Heavy Snow and High Wind in Utah, December 1 to 2, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
511 AM MST Thu Dec 1 2022

.SYNOPSIS...Southwest flow aloft will increase across the area
today ahead of the next winter storm which will push a strong
cold front through the state Thursday night into Friday. Unsettled
weather then continues through the weekend and into early next
week.

&&

.SHORT TERM...Satellite imagery early this morning depicts a broad
trough moving onshore to the West Coast, with a notable baroclinic
leaf extending from Northern California to Montana. The southern
extent of the feature of colder cloud tops is just beginning to
brush far northern Utah. Area radars show that precipitation
associated with this feature remains upstream (north) of our area
as of this writing. At the surface, south winds have picked up for
many locations in advance of the trough approaching our area. As
is common with overnight winds in complex terrain, the elevated
winds aloft have not mixed down to all locations, with some
valleys remaining decoupled and cold while other spots have seen
gusts in the 50 mph range and remain much warmer as a result. For
example, the Logan airport had temperatures in the lower 20s
earlier tonight but has since shot up to the mid 40s as south
winds developed overnight.

The aforementioned upper-level trough will continue to push inland
today into tonight, with IVT values showing appreciable moisture
transport across the Great Basin and into our area despite a
trajectory intersecting the Sierra Nevada. HREF means show a wide
area of 50kt+ winds ahead of the trough axis as it pushes into
our area, especially along and west of the central spine of Utah
mountains. Based on yesterday's observations, expect these winds
will have no problem mixing to valley floors along and west of the
I-15 corridor. Western valleys are currently under a wind
advisory as a result starting at 11am, which may have to be
started sooner by the next shift based on overperforming winds
overnight. The HREF means also depict a secondary 700MB jet
developing over southern Utah tonight in advance of the cold
front. With advisory-level winds now forecast from roughly Kanab
to Hanksville, have issued another wind advisory to cover that
area as well Friday night. The lower elevations of southern Utah
such as Zion NP to St George and the Lake Powell vicinity are not
under a wind advisory as of yet; guidance strongly suggests that
elevated winds aloft will not be able to mix down to the surface
in these areas, although this will be another area to keep an eye
on.

Cloud cover will increase north to south today as the upper-level
trough approaches, with precipitation beginning over the higher
terrain east of I-15 by later in the day. A vigorous cold front
will then cross into northwestern Utah near midnight Friday and
race through the I-15 corridor to Cedar City by sunrise. This
cold front is expected to have a band of moderate to heavy snow
(exceeding 1 inch/hour rates) and potential to create snow squall
conditions for 1-3 hours. There is a chance that valley
locations below 5000 feet could start as a rain/snow mix then
quickly change to all snow, though the 00Z HREF precip type is
favoring all snow. With surfaces being cold and high precipitation
intensities, expect that snow will readily accumulate on paved
surfaces.

Tonight's CAMs have trended quicker overall with the cold front
passage, moving it well before the morning commute for northern
Utah / southwest Wyoming, and about in line with the typical
commute time from about Fillmore southward. That said, a major
lingering uncertainty is how long precipitation continues after
the front passes. The HRRR including its 6Z cycle continues to
suggest a broad area of lingering post-frontal precipitation for
central and especially northern portions of the forecast area,
lasting for 3-6 hours post frontal. This would have major
implications as to whether the transportation impacts linger into
the morning commute (including a potential large contingent of
folks heading south on I-15 for the Pac-12 Championship Game).
Due to this uncertainty and the need to coordinate with
transportation partners arriving during daylight hours, have opted
to leave the inherited Winter Storm Watch highlights as is until the
aforementioned collaboration can take place. The quick-hitting
scenario favored by the FV3/NAMNest would lend itself to more
advisory-level impacts, while the longer-lasting scenario of the
HRRR might suggest warning-level impacts.

In terms of snow amounts, we are still looking at a most likely
forecast of 2 to 5 inches for northern valley floors and 4 to 8
inches for benches, the Wasatch Back (except up to 12 inches Park
City) and Uinta County, WY. For the northern mountains, a general
12 to 18 inches, with locally higher amounts. Central and southern
mountains will see closer to 6 to 12 inches. Valleys of central
and southern Utah are looking at 1-4 inches. Areas east of the
central spine of Utah mountains (e.g. Uinta Basin southward to
Lake Powell) will be lucky to pick up much more than a trace of
precipitation).

The trough axis fills and moves eastward quickly Friday, causing
any lingering orographic precipitation in the mountains to
rapidly diminish by the early afternoon hours. Any hints of lake
effect in CAMs also wind down quickly with drier air cutting off
the potential. A much colder air mass then settles into the area
(700MB temps on the order of -15 to -18), with a cold night in
store Friday night. There are some hints of an easterly wind event
Friday night from Ogden to Evanston that will need to be
monitored.

.LONG TERM (After 12Z Saturday)...The forecast for this upcoming
weekend has changed resulting from impacts from an area of low
pressure off the west coast. Saturday now brings a swatch of
atmospheric river moisture in from the southwest, which looks to
impact southern Utah initially and then spread northward through the
day. Snow levels across southern and central Utah increase into the
range of 7,000 to 8,000 ft across the southern mountains which would
lend itself to a mixture of rain and snow in areas (even in the
mountains). The forecast for Sunday is quite a challenge. Since
yesterday, the forecast for this day has switched with Saturday and
it potentially may be the one day's break in between weather
systems. That said, it's just as likely (50/50 split) that a mixture
of rain/snow will continue across the state. Thus making the
forecast a challenge and our confidence in this forecast lower.

A favored mountain snow/valley rain event remains in the forecast
for late Sunday into early Monday across much of the state while the
low makes its way inland. The exact timing of when this occurs does
vary between ensembles with the majority indicating the low moving
inland Monday morning across northern California and about 30%
indicating a slower, more gradual progression inland further south.
Currently, 50 to 80 percent chances of receiving 6 inches or more of
snow along the Wasatch Mountains are forecast for this event. Weak
high pressure is forecast to build into the region in the middle of
the week which would allow for drying conditions.

&&

.AVIATION...KSLC...Windy southerly winds are forecast for much
of the day with wind gusts around 35 knots. VFR conditions are
expected though CIG will be lowering. Snow showers are forecast to
be in the vicinity of the terminal as early as 02Z this evening. A
band of heavier snow is forecast to arrive around 09Z, quickly
dropping conditions to LIFR and shifting wind directions out of
the northwest. Wind gusts of 30 knots or higher are forecast along
this band and blowing snow is possible as a result. The heavier
band is expected to be relatively short lived with improving winds
within an hour or two but lowered CIG and VIS is likely to
persist through the early morning hours. Improving conditions are
forecast mid to late morning.

.REST OF UTAH AND SOUTHWEST WYOMING...Breezy conditions are forecast
mid-morning after LLWS early this morning across much of northern
Utah and southwest Wyoming. Look for gusts in the 25 to 30 knot
range during the day with wind gusts upwards of 40 to 50 knots
across southwest Wyoming. In these same areas, a band of heavy snow
is forecast to arrive between 06Z and 09Z, which will quickly and
briefly drop conditions to IFR/LIFR conditions. This band is
expected to reach the southern end of the Wasatch Front between 09Z
and 11Z. In addition to snow/heavy snow, blowing snow is also
possible. Improving conditions are forecast mid to late morning.

Across southern Utah terminals, breezy southerly winds are expected
to develop mid-morning with VFR conditions. A snow shower may appear
in the vicinity of BCE later this evening.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from noon today to 1 AM MST Friday for UTZ101-102-
     105-115-122.

     Winter Storm Watch from this evening through Friday morning for
     UTZ102>109-113-116-117-125.

     Winter Storm Watch from this evening through Friday afternoon
     for UTZ110>112.

     Wind Advisory from 10 PM this evening to 10 AM MST Friday for
     UTZ128-130.

WY...Winter Storm Watch from this evening through Friday morning for
     WYZ021.

&&

SHORT TERM...Van Cleave
LONG TERM...NDeSmet
AVIATION...NDeSmet

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