Winter Storm in Utah, December 26, 2021

Area Forecast Discussion
National Weather Service Salt Lake City UT
934 PM MST Sat Dec 25 2021

.UPDATE...
Update this evening to expand wind advisories over a much wider
area. Forecast confidence continues to increase that this will be
a quite windy storm system, both wit pre-frontal southwesterly
winds (across much of the region) and post-frontal westerly winds
(across mainly northern UT and southwest WY). The wind field is
quite impressive, with 50-60 knot 700 mb winds aligned with a
tight surface pressure gradient setting up after midnight night,
peaking just ahead of the surface front mid Sunday morning. While
increasing ascent will fight mixing these 700 winds completely to
the surface, the well aligned surface pressure gradient should
support fairly widespread advisory level (45-55 mph) winds. There
is chance that Milford and Cedar City briefly over perform ahead
of the front early Sunday morning with a few gusts around 60 mph
possible. However, the given the pre-frontal ascent and early day
timing, not confident this will happen, however will need to
monitor this area for possible upgrade to warning.

The surface frontal passage will be forcefully driven from west to
east across northern Utah by the vort max aloft, with the
combination of cold air advection, and strong surface pressure
rises behind the front working to mix the 40-50kt H7 wind downward
in the subsidence behind the front. This will continue breezy to
windy conditions across northern Utah and into southwest Wyoming.
Two locations that could over perform include Wendover and Uinta
County Wyoming. The aforementioned setup can favor downslope winds
in Wendover, however, the gradient remain more west-southwest, and
Wendover does better with downslope in northwesterly winds.
Otherwise, the higher elevation terrain of Uinta County could make
it easier for 50 knots to surface. Will need to keep an eye on
these two areas as well.

&&

.SYNOPSIS...The next in a series of winter storms will impact much
of Utah tonight through Sunday morning. Another storm will cross
the state Monday through Monday night. The active pattern looks
to continue through early in the new year.

&&

.SHORT TERM (Through 12Z Tuesday)...An active short term period is
on tap as a pair of shortwave troughs pass through the region.
Currently the flow aloft is backing in response to the first of
these waves, which is currently deepening as it makes landfall
along the Pacific Coast. As this wave moves inland tonight and
approaches the forecast area precipitation will develop within
orographically favored areas in southwesterly flow regimes, from
the Pine Valley/Brian Head areas northward to around
Sundance/Provo Canyon. Winds aloft will also strengthen,
particularly across southwest Utah, however increasing
precipitation will likely limit these winds somewhat. With colder
air/lower snow levels do not anticipate the same hydrologic
response within the Virgin River system as was observed Thursday
night into Friday. However, lower snow levels will also result in
increased travel impacts, most notably I-15 through Black Ridge
Canyon, and have added a Winter Weather Advisory for this area
through Sunday morning.

The shortwave trough is forecast to quickly lift across the
forecast area Sunday morning, accompanied by a fast moving frontal
boundary. Hi-res models are in fairly good agreement developing a
band of heavy precipitation along this fast moving boundary as it
crosses northern Utah toward sunrise. As such anticipate a quick
burst of snowfall, potentially intense, early Sunday morning.
Given the rapid movement and short duration, anticipate only
minor impacts if any along the Wasatch Front, however where
surface temperatures remain colder along the Idaho border as well
as east of the Wasatch Crest, anticipate a better chance for
accumulating snowfall, and have added Winter Wx Advisories for
these areas as well. Anticipate a 2-3 hour window of heavy
snowfall along the terrain with this boundary and in its wake
before precipitation turns showery by late morning.

With the passage of this wave, a tightening gradient associated
with pressure falls east of the Continental Divide will result in
strong westerly winds across much of the forecast area Sunday,
particularly exposed areas as well as downslope prone areas such
as the western Uinta Basin. As such have added a Wind Advisory for
much of the day Sunday for most eastern valleys along with the
Wasatch Front.

A brief break in precipitation is expected late Sunday into early
Monday, before the next shortwave trough digs into the region.
This wave will be accompanied by a much colder airmass, with the
surface front pushing through northern Utah late Monday, then
through the remainder of the area Monday night. This will bring a
period of accumulating snowfall as it crosses the area, including
potentially some accumulation to the valley floor around St
George.

.LONG TERM (After 12Z/5am Tuesday)...Cold cyclonic flow will be the
dominate weather pattern through the extended period with a couple
of embedded shortwaves rotating through to increase PoP potential.
The first wave will be moving through central and southern Utah
Tuesday for mainly chance PoPs across the mountains and the I-15
corridor. The ensemble members and the operational GFS and EC are in
pretty good agreement with this feature.

The cold trough over the western CONUS gets reinforced as another
shortwave moves southward along the PacNW Coast late Tuesday. This
will cause the winds over Utah to back with weak warm advection
developing late Tuesday night through Wednesday. This should bring
extensive cloud cover across much of the region so have boosted sky
cover in many areas from partly cloudy to mostly cloudy. GFS/EC and
ensemble members are not very enthusiastic for creating precip so
didn't get excited with PoPs either although would not be surprised
if PoPs for Wednesday gets increased for the areas that are favored
by warm advection/southwest upslope flow such as the northern
Wasatch Mtns, Cache Valley, and the southwest portions of Utah,
respectively. Part of the discrepancy in how things will play out
later Wednesday and Thursday will be to what extent of splitting of
the trough will occur over the western CONUS. The EC ensemble mean
is more consolidated without a closed low forming off the southwest
California coast vs the GFS and Canadian which are both showing more
of a closed low feature forming. This farther SW circulation, if
comes to pass, will have less chance to impact the southwestern
portion of the forecast area.

By Friday the ensemble means are back in better alignment,
especially with the next trough dropping southeastward across the
PacNW/Northern Rockies. The operational models are in sync as well
and bring a fairly decent baroclinic zone across the region. Due to
the good agreement of models and ensemble means have boosted PoPs a
tad across the forecast area, especially those areas favored by NW
flow such the Wasatch Front and southward along the I-15 Corridor as
well as all mountains.

Confidence is high that temperatures will below to well below normal
through this extended period. Most areas will have max temperatures
that are 10-15 degrees normal.

&&

.AVIATION...KSLC...The SLC terminal will see VFR conditions
through 08Z before intermittent MVFR/IFR conditions accompany VCSH
ahead of an impending cold front. In addition to the pre-frontal
VCSH, strong southerly winds gusting over 35kts are possible. The
cold front is expected to arrive around 13Z. Behind the front
strong westerly winds along with a quick period of moderate to
heavy snow lasting between 1-2 hours will drop conditions to IFR.
Once this heavy band a precipitation passes conditions will
quickly improve back to MVFR/VFR, but winds will remain strong out
of the west for the remainder of the period along with some
mountain obscuration.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will prevail
over much of the airspace through 06Z before pre-frontal
precipitation begins to spread over the CWA ahead of a powerful
and quick moving cold front passing over central and northern
Utah/SW Wyoming. Conditions will drop to MVFR/VFR when
precipitation is over the airfields. In addition to the
precipitation, strong southerly winds will gust close to 40kts, at
times. This could lead to brief periods of low level wind shear
for KENV, KHCR, KEVW, and KBCE prior and during cold front
passage. The cold front arrives from west to east between 10-15Z
along with a quick burst of heavy snow lasting 1- 2 hours. KCDC,
KBCE, and KSGU will most likely avoid this heavy band of
precipitation, but maintain MVFR/IFR conditions during VCSH
through 18Z. This front, along with most precipitation will be
through the CWA by 18Z. Winds will stay gusty out of the west, but
CIGs will raise to low-end MVFR and VFR, however mountain
obscuration will still be likely for the northern airfields.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 2 AM to 2 PM MST Sunday for UTZ106-115-116-
     119-122-126-127.

     Winter Weather Advisory until noon MST Sunday for UTZ103-107>109.

     Wind Advisory from 2 AM to 7 PM MST Sunday for UTZ102-104-105.

     Wind Advisory from 8 AM to 7 PM MST Sunday for UTZ101-114-121-
     128>131.

     Winter Weather Advisory until 9 AM MST Sunday for UTZ122.

     Winter Storm Warning until 5 PM MST Sunday for UTZ125.

     Winter Weather Advisory until 8 PM MST Sunday for UTZ113-117.

     Winter Storm Warning until 8 PM MST Sunday for UTZ110>112.

WY...Winter Weather Advisory until noon MST Sunday for WYZ021.

&&

Church/Seaman/Struthwolf/Mahan

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