Heavy Snow and Winter Weather in Utah, December 23 to 25, 2021

Area Forecast Discussion
National Weather Service Salt Lake City UT
1039 AM MST Thu Dec 23 2021

.UPDATE...
In excellent news this morning, the increasing southwesterly flow
ahead of the incoming storm system has proven to be very
efficient and eroding and mixing out valley inversions this
morning. Most locations have seen temperature spike and air
quality significantly improve, along with the increased southerly
winds. As of 1030 AM just some lingering inverted areas near
Wendover, and in eastern valleys of the Uinta Basin and Castle
Country. Eastern valleys are unlikely to break out of the inverted
conditions ahead of the incoming storm system, however expected
Wendover to break out this afternoon as the south-southwesterly
pressure gradient becomes better aligned with the 700 mb winds.

Otherwise, little to no change in the thinking for the incoming
winter storm system. If anything, confidence continues to increase
on timing of the most impactful periods across the mountains.
00-03Z this evening precipitation will fill in and become heavy
across the northern Wasatch and Bear River Range, then shift south
and lock in across the central Wasatch, western Uintas and Wasatch
Plateau/Book Cliffs between 03Z and 09Z. After 09Z, this axis will
start to shift southeastward filling in heavier snow across the
central mountains before diminishing in intensity Friday
afternoon.

&&

.SYNOPSIS...A trough off the West Coast will be ejected eastward
across California today and across Utah late tonight. A very
moist SW flow will prevail through Friday morning ahead of this
short wave trough. The long wave cold trough will set up over the
Pacific NW Friday through the extended period with multiple storm
systems expected to cross over Utah through next week.

&&

.SHORT TERM (Through 12Z/5am Sunday)...The inversion is being
overturned already across much of western Utah except for near the
Great Salt Lake and across the Great Salt Lake Desert and the
Cache Valley. Winds have broke through in many areas with gusts
between 20 and 30 mph and locally over 35 mph. Temperatures have
jumped 10 to 20 degrees in the past 2 hrs in areas where the
inversion had be holding temperatures hostage the past few days.
Temperatures are already in the 40s in many place so have just
updated the max temperature forecast by several degrees to
accommodate this temperature jump already.

There still remains a concern for areas mentioned above in the
Cache Valley and around the GSL desert which have the potential
to have a wintry mix later this morning into this afternoon if the
inversion does not get scoured out. Moreover, my greatest concern
is for the Cache Valley (which a couple of models show not having
temperatures in the vertical column rising enough above freezing
to allow for a change over to rain). Therefore, a careful eye will
need to be kept on p-type in the Cache where if it remains all
snow could end up with 5-10 inches of a wet heavy snow. There is
about a 25% chance of this occurring.

The elongated trough off the West Coast this morning will make
landfall across California early today and then it will move east
across the Great Basin into Utah by tonight. Abundant moisture
ahead of this system will spread across the region today with most
of the lift being initially provided by orographics. However, as
the trough approaches the jet dynamics will increase lift across
northern Utah late today through tonight and across central and
southern Utah tonight through midday Friday. All northern
mountains, the Wasatch Back of Utah and Uinta County Wyoming are
under a Winter Storm Warning due to the prolonged southwest flow
and temperatures cold enough for all snow. However, areas that are
not as favorable to southwest orographics or are shadowed by this
flow direction in northern Utah are under a winter storm
advisory, including the Bear River valley and western Uinta
Basin. While the jet will drop southward across the central and
southern mountains later tonight into Friday, the duration will be
less and therefore they have been placed under an advisory for
lesser snowfall amounts.

The cold trough that is responsible for ejecting the elongated
trough across the Great Basin will enter into the PacNW Friday
night. However, a weak shortwave at the leading edge of this main
trough will move across northern Utah Friday night with just
enough cold air to cause precip to mix with or change over to snow
showers. Because the main trough is punching more southward than
southeast a southwestward flow aloft will return Saturday with
gusty winds over 20 mph likely across the western valleys. The
eastern valleys will not be as breezy.

The main cold trough will march into Utah Saturday night into
Sunday with colder air such that most valleys will have precip in
the form of snow, except Washington Co and southeast portions.

.LONG TERM (After 12Z/5am Sunday)...An active pattern looks to
continue throughout the long term period as a series of troughs
impact the area. Guidance shows the next cold system moving
through late Saturday into Sunday with the majority of ensemble
members showing the wave primarily impacting northern portions of
the forecast area. With 700 mb temperatures in the -10C to -14C
range, snow levels would easily fall to northern Utah valley
floors, though the potentially progressive nature of the system
could limit associated accumulations.

Another wave looks to follow in the vicinity of late Monday into
Tuesday. However, spread in the guidance increases noticeably with
this system as well as the various shortwaves that follow through
day seven. Overall, confidence is increasing in a continued
unsettled pattern with above average precipitation for most or all
of the area, particularly the higher terrain. However, confidence is
low in the timing of any particular system and associated
precipitation, as well as specific amounts for any part of the area.
With this in mind, have kept POPs predominately in the slight
chance/chance range for most valleys and in the chance/likely range
for most mountain locations. Also, as one would expect in a cool and
unsettled pattern, have kept temperatures primarily 5-10F below
seasonal normals throughout the long term period.

&&

.AVIATION...KSLC...Conditions at the SLC terminal have improved to
low end VFR with the trend of improvement continuing during the
morning as winds increase and the inversion mixes out. Cigs,
however, will be decreasing as the storm system approaches, with
values at or below 6000 feet AGL anticipated by the early afternoon.
Southerly winds are expected to become increasingly gusty by this
afternoon.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions should prevail
across the airspace this morning. Inversion conditions will keep
visibilities in the MVFR range during the morning in some prone
areas including the Cache Valley, but increasing winds will mix out
most locations. As precipitation increases throughout the day,
mountain obscurations will become more common, with areas of MVFR
and IFR conditions in mountain and mountain valley locations.
Precipitation will become more widespread this evening and
overnight.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 5 PM this afternoon to 5 AM MST
     Saturday for UTZ117-125.

     Winter Storm Warning until 5 AM MST Saturday for UTZ108-110>113.

     Winter Weather Advisory until 5 AM MST Saturday for UTZ109-114.

WY...Winter Storm Warning until 5 AM MST Saturday for WYZ021.

&&

Church/Struthwolf/Traphagan

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