Winter Weather in Wyoming, December 16 to 17, 2021

Area Forecast Discussion
National Weather Service Cheyenne WY
1040 AM MST Thu Dec 16 2021

.SHORT TERM...(Today - Saturday)
Issued at 423 AM MST Thu Dec 16 2021

Wind gusts have diminished below warning criteria across the south
east Wyoming wind corridors early this AM. Despite a few lingering
gusts approaching 50 MPH near Arlington, we have allowed High Wind
Warnings to expire for these areas. 700 & 850 mb CAG-CPR gradients
remain elevated around 50 meters through 18z, but probabilities of
high wind exceedance to decrease over the next few hours per local
guidance based on GFS inputs. Expect to see continued gusts around
50 MPH until 18z per local enhancements, but a breezy day is still
expected courtesy of steep lapse rates along with widespread 30 kt
flow in the 700-800 mb layer.

Morning GOES-16/17 water vapor satellite imagery already shows the
next significant short wave digging into northern California. This
disturbance will move rapidly east today w/ dynamic lift likely to
increase markedly by 00z Friday. Orographics should favor mountain
snow for the Snowy and Sierra Madre ranges. The heaviest snow will
likely be favored in the Sierra Madres courtesy of deep, moist WSW
flow in place. Model QPF averaging 0.3 to 0.5 inch w/ expected SLR
values up to 20 to 1 suggests potential for 5 to 10 inch snow fall
amounts in the high country. Have issued a Winter Weather Advisory
for both mountain zones from 5 PM today until 8 PM MST Friday.

What comes next should surprise absolutely no-one. Any guesses? It
could not possibly be more wind, could it? Unfortunately, both 850
and 700 mb CAG to CPR gradients are set to climb back into the 65-
75 meter range late Friday night through Saturday. Arlington & Elk
Mountain will quickly respond by ramping the gusts up yet again to
65-75 MPH. In-house probabilistic guidance points toward a low end
potential of reaching high wind criteria late Friday night, before
peaking around 60 percent Saturday afternoon. BRX along I-25 could
see higher gusts as well, assuming the surface trough advances far
enough to the east.

Temperatures over the next few days may be on the cooler side with
a very active pattern bringing several disturbances across the CWA
through the weekend. In general, look for daytime highs 35-45 over
the eastern plains with upper 20s or low 30s for areas to the west
of the Laramie Range. Low temperatures will fall to 8-12 deg F for
many on Friday night with the passage of a cold front.

.LONG TERM...(Saturday Night - Wednesday)
Issued at 423 AM MST Thu Dec 16 2021

Much of the long term forecast has remained on track with minor
adjustment required, mainly in the wind fields. Mostly dry
conditions, outside of some mountain snow possible, and gusty to
strong winds expected in the long term forecast, with an upper level
ridge pushing eastward from the west coast. Temperatures should
continue to make a recovery in the high 40s to low 50s on Sunday,
staying above seasonable averages through late next week. Upper
level ridging pattern will quickly flatten out as it treks eastward,
with shortwaves disturbances embedded within the flow pulsing
through the region. Not expecting much out of these, outside of
kicking up wind speeds to high wind criteria in our wind prone areas
along the I-25 corridor. Specifically looking at Saturday night into
Sunday morning, NBM v4.1 Probability of Exceeding 48kts indicate a
high likelihood of stronger winds along near the Central Laramie
Range and eastward into Bordeaux. CAG-CPR gradients have stayed
consistent with this feature jumping from 60 to 70 meters on
Saturday evening and upwards to 80 meters through Sunday morning. As
a result, went ahead and started trending wind speeds towards
criteria wind gust levels for Bordeaux and started nudging those
speeds up for the Laramie Range as well. With the strong westerly
winds, increase in temperatures is likely with a good downsloping
component, and almost a 10 to 15 degree difference in temperatures
for areas west of the Laramie Range compared to areas further
eastward. Upper level ridging is progged to become nearly stationary
and encompass most of the CONUS, with an upper level trough
developing just off the western seaboard, effectively halting any
significant weather into the region through Midweek. Next round of
mountain snow is expected Late Wednesday into Thursday, with decent
energy from the Pacific pushes eastward and a fair amount of
moisture advection across the Rockies.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1035 AM MST Thu Dec 16 2021

VFR conditions expected for area terminals through the forecast
period with gusty winds this afternoon for SE WY terminals. While
KCYS could see gusts around 20 kts, KRWL has already observed
gusts near 40 kts late this morning. Expect these gusty
southwesterly winds to continue this afternoon. KRWL is the only
site that could also see brief reductions in flight conditions
with snow shower activity this afternoon and evening. Showers
should be brief and most should remain over the higher terrain
south of KRWL so retained VCSH from the previous TAF. Otherwise,
BKN upper level clouds expected this evening and overnight across
area terminals before clearing out tomorrow morning.

&&

.FIRE WEATHER...
Issued at 423 AM MST Thu Dec 16 2021

No fire weather concerns given cool temperatures & fuels generally
not favorable for large fire growth.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 5 PM this afternoon to 8 AM MST
     Friday for WYZ112-114.

NE...None.
&&

SHORT TERM...CLH
LONG TERM...MD
AVIATION...MB
FIRE WEATHER...CLH

Hazardous Weather Outlook
National Weather Service Cheyenne WY
623 AM MST Thu Dec 16 2021

WYZ106-110-116-117-171330-
Central Laramie Range and Southwest Platte County-
North Snowy Range Foothills-South Laramie Range-
South Laramie Range Foothills-
623 AM MST Thu Dec 16 2021

This hazardous weather outlook is for portions of south central
Wyoming and southeast Wyoming.

.DAY ONE...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

Strong winds are expected for the wind prone areas of southeast
Wyoming from Friday night through Saturday night.

.Spotter information statement...

Spotter activation will not be needed.

WYZ112-114-171330-
Sierra Madre Range-Snowy Range-
623 AM MST Thu Dec 16 2021

This hazardous weather outlook is for portions of south central
Wyoming.

.DAY ONE...TODAY AND TONIGHT

Mountain snow is expected this evening through early Friday. A
Winter Weather Advisory is in effect.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

The probability for widespread hazardous weather is low.

.Spotter information statement...

Spotter activation will not be needed.

NEZ002-003-019>021-054-055-095-096-WYZ101>105-107>109-111-113-115-
118-119-171330-
Dawes County-Box Butte County-Scotts Bluff County-Banner County-
Morrill County-Kimball County-Cheyenne County-
Northern Sioux County-Southern Sioux County-
Converse County Lower Elevations-Niobrara County-
North Laramie Range-Ferris/Seminoe/Shirley Mountains-
Shirley Basin-East Platte County-Goshen County-
Central Carbon County-Southwest Carbon County-
Upper North Platte River Basin-Laramie Valley-
Central Laramie County-East Laramie County-
623 AM MST Thu Dec 16 2021

This hazardous weather outlook is for portions of panhandle
Nebraska...east central Wyoming...south central Wyoming and
southeast Wyoming.

.DAY ONE...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

The probability for widespread hazardous weather is low.

.Spotter information statement...

Spotter activation will not be needed.