Heavy Snow and High Wind in Utah, February 26 to 28, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
1050 PM MST Sat Feb 25 2023

.UPDATE...Primary update with this evening's package was to issue
Winter Storm Watches for the northern Utah mountains, Wasatch
Back, Bear River Range, Northern Wasatch Front and Cache Valley.
Model consensus is well-clustered around the idea of snow showers
developing Sunday afternoon across the northern Utah mountains and
by late afternoon/early evening across adjacent valleys. This is
in response to an incoming PacNW trough. With this first wave, highest
snowfall rates are expected Sunday night into early Monday
morning. Transportation partners are concerned about Monday
morning commute impacts on routes from I-15 eastward (including
US 89 in the Northern Wasatch Front) across the Wasatch Back, Bear
River Valley and south to Solider Summit. Snow is then expected
to continue across at least the northern mountains and Wasatch
Back through Wednesday, so have issued an unusually long Watch
through this timeframe. A second wave is forecast to propagate
east across Utah and SW Wyoming Tuesday, with an attendant uptick
in snowfall rates across all mountain zones as well as the
entirety of the valleys from the Wasatch Front south to Cedar
City. Given this period is still over two days out, have
constrained watch issuance to areas with highest confidence of
either reaching warning criteria and/or having Monday morning
commute impacts. Previous discussion follows.

&&

.SYNOPSIS...A series of low pressure systems will maintain an
active weather pattern across the region through the middle of
the upcoming week.

&&

.SHORT TERM (Through 00Z Tuesday)...An anomalously deep and cold
upper low spinning along the CA coast will move inland this
evening, before quickly translating east along the UT/AZ border
region Sunday. As this occurs the occluded frontal band will lift
into southwest UT overnight, accompanied by a period moderate to
heavy precipitation. With the exception of St George which should
remain rain through much of the event, a 3-4 hour window of
moderate to heavy snow can be expected across much of southwest
and south central UT overnight. Have expanded the winter wx
advisory into southwest UT including Cedar City, where snow will
impact travel along I-15 particularly through Black Ridge Canyon
just south of CDC. This decaying band will spread across central
and eventually northern UT Sunday morning, with lingering
precipitation becoming more disorganized and showery with time.

As the upper low accelerates away from the region and into the
southern Rockies Sunday afternoon, increasingly moist warm
advection within a developing southwesterly flow aloft will
overspread northern Utah. This ascent will allow for widespread
snow late Sunday afternoon into Sunday evening across the northern
mountains, as well as eventually northern valleys favored in
southwesterly flow such as the northern Wasatch Front and Ogden
Valley. This warm advection pattern will remain in place through
the day Monday, with occasional enhancements in precipitation
owing to subtle embedded shortwave features translating through
the flow. At this point uncertainty in timing of heavier periods
of precipitation and the resultant impact are uncertain, as
increasing sun angle/surface heating by late February tends to
limit daytime impacts across the valleys outside of heavier
periods of snowfall.

.LONG TERM (After 00Z Tuesday)...Unsettled weather pattern
continues into the start of the long term forecast period. As a
shortwave digs into the PacNW, west-southwesterly flow is
reinforced over the forecast region. PVA associated with weaker
embedded impulses ahead of the aforementioned encroaching
shortwave in combination with the stronger synoptic wind field
will yield reinvigorated precipitation potential across the
forecast region, particularly for higher / orographically favored
terrain. This pattern is largely continued through at least late
Wednesday into Thursday prior to the passage of the primary
shortwave and trough axis, along with associated surface
reflection. Of note, some uncertainty remains in regards to
passage of the trough axis, with around 60% of ensemble members
pushing the trough through by Thursday, and around 40% depicting a
slightly slower transit and as a result some lingering effects.

Overnight into Tuesday morning, antecedent airmass appears to
remain cold enough for this precipitation to fall as snow across
the vast majority of the forecast region, with the exception being
portions of Lower Washington County / St. George where rain (to
at times some potential for wintry mix) is largely expected. With
more prolonged broad scale forcing for ascent rather than a
sharper boundary or more potent forcing mechanism, generally
expecting lighter prolonged snow accumulations (aside from
mountains which will see more moderate to heavy snow) prior to
final trough passage. That said, will need to keep monitoring for
potential smaller scale features which could exacerbate precip
rates, which in turn will impact exact snow totals. To that end,
moderate uncertainty is still noted in QPF amounts. For example,
using the NBM QPF for CDC, the IQR (difference between 75th
percentile and 25th percentiles) is 0.13" for the 24- hour period
5pm Mon to 5pm Tue, and 0.21" for the 24-hour period 5pm Tue to
5pm Wed. By late Wednesday into Thursday, expect the trough axis
to begin to pass to the east, with precipitation gradually waning
as a result. During this time, could see enough cold air with the
main trough axis to allow a brief window of predominately snow in
the greater St. George area, though certainty is low, and even
then any accumulations are expected to be very light.

An additional note for the mid-week system is the potential for
gusty winds in southern Utah on Tuesday as the low level synoptic
wind field amplifies. While higher elevations and mountains will
see the better probability of gusts exceeding 55 mph through early
Tuesday morning, current forecast maintains a period of strong
gusts in the 30-40 mph range for much of the lower lying areas as
well. With elevated snow chances forecast concurrent with the
wind, blowing snow will become a possibility, and those planning
travel on any of the more exposed / vulnerable routes will want to
monitor the forecast ahead of time and be prepared for
deteriorated conditions.

Colder air moves in following the trough, with forecast lows
Thursday morning expected to be the lowest of the forecast period.
Temperatures thereafter are expected to remain well below
climatological normal for the time of year as well. Precip chances
largely remain lower, though deterministic ECM/Canadian do show
potential for a quick grazing shortwave which nudges PoPs upwards
for northern Utah / southwest Wyoming Friday. By the weekend
onward, there is some signal for yet another deeper trough digging
into the PacNW, but certainty in any sort of details remain low
at this time.

&&

.AVIATION...KSLC...VFR conditions will persist through the TAF
period at KSLC. Light northwesterly winds will remain into the
overnight before turning back to southerly early on Sunday where
they will remain for much of the day. Skies will gradually fill in
over the period with the returning of vicinity showers by Sunday
afternoon on the toes of the next winter storm.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will persist
for northern terminals through the TAF period while southern Utah
sites see periods of MVFR/IFR. Light diurnally driven winds effect
most sites while CDC, SGU, and BCE could be breezy today ahead of
the incoming storm. Skies will gradually fill in over the period
with the returning of vicinity showers for northern terminals by
Sunday afternoon on the toes of the next winter storm. Southern
sites are expected to receive moderate to heavy snowfall during the
overnight and early morning hours that could bring very low CIG/VIS.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Sunday afternoon through Tuesday
     afternoon for UTZ103-104.

     Winter Storm Watch from Sunday afternoon through Wednesday
     afternoon for UTZ107>109.

     Winter Storm Watch from Sunday morning through Wednesday
     afternoon for UTZ110>112.

     Winter Weather Advisory until noon MST Sunday for UTZ122-124-127-
     128.

     Winter Weather Advisory until 5 PM MST Sunday for UTZ125.

WY...None.
&&

ADeSmet/Seaman/Warthen/Selbig

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