Heavy Snow and High Wind in Utah, December 1 to 4, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
922 PM MST Thu Nov 30 2023

.UPDATE...Made minor updates to the going forecast, mainly to
increase PoPs across southwest Utah where snow showers have
produced spotty accumulations. Also maintained haze through the
night across some northern and central Utah valleys where
visibilities remain around 5 miles and there is little reason for
significant improvement before the next storm arrives tomorrow.

&&

.SYNOPSIS...Unsettled conditions will develop through the weekend
as a series of storm systems impact the area, bringing potentially
significant snow, especially for northern Utah. High pressure is
expected to return to start the upcoming work week.

&&

.SHORT TERM (Through 00z Monday)...Early afternoon satellite and
H5 analysis indicates a broad long wave trough encompassing much
of the CONUS, with its western periphery extending upstream across
the eastern pacific. A strong upper level jet can be noted off
the PacNW coast...trends encroaching southeastward...with the
first of a series of short wave troughs embedded impinging on the
Oregon coast attm. A sharper short wave is just upstream of this
feature, with orientation near 145W. These waves are expected to
translate across the northern Great Basin and into namely
northern/central portions of the forecast area Friday, and again
Friday night, bringing light to at times moderate snow to
valleys/mountains alike...with snow most widespread during each of
the mid level axis passages (Friday afternoon and again later
Friday night). Anticipating our first somewhat widespread
potential for low elevation road snow/slush impact along the
Wasatch Front given cold antecedent conditions at precip onset,
namely late Friday into Saturday morning as the aforementioned
trailing wave crosses the region. Have issued winter weather
advisories for the northern valleys due to this potential, even
though accumulations should remain on the light side, at least
initially.

The mode of this upcoming unsettled period begins to shift later
Saturday, and especially leading into Sunday, as a transition
from consistent (but modest) cold advection from these first two
waves phases neutral, then into fairly stout warm advection for
the remainder of the event (Sunday on into Monday morning).
Coincident with this is decent IVT penetrating inland and across
northern Utah within an anticyclonic northwesterly flow aloft,
this remnant from the atmospheric river impacting areas upstream.
Favorable jet support aloft coupled with overrunning of cold
pools in place, and upglide coincident with this moist WAA, will
maintain heavy at times precip Sunday into the overnight hours
across northern Utah. Heavy mountain snow with increasing density
trends can be expected across the central/northern Wasatch on
into the Bear River mountains yielding our first decent thump this
early winter (20-30" storm totals, 1.5-2.5" SWE)...have issued a
winter storm warning has been issued for these mtn ranges as such.
Have also issued a warning for the Wasatch Back as favorable
flow/overrunning is expected to produce high enough rates to
maintain isothermal snowfall through Sunday, even when temps
aloft warm enough to not usually support it. Storm totals of
12-18" is not out of the question for the Liberty/Eden areas,
lesser amounts further south into the Coalville/Park City regions.

The aforementioned warm advection will eventually pose a p-type
challenge with lower valley snow transitioning to more of mixed
precip and eventually rain along the Wasatch Front first, then the
Cache. The challenge with this is primarily tied to timing of
this eventual occurrence, and additional snow accumulations will
be directly tied to the switchover. In large, expecting a south to
north transition with the Salt Lake/Utah Valley areas potentially
mixing by Saturday night, the northern Wasatch Front sometime
Sunday morning, and finally the UT/ID border region and Cache
sometime Sunday afternoon. Still, uncertainty exists with this
timing, as PI rates will heavily influence maintenance of
isothermal snowfall potential, or lack thereof. Expiration of the
winter weather advisories are largely tied to this anticipated
timing, but could easily see some areas transitioning away from
snowfall sooner...especially south of Ogden. Future shifts will
keep tabs on this.

Winter weather advisories were also issued for the western Uintas
and central mountain for modest snow accumulations during this
period, with expirations occurring sooner across the central
mountains due a the moist WAA period becoming more focused across
the north in time.

Lots of details/subtleties were needed to be dissolved in this
short term forecast...and I'm looking forward to seeing how it
all plays out in time. Regardless, it will be a net positive for
building on our early season mountain snowpack for many reasons!
This said, if planning on getting out in the backcountry, make
sure to keep tabs with the latest observations and forecasts from
the Utah Avalanche Center as this winter season is ramping up.

.LONG TERM (After 00Z Monday)...Beginning of the long term picks up
at the tail end of this weekends storm. Some valley rain showers and
mountain snow showers could continue into the morning hours on
Monday. Not much is expected in the way of snow accumulation, maybe
a few inches during this time period.

After this storm moves out of the area by Monday morning, there
continues to be good agreement in both deterministic and ensemble
guidance that a ridge will be back in place through the middle of
the week. With this strong subsidence in place, shown by the
ensemble mean 700mb temperature of +3C by Wednesday, inversions
will return, especially in valleys that have snow cover. Compared
to the current inversion conditions, the ridge seems to be
stronger as forecasted temperatures are between 5F and 10F above
normal. These temperatures though would be affected by snow cover
and inversions if one or both of these are in place. Towards the
end of the week, guidance is honing in on another trough moving
through the Great Basin area that could get rid of any inversions
in place.

&&

.AVIATION...KSLC...MVFR conditions owing to a low stratus deck
and hazy conditions will gradually transition to periodic VFR
conditions this evening/overnight as CIGs and VIS rise ahead of
tomorrow's storm. VIS will remain between 5-6SM as inversions don't
completely clear overnight, and there is ~30% chance VIS goes <3SM
briefly overnight. Winds will remain light and variable overnight
with a rogue light snow shower possible (~10% chance).

.REST OF UTAH AND SOUTHWEST WYOMING...Across northern Utah, a low
stratus deck and hazy conditions will gradually improve overnight as
our next storm approaches. This will transition MVFR conditions to
periodic VFR conditions as CIGs and VIS rise, but mountain
obscuration will persist. Snow could begin across northwestern Utah
as early as 12Z. Winds will remain light and variable. Across
southern Utah, mainly VFR conditions with brief periods of reduced
categories owing to isolated mountain snow showers. CIGs <7kft will
keep mountain tops obscured.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 11 AM Friday to 11 AM MST Sunday
     for UTZ103-104-107-109.

     Winter Weather Advisory from 11 AM Friday to 5 AM MST Sunday for
     UTZ105-106-113-117.

     Winter Storm Warning from 11 AM Friday to 5 PM MST Sunday for
     UTZ108.

     Winter Storm Warning from 11 AM Friday to 5 AM MST Monday for
     UTZ110-111.

     Winter Weather Advisory from 11 AM Friday to 5 AM MST Monday for
     UTZ112.

WY...None.
&&

UPDATE...Cheng
PREVIOUS DISCUSSION...Merrill/Cecava/Mahan

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http://weather.gov/saltlakecity