Winter Storm Warning 12, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
336 AM MST Sat Dec 10 2022

.SYNOPSIS...Another winter storm will impact the region Sunday
into early next week, bringing the potential for heavy mountain
snow and accumulating valley snow.

&&

.SHORT TERM (Through 00Z Wednesday)...Another spicy short term
forecast to discuss this morning...with the potential for
significant mountain snow and even some heavy valley snow. It
really hinges on a few specific details.

Morning upper air and satellite analysis indicates an active polar
jet with a 110+kt jet maxima approaching the California Coast. A
~520 dam upper level low is located near the British Columbia
coast. This upper level low and associated shortwave trough will
continue to shift south and east today into Sunday.

First, southern Utah...by Sunday afternoon, strong PVA and upper
level diffluence will provide more than sufficient forcing for an
area of heavy precipitation across southwestern Utah, shifting
east with time through Monday morning. This area of precipitation
will be associated with a substantial IVT anomaly, following the
preferential around the southern Sierras pathway for heavy
precipitation across southwestern Utah.

Areas most prone to heavy precipitation during the ~Sunday 20Z to
Monday 09Z period will be related to those areas that typically
see enhanced southerly flow orographic enhancement such as the
Pine Valleys and near the Brian Head area. This could bring a
quick 8 to 15 inches of snow (or more) to portions of the
southern mountains during this timeframe.

Snow levels are expected to remain near to above 5000-5500 feet
during this time owing to the relatively "warm" air associated
with this atmospheric river event. This will reduce any threat of
accumulating snow to the southern valleys. Precipitation across
southern Utah is largely associated with this period of enhanced
forcing and comes to an end in much of the guidance by Monday
afternoon or early Monday evening.

For northern Utah...expect broad synoptic lift to also support an
area of moderate to heavy precipitation across northern and
central Utah, beginning Sunday afternoon into Sunday evening. IVT
anomalies will also be elevated across northern Utah...though
not on par with southern Utah. Snow may be heavy at times,
especially for higher elevation areas favored in southerly flow.
Snow levels through Sunday evening will remain near 5000-5500 feet
across northern and central Utah.

Now comes the somewhat lower confidence portion of the storm
across northern Utah. There are two main solutions in the guidance
envelope. Solution 1 brings the main upper level low across
northern/central Utah Monday into early Tuesday. This sets up
northern Utah to be in an area of prolonged wraparound under the
main low...with moist westerly 700mb flow aiding in orographic
upslope. Some portion of this solution set extends this
precipitation to central Utah. This solution represents a
significant portion of the GEFS and Canadian ensemble systems. If
this solution were to occur, precipitation may continue through
the day Monday, Tuesday and even potentially into Wednesday
morning. Note this doesn't mean it would snow constantly this
entire period, but there would be periods of light snow. This
solution also represents the heaviest threat for snow for the
northern Utah valleys, with snow levels likely falling to valley
floors Sunday night.

Solution 2 is represented in a minority of the guidance envelope,
though it is heavily represented in the EPS. Instead of crossing
northern/central Utah, the upper level low follows more along the
Snake River Plain, with the wraparound precipitation remaining
well north of Utah...and thus this solution is much drier. This
solution set would largely end precipitation across much of the
state Monday morning, with the potential for an area of light
precipitation Tuesday.

The current forecast is heavily leaning toward solution 1...which
would be a much heavier snow for both the northern and central Utah
valleys and mountains. Northern valleys could see 4 to 8 inches,
with higher amounts on benches. Northern mountains could see 1 to
2 feet, with locally higher amounts. If the outcome is closer to
solution 2, these amounts will be considerably lower. So while
these numbers are exciting if you love snow, keep in mind that
solution 2 is still a possibility.

As far as impacts from wind, ensemble systems indicate a high
probability (greater than 60%) of at least a period of wind gusts
in excess of 45 mph across the western valleys Sunday morning
into Sunday afternoon. While the models have been trending later
with development of precipitation Sunday (and thus increasing the
threat of 50+kt 700mb winds mixing down across western
Utah)...don't see a sufficient threat for gusts in excess of 58
mph to issue a high wind watch with this package.

Winter weather headlines will be coordinated with UDOT and issued
with the package later today.

.LONG TERM (After 00Z Wednesday)...Beginning of the long term
will have some lingering snow showers in the northern UT mountains
from the storm that will effect the area early in the week. These
snow showers are expected to end sometime Wednesday morning. In
the wake of this system leaving our area, very cold temperatures
will follow. High temperatures will be 10 to 15 degrees below
normal. Low temperatures are expected to be in the single digits
for high elevation locations and the teens for most other
locations. Lower Washington County will see low temperatures in
the mid to upper 20s.

From Thursday through the rest of the long term period, the
forecast remains quite uncertain. The GFS, EC, and CMC
deterministic models all have another storm hitting our area but
the timing is off for when it will effect the CWA. Ensemble
guidance is keying in on another storm developing off the CA
coast, however the strength of this storm is still uncertain.
Currently only 40% of all ensemble members support a wet solution
while the other 60% support a drier solution. Regardless of what
is to come at the end of the long term, one thing is certain and
that is temperatures will be cold and well below average during
this period.

&&

.AVIATION...KSLC...South to southeast winds will continue for the
TAF period. VFR conditions expected. CIGS will remain around
10kft for the majority of the forecast period. CIGS are expected
to drop to around 7kft Saturday night into Sunday morning, so
mountain obscuration will be possible during this time.

.REST OF UTAH AND SOUTHWEST WYOMING...Light and variable winds
will continue until the mid morning hours Saturday when southerly
winds will become the dominant direction. Some areas of western UT
may see gusts as high as 40mph. These gusty winds will move
further northward as the day progresses. CIGS are expected to
remain between 10kft and 15kft for the period.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Sunday morning through Monday afternoon
     for UTZ125.

WY...None.
&&

Kruse/Cecava

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