tonight also.
While the main event won`t get going until Thursday afternoon,
decided to start the watch for the I-80 summit a little earlier, at
03z tonight to account for the low confidence first period.
Transient ridging moves overhead late Thursday into Friday, briefly
surging 700-mb temperatures to peak around 12z Friday morning.
Westerly flow returns late Thursday afternoon with a strong cross-
barrier pressure gradient thanks to the persistent northwest
Colorado surface high. Low level pressure gradients creep up in the
daytime Thursday with 700-mb gradient surging after sundown.
Gradients look sufficient for good confidence in high winds for the
wind prone areas beginning mid to late afternoon Thursday and
continuing into Friday morning. There is some uncertainty in the
start and end times, but the core overnight period has a clear high
wind signal, and high probabilities on internal guidance. As a
result, decided to issue a High Wind Watch for all wind prone areas.
Blowing snow will be a concern yet again for Carbon and Albany
counties. Watches may need to be expanded again to Rawlins and
Cheyenne, but confidence is too low to do so at this time.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Issued at 345 PM MST Wed Jan 22 2025

Friday night into Saturday, A low pressure system
continues to dive along the western coastline to continue give the
Rocky Mountains and High Plains chances for precipitation into the
weekend. There`s a 500mb vorticity gradient that runs from the
parent low over the Great Lakes region to the low pressure system
diving along western coast of the United States. This gradient plus
some weak isentropic level will provide sufficient forcing to start
the snow production process Friday into Saturday. Early Saturday
morning the pattern switches to a temporary easterly flow at the
700mb level. This direction is favorable for upslope along the High
Plains continuing to aide in more widespread snow chances east of I-
25. However, as this system dives further south along the coast it
will drag down some of the dry arctic air and end the snow
production for most areas Sunday morning into the afternoon. With
this arctic air over us temperatures will likely drop in to the
teens and single digits overnight into Sunday morning. The ensemble
disbursement is fairly narrow thus increasing confidence in the
temperatures through Saturday and Sunday. Over the day Sunday
the pattern does switch to our usual westerly flow, favoring
downsloping off the mountains to help us warm up into the 20`s
by the evening. Looking into the beginning of next week, an
emerging Rex Block is developing with the closed low showing
little movement across the Desert Southwest and upper level
ridging slowly making its way inland from the Western Seaboard.
Persistent dry weather is likely to make its way into the region
Monday onward, due to a positively tilted ridge over our
region. Looking at cluster analysis, the global ensembles are
still uncertain on what to do with low pressure system for the
rest of the week. The low pressure system seems to cut off over
the Desert Southwest and its up in the air as to when the system
rejoins the flow.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1028 AM MST Wed Jan 22 2025

MVFR to VFR ceilings will be moving through the terminals as
lower clouds will be pushing south. Banded snow showers will be
passing through the area but unsure if some of the Wyoming
terminals will be hit. So Prob 30 was used to high light this
uncertainty as it will be tied to an incoming shortwave. These
periods of mid to lower clouds should be expected for today and
tomorrow looking at upstream observations on satellite.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 5 AM MST Thursday for WYZ102.
     High Wind Watch from Thursday afternoon through Friday morning
     for WYZ104.
     High Wind Watch from Thursday afternoon through Friday morning
     for WYZ106.
     High Wind Watch from Thursday morning through Friday afternoon
     for WYZ110.
     High Wind Watch from 8 PM MST this evening through Friday
     morning for WYZ116.
     High Wind Watch from Thursday afternoon through Friday morning
     for WYZ117.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...MM
AVIATION...MM