920 
FXUS65 KCYS 012309
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
409 PM MST Thu Dec 1 2022

.SHORT TERM...(This evening through Saturday night)
Issued at 310 PM MST Thu Dec 1 2022

A upper low was located off the WA coast, with a fast southwesterly
flow aloft downstream over the Rockies and High Plains. Visible 
imagery showed wave clouds over and downwind of the mountains. A 
surface trough extended from the Big Horn Range through the Snowy 
Range into northwest CO, with the gradient tightest along and west
of the trough. Southwest winds have been quite strong since early
this morning along I-80 between Laramie and Rawlins and I-25 north
of Wheatland to Douglas. Peak gusts of 65 to 80 MPH range verified
the High Wind Warnings in these areas.

High winds, accumulating snow and temperatures are the primary forecast
concerns through the short term.

The surface trough aligned north to south just west of the Laramie
Range will slowly shift east in advance of an approaching strong
cold front from the west tonight. The tightest low level pressure
gradient will begin to drift east over and just east of the Laramie
Range tonight, with the strong southwest winds affecting the I-25
corridor from Cheyenne to Douglas. The cold front will be moving
into western WY this evening, and quickly through southeast WY
Friday morning, and the NE Panhandle Friday afternoon. The presence
of a 120-kt 250mb jet with 65-80kt westerly 700-800mb flow will
maintain very strong winds with gusts to 80 MPH west of I-25 tonight.
Strong post-frontal cold air advection and deep layer subsidence 
will aid in mixing of strong winds aloft to the surface along and 
east of I-25. West winds gusting to 65 MPH are likely over Platte,
Goshen and Laramie counties overnight into early Friday morning,
spreading into the southern NE Panhandle later Friday morning. That
said, High Wind Watches for areas along and east of I-25 were upgraded
to Warnings. High Wind Warnings west of I-25 were extended until 5
PM Friday as the post-frontal gradient will be tight enough to produce
periodic low-end high wind gusts through the afternoon.

Besides the high winds, strong orographic lift will be sufficient to
generate moderate to briefly heavy snowfall over the southern WY 
mountains. The 18Z WPC QPF came in a tad higher for the Snowy Range
with 3 to 7 inches, hence the Winter Weather Advisory for this 
zone. The Sierra Madre Range can expect 5 to 10 inches from late 
tonight through Friday afternoon when the snow begins to taper off.
Winds gusting to 60 MPH will produce low visibility in blowing snow.

Friday's high temperatures will occur early in the morning prior 
to FROPA, then fall in the post-frontal airmass through the rest
of the day. Winds will diminish Friday night, with overnight low
temperatures bottoming out in the single digits and lower teens.
Saturday's high temperatures will moderate slightly, with 30s to 
lower 40s. Shortwave energy embedded within the southwest flow
aloft will move into southern WY Saturday afternoon and evening.
This feature will aid the orographic lift for snow over the
Snowy/Sierra Madre ranges with 1 to 3 inches possible.

.LONG TERM...(Sunday through Thursday)
Issued at 310 PM MST Thu Dec 1 2022

Models have been holding steady in the long term for potential high 
winds in the wind prone areas of southeast Wyoming, accumulating 
mountain snow, and an artic front trying to creep its way into the 
CWA. 

A near zonal jet will lead to an active pattern for the mountains. 
Continuous upslope flow and good 700 mb moisture will lead to 
prolonged orographic snow showers for the Sierra Madre and Snowy 
Ranges. Although the newest 12Z deterministic models have toned down 
slightly on QPF amounts, the long duration of the event will likely 
lead to solid accumulation amounts. Between Saturday night and Monday,
the Sierra Madres could see Warning level accumulations and the 
Snowys will likely be shadowed and see Advisory level accumulations.

Of course, it would not be a Wyoming forecast without the mention of 
high winds. Low pressure develops in lee of the Laramie Range causing
pressure gradients to tighten along and west of the Laramie Range
Sunday night. 700 mb winds will increase Sunday night and into the
day Monday. Could see a brief high wind event during this time as
55 kt winds exist over the wind prones with great downward omegas.
In-house guidance is already picking up on the potential for high
winds during this time as well. Next high wind event comes Tuesday
night into Wednesday. This event has a better signal for the wind
prones with even stronger winds aloft and steeper CAG-CPR 700 mb 
height gradients.

Still keeping an eye out on the potential back-door arctic front 
Monday night. Deterministic ECMWF wants to bring the colder air 
farther south than the GFS. Regardless, both ensembles have plenty 
of members showing sub-freezing temperatures for highs on Tuesday, 
mainly for the northeastern corner of the CWA, including Douglas and 
Chadron. As a result, Tuesday will likely be the coldest day of the 
extended forecast.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 404 PM MST Thu Dec 1 2022

Strong winds for our southeast Wyoming airports KRWL and KLAR this
evening. Strong winds still expected to spread east across the 
Laramie Range tonight into Friday. Will likely need to increase 
winds at KBFF and KSNY on the 06Z TAFs tonight for Friday 
afternoon. Snow develops out by KRWL and KLAR early Friday 
morning...leading to IFR/VLIFR conditions.

&&

.FIRE WEATHER...
Issued at 310 PM MST Thu Dec 1 2022

Strong southwest winds this afternoon and tonight will shift to 
the west with the passage of a cold front Friday. Gusts in excess
of 60 MPH are likely for much of Southeast Wyoming and the southern
Nebraska Panhandle. Snow will develop along and behind the front 
late tonight and Friday morning, tapering off in the afternoon. 
Cool daytime temperatures combined with snow cover will keep 
minimum relative humidities well above critical levels through the
next 48 hours. An active weather pattern will produce periods of 
strong winds and chances for snow this weekend into early next week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 5 PM MST Friday for WYZ101-103>106-109-
     110-113-115>117.

     High Wind Warning from 2 AM to 5 PM MST Friday for WYZ107-108-
     118-119.

     Winter Weather Advisory from 11 PM this evening to 5 PM MST 
     Friday for WYZ112-114.

NE...High Wind Warning from 8 AM to 5 PM MST Friday for NEZ019>021-
     054-055.

&&

$$

SHORT TERM...MAJ
LONG TERM...SF
AVIATION...GCC
FIRE WEATHER...MAJ