Heavy Snow in Utah, December 9 to 10, 2021

Area Forecast Discussion
National Weather Service Salt Lake City UT
1003 PM MST Wed Dec 8 2021

.UPDATE...Spotty precipitation has already developed across
northern Utah and southwest Wyoming in conjunction with the
arrival of the upper jet and an initial weak shortwave trough.
The airmass was relatively well mixed today and valleys remain
warm currently. Snow levels have thus far remained above the lower
valleys, while elevations above around 6kft and higher have
started to see snow, which is reasonable given light precipitation
and H7 temperatures between -4 to -6C. Moisture will continue to
increase through the night with generally light valley
rain/mountains snow before a cold front associated with the next
trough arrives tomorrow morning. Heavier precipitation and lower
snow levels will focus initially along an area of frontogenesis
across northern Utah tomorrow morning before the front works its
way south through the day. Have updated the forecast mainly to
increase PoPs across the north this evening, otherwise the
evolution of the storm remains similar to that previously
discussed below.

&&

.SYNOPSIS...A winter storm will impact the region from late
tonight through Thursday, with a secondary and colder storm system
impacting northern Utah Friday. High pressure will return this
weekend through early next week, before another storm system
approaches mid-next week.

&&

.SHORT TERM (Through 00z Saturday)...The nose of a 120kt
anticyclonic northwesterly jet is just now impinging upon
northern Utah this afternoon. Attendant is a net uptick of deep
layer moisture which stretches near zonally in advance of a trough
just now beginning to amplify off the west coast. With this net
increase of moisture a moist southwesterly crest level flow is
developing which will allow namely orographically driven light to
moderate snow across the northern mountains and Wasatch Back
tonight, especially after midnight, as jet support aloft becomes
even more favorable for maintenance of deeper layer lift across
the north. This however, is only the beginning of near statewide
winter storm which will impact the forecast area through Thursday
and into the overnight with the passage of the aforementioned
trough and attendant cold front which will trend somewhat
frontogenic during the day.

In large guidance has changed minimally over the last few runs
outside of a continuation of a slight northward shift of the
h7 thermal gradient over northern Utah as the mid level front
trends frontogenic along the leading edge of the trough. Strong
convergence into this baroclinicity both from speed in the moist
southwesterly flow in advance, and the veering of winds on the
backside, will promote heavy at times precip development from
roughly a Dugway/Davis County/SW Wyoming line and points south as
the boundary pivots and shifts south trough the day w/trough
passage. Mountain areas along and south of I-80 have been upgraded
to Winter Storm Warnings primarily due the first widespread and
prolonged period of winter driving conditions, though storm total
amounts for most areas fall closer to a stout Advisory level event
and within the 10-20" range over a 24 hour period. Have also
included the Wasatch Back in a warning as favorable low/mid level
flow in the pre-frontal environment coupled with the deeper
forcing of the frontal environment will likely bring 6-12" of snow
from the Heber Valley north to I-80.

For the valleys initial p-type continues to look to largely begin
as rain later tonight into tomorrow morning along the Wasatch
Front prior to snapping to snow by mid morning due to a coupling
of increased PI and CAA with frontal passage. With this,
increased insolation and warm ground temps are expected to limit
travel impacts for much of the day within the SLC metro area and
Davis County, but impacts for valley areas south of Provo are
expected to increase due to a later frontal passage timing and
loss of solar mid to late afternoon and into the evening hours.
Have issued Winter Weather Advisories for the valleys from Davis
County and points south along the I-15 corridor for Thursday as
winter driving conditions will be present and accumulations look
to be modest, travel impacts greatest central/south. The passes
along I-15 near Cove Fort/Beaver are within the central/southern
mountain warning as impacts look to be especially significant at
those higher elevations of I-15.

This initial trough and cold front will quickly exit the area
from west- east Thursday evening with post frontal subsidence
driving a rapid decrease of snowfall for most all areas outside of
portions of the I-15 corridor and bordering mtns favored by low
level northwesterly upslope. CAMS continue to suggest rates and
areal of extent of this will taper rapidly during the latter half
of the night however. Have allowed the bulk of the winter weather
headlines expire at midnight as such...all but the SLC metro/Davis
County areas and central Wasatch mountains however were those
headlines continue through Friday afternoon.

A secondary and sharpening short wave within cyclonic
northwesterly flow will cross northern Utah Friday will ramp snow
back up across northern Utah by dawn, with growing potential for
lake enhanced snowfall and or banding within the Salt Lake or
Davis County areas. H7 temps trending as low as -15C coupled with
steep lapse rates and short wave passage will drive at the least
light to periodic moderate snowfall favoring easterly bench areas
across the region, while warm lake temps and increasing potential
for convergent low level flow over the GSL just may produce heavy
banding to its southeast. If this occurs, those valley areas
mentioned may have to be upgraded as heavy snow potential can't be
ruled out. Regardless, timing of even light snow onset around
dawn and the airmass advecting in looks likely to produce a more
impactful commute Friday morning across the metro. Something to
definitely keep tabs on moving forward as higher-res guidance
becomes available and confidence increases on potential and or
impacts.

.LONG TERM (After 00z Saturday)...All precipitation will have exited
the forecast area by Friday evening, with the exception of lingering
snow showers in the high elevations of the Wasatch and Bear River
mountains. A broad ridge will quickly build in behind the exiting
storm which will bring a limited inversion potential to valley
locations. This ridge is short-lived as another trough enters the
western CONUS by Monday, which looks to impact Utah and SW Wyoming
with another round of mountain snow and valley rain/snow by Tuesday.

As precipitation exits the area Friday evening a broad flat ridge
builds into the area for the weekend. Skies will be mostly clear for
southern and central Utah, but clouds will linger through Saturday
afternoon for northern Utah. The fresh snow and clear skies for
southern and central Utah will result in single digit temperatures
in the valleys on Saturday morning. Northern Utah valleys will drop
into the teens, as a result of lingering clouds. A few mountain
locations could go sub-freezing.

Inversion conditions could develop on Saturday for valley locations,
especially those that received high snow amounts. However, inversion
conditions won't be particularly strong given the location and low
amplitude of the ridge. On Sunday a shortwave will scrape far
northern Utah which will result in more clouds and less inverted
conditions.

Clusters depict good agreement amongst most EC, GEFS, and CMC
members in terms of the position and depth of the aforementioned
trough. As this trough digs off the west coast it will tap into an
atmospheric river that could be pulled into Utah from southern
California. Pairing this atmospheric river with strong synoptic
forcing could result in another significant storm for the entire
forecast area. Mountain snow and valley rain/snow could impact the
area as early as Tuesday of next week.

&&

.AVIATION... KSLC...VFR conditions are expected to continue through
03z, however, ceilings are expected to lower below 6000ft. MVFR CIGs
are expected around 06z with ceilings around 3000ft. After ~03Z,
vicinity showers will develop, with -RA impacting the terminal
beginning around 10z and low end MVFR CIGs developing. IFR
conditions will begin to develop as rain transitions to snow around
12-13Z. South winds will remain in place through ~12-14Z, switching
to a northerly direction thereafter. Otherwise, expect flight
conditions to remain poor throughout the remainder of the TAF period.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will continue
for the majority of the area, with gusty winds in Evanston, Cedar
City, and Bryce Canyon, where gusts have been near 30kts at each
site. Expect increasing clouds and precipitation across the northern
sites, where terminals will approach or reach MVFR conditions
between 03-08z. Showers will begin to increase in coverage between
08-10z in the northern sites, with mountain obscuration becoming
more widespread. Southern Utah terminals will stay in VFR
conditions, with higher elevation areas, such as Bryce Canyon,
remaining in MVFR conditions.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 8 AM to 11 PM MST Thursday for
     UTZ102-106.

     Winter Weather Advisory from 8 AM Thursday to 4 PM MST Friday
     for UTZ104-105.

     Winter Storm Warning from 5 AM to 11 PM MST Thursday for UTZ117-
     125.

     Winter Weather Advisory from 11 AM to 11 PM MST Thursday for
     UTZ115-116-118-122-126-127.

     Winter Weather Advisory from 5 AM to 8 PM MST Thursday for
     UTZ109.

     Winter Storm Warning until 11 PM MST Thursday for UTZ108-112-113.

     Winter Storm Warning until 4 PM MST Friday for UTZ111.

     Winter Weather Advisory until 4 PM MST Friday for UTZ110.

WY...Winter Weather Advisory from 5 AM to 8 PM MST Thursday for
     WYZ021.

&&

Cheng/Merrill/Mahan/Cunningham

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