Heavy Snow in Utah, December 27, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
407 AM MST Tue Dec 27 2022

.SYNOPSIS...Utah and southwest Wyoming will see an active weather
pattern develop to close out the year. A mild and moisture rich
storm system will impact the region today through early Thursday,
with another system expected to arrive late this week through the
upcoming weekend.

&&

.SHORT TERM (Through 12Z/5AM Thursday)...A strong Pacific jet and
attendant atmospheric river is currently impinging on
northern/central California, while an associated occluding wave
is moving onshore along the Oregon/Washington coastline. These
features will bring widespread precipitation to the forecast area
through the short term period...the first in a series of moisture
rich systems to impact the region.

Low-mid level moisture advection along the leading edge of this
plume is currently spreading across the northern Great Basin/Snake
River Plain, and will spread across northern Utah mid to late
morning. This will bring a period of valley rain/mountain snow
with snow levels rising above 7000 feet by midday. At the onset of
the event may see a brief period of freezing precipitation across
the Cache, Ogden, and other sheltered valleys before shallow cold
pools mix out by late morning.

Behind initial surge of moisture advection, a window of
relatively enhanced IVT within a mean southwesterly flow aloft
will overspread the forecast area this afternoon and evening,
resulting in a window of widespread valley rain/mountain snowfall,
with rainfall/SWE rates approaching 0.20/hour across
orographically favored locations. Snow levels will remain near 7
kft through the evening, before gradually lowering from north to
south overnight into Wednesday morning, as the upstream trough
translates across the region through Wednesday morning. This will
lower snow levels to the valley floors toward the tail end of the
precipitation event, and as such forecast snowfall totals across
the valleys remain minimal through Wednesday.

In the wake of the mid level trough axis, a favorable window for
orographic snowfall east of the I-15 corridor will result in a
period of additional snowfall accumulation across the higher
terrain through Wednesday evening.

Given the relatively mild nature of the air mass through this
evening, the event looks to unfold as a long duration advisory
level event across the higher terrain. Have expanded this advisory
into the western Uinta Basin where a cold pool remains in place
which should result in snow as the predominant precip type based
on model forecast soundings. Additionally, will need to monitor
the Wasatch Back closely, particularly this evening, as snow
levels may flirt with the valley floor.

LONG TERM (After 12Z/5AM Thursday)...The long term remains
saturated with active weather continuing throughout the period.
Substantial moisture transport coupled with multiple energetic
shortwave troughs embedded in a mean Pacific longwave trough will
drive significant snow/water accumulations. Reiterating some
impressive numbers relative to climatology, the upper Cottonwood
Canyons could see in excess of 10% of annual SWE (nearly 4+ inches)
through day 7 (next Tuesday). For the Pine Valley mountains, nearly
20% of annual SWE is possible over the same time period. Confidence
in this active and wet pattern is high, though given some
inconsistent output run-over-run, confidence in exact amounts is not.

After a brief break in precipitation Thursday morning, the next
round kicks off by midday across the state. Initially, heaviest
precipitation will be confined to the mountains with lighter amounts
in areas requiring better broad scale dynamics. These finally arrive
with the advection of an elongated vort max and breaking down of
upper level zonal flow by Friday morning. Widespread heavy
precipitation across northern Utah, southwest Wyoming, and the
majority of central and eastern valleys is expected. At first look,
this system appears to be much colder at the onset than the
previous, but nuances between the GEFS and EPS highlight the impact
of a slightly faster passage of the thermal trough and quick return
to warm advection behind its axis. This could lead to prolonged
periods of valley rain/snow mix, if not entirely rain. Current temps
and snow levels largely prevent any snowfall accumulation beyond
Thursday night which should be quickly wiped out by heavy rain
Friday morning. That said, will need to watch trends as a shift in
the ensemble mean towards a colder solution could greatly affect
this forecast. Still, the Thursday-Friday event is expected to be
the weakest of this series - still bringing impressive totals as
most valleys rack up another quarter to half inch of water and
mountains in excess of another inch.

Not much of a break in the action between Friday and Saturday as yet
another wave moves into the Intermountain West. Likely another
initially warm system, models lift a broad warm front ahead of the
low, with strong overrunning by a saturated airmass driving periods
of very heavy precipitation all the way from the southwestern border
up through southwest Wyoming. Yet again, the weekend system is
expected to produce substantial snow/water totals as the result of a
southwest to northeast oriented atmospheric river. Current forecasts
follow most likely outcomes from ensemble members, which place the
best moisture transport around the southern flank of the Sierra
Nevada - an ideal orientation. A shift to the north or more zonally
oriented moisture plume could lead to drastic reductions in total
water amounts for the weekend storm.

No matter the nuances, expecting signficiant precipitation totals
across the forecast area through early next week. Precipitation
type, and thus impacts, for valleys remains highly uncertain as a
rain-snow mix dominates much of the forecast. Snowfall amounts will
be highly elevation-dependent, which in some cases could leave lower
elevation valley floors soaked and snow-free while benches rack up
6+ inches through the whole period.

&&

.AVIATION...KSLC...VFR conditions with south winds will prevail
through 16Z, with a 30 percent chance of gusts in excess of 25
kts. MVFR cigs with periods of light to moderate rain will become
predominant after 17Z, and likely persist through the remainder of
the afternoon and evening. Winds will remain southerly through the
day, but could become variable in direction at times due to the
precipitation.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions through 16Z
will deteriorate to predominant MVFR and occasional IFR or worse
from north to south through the remainder of the day. Widespread
valley rain/mountain snow with snow levels near 7000 feet MSL can
be expected through this evening across the region, with snow
levels lowering to 6000 feet MSL overnight.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 9 AM this morning to 9 AM MST
     Thursday for UTZ110>112.

     Winter Weather Advisory from 2 PM this afternoon to 9 AM MST
     Thursday for UTZ113-117-125.

     Winter Weather Advisory from noon today to 4 PM MST Wednesday
     for UTZ114.

WY...Winter Weather Advisory from 9 AM this morning to 9 AM MST
     Thursday for WYZ021.

&&

Seaman/Wessler

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