Winter Storm Warning 1, NWS Salt Lake City, UT

Beaver County, UT, Box Elder County, UT, Cache County, UT, Carbon County, UT, Davis County, UT, Duchesne County, UT, Emery County, UT, Garfield County, UT, Iron County, UT, Juab County, UT, Kane County, UT, Millard County, UT, Morgan County, UT, Piute County, UT, Rich County, UT, Salt Lake County, UT, Sanpete County, UT, Sevier County, UT, Summit County, UT, Utah County, UT, Wasatch County, UT, Washington County, UT, Wayne County, UT, Weber County, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
354 PM MST Wed Jan 4 2023

.SYNOPSIS...High pressure will slowly deteriorate across the
eastern Great Basin as another storm system begins pushing into
the region. A mix of rain and snow is expected across much of the
area late Thursday through Friday. High pressure attempts to
rebuild thereafter, but will struggle as an active Pacific storm
track is maintained.

&&

.SHORT TERM (Through 5PM Friday / 00Z Saturday)...Mid-level
ridging has build into the region today alongside a brief period
of drying. Miraculously, the sun has shined on much of the area
today, with the exception of the northern Bear River range and
Bear Lake area. Water vapor imagery, however, shows increasing
mid and upper level moisture pushing into the area which is
leading to an influx of mid and high level cloud cover.
Additionally, an area of weak showers have formed in southwestern
Utah along the leading edge of moisture in the lower levels. This
area of showers will continue as it tracks north and east, but
will gradually weaken under increasing subsidence as the ridge
axis moves overhead.

With the ridge axis downstream of the forecast area by Thursday
morning, an avenue of increasing moisture will open as deeper
southwesterly flow establishes ahead of an upstream trough. A deep
offshore trough/ cutoff low currently noted near 135W will push
inland through the morning tomorrow, helping to increase both
moisture and forcing over the eastern Great Basin as we head
through the day. Model trends continue to favor the best moisture
availability and paired upper forcing over the southern half of
Utah as the inbound trough splits, especially from about 8PM
through 2AM. during this brief period, thinking that heavy, wet
snowfall will be possible for elevations above 6000ft. Initially,
warm advection ahead of the trough axis will help bump snow levels
up to ~7000ft, but those levels should quickly drop as the axis
swings through and daytime solar influence decreases. Colder
temperatures in the wake of the trough axis will begin to favor
snow levels to valley floors outside of lower Washington County,
but moisture will be limited, so any significant valley
accumulations seem unlikely at this time.

For the northern half of the forecast area, moisture will still be
readily available for solid mountain snow. As stated above, best
dynamics will be favored for southern Utah, but the splitting
trough should maintain enough energy across the northern tier to
favor a brief period of heavy snowfall for the mountainous
terrain. Thinking that a widespread 5-10 inches still seems well
within reason across the northern Utah mountains, will upwards of
10-14 for the higher central Wasatch Peaks. Valley areas west of
the Wasatch Crest, similar to southern Utah, will see rain
initially in the pre- trough axis/ pre- frontal environment,
transitioning to an environment ripe for valley snowfall early
Friday morning. That said, as the axis of the trough moves
downstream, access to deep moisture will move downstream as well.
Colder, northwesterly flow and trailing shortwave energy will have
to work with lingering lower and mid- level moisture, which
should work out well for the mountainous terrain. Valley areas are
likely to see more showery type weather as broad synoptic forcing
will be lacking, but still thinking there is strong possibility
for a snowy morning commute on Friday.

For the Wasatch Back the story becomes a little different. While
snow levels will begin initially higher than valley floors, stable
conditions and recent snowfall seem to have helped develop strong
valley cold pools which may be difficult to overcome with the warm
advection period. Have maintained a forecast of rain-snow mix
during the first half of the round of precipitation for the lower
lying areas of the Wasatch Back considering this possibility,
trending toward snowfall overnight. Thinking that advisory level
accumulations could come about in this scenario. A similar
scenario with strong valley cold pools and potential for the event
to be entirely snow exists for the western Uinta Basin... have
similarly maintained advisory level snowfall in this area.

Have decided to hold off on the issuance of headlines with this
forecast package, but anticipate updated headlines, at least over
the southern mountains, as the overnight shift gets a solid look
at the latest model trends.

Mid-level drying is anticipated through the morning hours on
Friday, bringing a transition to primarily orographically driven
precipitation through the remainder of the day. During this period
from ~11AM through 5PM Friday, colder temperatures will help
provide potential for lower density snowfall. So while lighter
precipitation intensity/ water amounts are anticipated, snowfall
should be able to stack up through the evening on Friday.

.LONG TERM (After 5PM Friday / 00Z Saturday)...Our next storm will be exiting the
region to start the long term period. A few slopes favored in
westerly flow could still have some lingering snow showers, but the
air mass will be quickly drying ahead of a transient ridge. This
ridge will keep the weekend mostly dry before a return to unsettled
weather late Sunday into early next week. A more potent storm
appears to arrive by midweek, however uncertainty on arrival time
and intensity does exist.

As the ridge builds in this weekend the CWA will be under
southwesterly flow with temperatures rising to near average, if not
slightly above across northern Utah. A return of moisture will
combine with weak ascent from a shortwave to bring precip chances
back late Sunday into Monday. QPF amounts appear unimpressive and
mostly confined to higher elevations. This is denoted in the fact
that 85% of ensemble members have drier solutions compared to just
15% presenting wetter solutions.

This shortwave comes ahead of a broader trough that will bring
another significant AR event to the west coast. The storm looks to
weaken and lose some moisture to terrain as it pushes ashore. The
storm is set to arrive across our CWA some time midweek, but
uncertainty exists in the arrival time as well as how much moisture
will accompany the trough. 60% of ensemble members suggest an
earlier arrival time with precipitation beginning statewide on
Tuesday. The remaining 40% hold off until Wednesday. Similar to the
past couple of storms this one looks to start as valley rain and
mountain snow with a possible transition to valley snow on the back
end if any moisture remains once colder air arrives. Expect
temperatures to remain near average throughout the period as cold
continental airmasses will continue to elude us in favor of warmer
coastal airmasses.

&&

.AVIATION...KSLC...VFR conditions with south winds will prevail
through the evening and overnight hours. There is a 20 percent
chance cigs lower below 6 kft between 03-09Z. South winds will
become gusty Thursday morning with a 40 percent chance gusts
exceed 25 kts after 18Z.

.REST OF UTAH AND SOUTHWEST WYOMING...Predominant VFR conditions
will prevail across the area through tonight. Cigs will begin to
lower below 6 KFT across most areas late Thursday as the next
storm system approaches the area.

&&

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

WY...None.
&&

Webber / Mahan / Seaman

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