High Wind in Utah, October 22, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
419 AM MDT Sat Oct 22 2022

.SYNOPSIS...The stretch of unusually warm and dry October
conditions will come to an abrupt end today as a potent cold
front brings wind, rain, significant mountain snow, and much
colder temperatures with a hard freeze. Unseasonably cool and
unsettled conditions will then linger through the week.

&&

.SHORT TERM (Through 12Z/6AM Monday)...A potent cold front will
sweep from north to south today, bringing an abrupt end to our
stretch of warm, dry weather and quickly catapulting us into late
fall/early winter conditions. A myriad of impacts will accompany
this system, including strong, gusty pre-frontal winds,
significant mountain and backcountry snow, hazardous travel
conditions for high-elevation seasonal routes, and lake effect
valley snow. Those with any outdoor plans this weekend should
be prepared for wintry conditions!

Early morning observational data depicts the presence of a
deepening (strengthening) trough of low pressure digging into the
PacNW. This trough will continue to strengthen as it digs into the
Great Basin tonight and early Sunday before briefly closing off
across the ID/WY border region on Sunday. By this time, the trough
axis will have shifted east of the area. This process will drive
a very potent cold front across Utah and SW Wyoming. The surface
front will enter NW Utah by mid-morning today, and quickly surge
southeastward, clearing all of the Wasatch Front, NW Utah and SW
Wyoming by early afternoon. High-resolution and global models
then depict the front slowing a bit as it continues its
southeastward trek across central Utah this evening before finally
exiting into Colorado and Arizona shortly after sunrise Sunday
morning.

As the front moves through northern and central Utah, a region of
robust low to mid-level frontogenesis will develop thanks to an
impressive thermal gradient along the front, and a strong frontal
circulation via right-entrance region jet dynamics courtesy of a
~120kt jet streak poised to propagate across Idaho, Wyoming and NE
Utah in advance of the digging trough. The robust frontogenesis
will aid in the development of a strengthening frontal
precipitation band with embedded convective elements (cross-
sections reveal favorable conditions for conditional instability
release). Lift will initially be used to saturate the column as
significant low-level dry air remains in place, however the
development of strong lift will overcome low-level dry air
along/behind the front. Thus, precipitation will begin to reach
the surface by 12Z (6AM) for areas along/near the Idaho border,
with the leading edge of precipitation approaching the I-80
corridor by 15Z (9AM). By 18Z (Noon) precipitation will have
filled in across the west deserts. Across the Salt Lake Valley,
will have to wait for low-level southerly downslope effects to
end, and HREF mean suggests the low-level wind shift from the S to
NW will take place between 18-19Z. Shortly thereafter,
precipitation will begin in earnest. By late afternoon, a broad
band of precipitation with embedded mesoscale bands and convective
elements will spread from SW Utah NE into SW Wyoming.
Precipitation will fall in the form of all snow above 8,000-9,000
feet. Behind the front, snow levels will crash to 6,000-7,000
feet. By late afternoon, rain will have changed to snow across SW
Wyoming and the Wasatch Back.

By this evening, impressive cold advection and dynamic cooling
will force a changeover from rain to snow across much of SW Utah
outside of lower elevations south of Cedar City and Zion. Between
9PM and 12AM, rain will change to snow as far south as Cedar
City. Orographic influences will aid in enhancement of
precipitation across favored NW flow areas in the post-frontal
environment. Snowfall rates will be impressive, especially from
near Cedar City to Cove Fort, with HREF indicating near 100%
probability of 1"/hr snowfall rates, with even a low-end
probability of a brief period of 3"/hr rates. Heaviest snow
along/behind the front will be relatively short-lived, lasting
less than 6 hours along/behind the front, but rates may be high
enough to overcome antecedent warm surface conditions, especially
when we consider that the heaviest snow will fall after sunset.
Thus, snow will quickly accumulate on elevated/grassy surfaces and
may even begin to accumulate on paved surfaces. After midnight,
precipitation production mode will transition from frontally-
forced precipitation to orographically- forced precipitation as
moist, slightly unstable deep-layer NW flow will develop. This is
a highly favorable regime for efficient snow production for those
favored high-terrain areas in moist NW flow.

A lobe of vorticity and pocket of cold air aloft (700 mb temps as
cold as -10C) will shift into NW Utah and SW Wyoming tonight. At
the same time, unidirectional NW flow will be in place behind a
departing trough axis while deep-layer moisture will be in place
throughout the DGZ. Snow levels will have crashed to the surface
by tonight, with surface temperatures falling into the low to mid
30s across northern Utah valleys. With Great Salt Lake surface
temps currently around 60F, sfc-700mb delta-Ts of 20-25C (insert
eyes emoji here) will develop after midnight tonight. Forecast
soundings suggest 1500-2000J of lake-induced CAPE will develop
after midnight tonight as well. Thus, the environment will be
primed to support the development of lake-effect snow downwind of
the Great Salt Lake, Utah Lake and Bear Lake. High resolution
models suggest a high probability of moderate to even locally
heavy snow band development late tonight and Sunday morning before
becoming more convective/cellular by Sunday afternoon. Considered
issuing a Winter Weather Advisory for portions of the Wastach
Front and/or Tooele Valley, but uncertainty exists regarding exact
placement of this activity, so have held off for now. Areas from
Bountiful to Tooele may be impacted by this lake effect snow, with
benches perhaps primed for potentially several inches of snow. We
can't rule out accumulating snow for valley locations either.
This will be dependent on how transient lake effect activity is,
as heavier activity will require some residence time over any one
area to overcome warmer antecedent surface conditions. The
potential certainly exists for valley accumulations as well. A
concern with this activity will be the weight of heavy, dense wet
snow on trees with existing leaves and powerlines, as damage to
both will be possible. An examination of the ensemble distribution
of snowfall suggests potential valley lake effect snow totals may
range from a trace to 4-6" -- with both scenarios possible at
this time. Orographic snow showers will continue Sunday evening
before gradually winding down Sunday night as 700mb-500mb moisture
gradually diminishes. Enough low-level moisture and instability
may promote a continuation of lake effect snow showers through
Sunday night as well, with additional accumulations possible.
Mountain areas, especially across the Northern Wasatch, are
primed for a banner early-season event, with model solution space
suggesting 1-2 feet of snow, with perhaps as much as 3 feet of
snow for favored areas.

Ahead of the front, strong southwesterly winds will develop, with
700mb winds strengthening to 55kt across southern Utah by this
evening. Ensemble mean winds suggest gusts in the 50-60 mph range
will develop this afternoon across much of southern Utah, with a
worst case scenario suggesting 60-70 mph gusts for favored areas
across south-central Utah this evening, as evidenced by ensemble
maxes. With some uncertainty regarding boundary layer conditions
and mixing, have opted to leave ongoing wind headlines in place,
but south- central Utah Wind Advisories may need to be upgraded to
High Wind Warnings. Blowing dust, difficult travel conditions,
isolated power outages are all ranges of potential impacts. Folks
should also secure loose, lightweight objects such as Halloween
decorations.

Finally, a hard freeze looks likely for most locations outside of
St. George and Lake Powell areas Sunday night into Monday
morning. Those with gardens in most Utah valleys should be
prepared for a hard freeze. While it is still warm today and
Saturday, it may be worth winterizing sprinkler systems, RV and
swamp coolers and getting ready to move vulnerable plants indoors.

.LONG TERM (After 12Z/6AM Monday)...A brief warmup is forecast
across the state early next week under a weak high pressure ridge. A
45-kt 700 mb shortwave is forecast to track from Idaho into northern
Utah Monday evening through Tuesday morning which will bring the
next round of rain and snow showers to the region. Snow levels are
currently forecast around 5,000 feet during the overnight hours.
This is not a very impactful boundary with at most 1 to 2 inches of
accumulating snow at the higher elevation peaks. The boundary
quickly dissipates during the day with very little chance for
additional showers across central Utah. A short break is then
forecast in between weather systems as the next system is on track
to move into the region quickly by Wednesday. There is uncertainty
regarding the exact timing and strength of this particular system
and thus its impacts to our area. The GFS is calling for a deeper
and slower moving low while the ECMWF is tracking a quickly moving
and weaker trough. The current forecast calls for the best chance
for rain and snow Wednesday evening. Building high pressure late
week will allow for drying conditions and gradual warming into the
weekend.

&&

.AVIATION...KSLC...The day will become increasing impactful with the
arrival of a cold front this afternoon. First, a wind shift from
south to northwest is forecast at 18Z. Two to three hours later,
moderate to potentially heavy rain is expected to arrive. This will
drop ceilings at least to MVFR status with potential for IFR status
(around a 15% chance). Visibilities, however, are expected to drop
conditions to IFR status for a few hours this afternoon. Winds will
decrease during this period and eventually shift back out of the
southeast around 03Z Saturday evening. A rain/snow mixture is
forecast before becoming an all snow precipitation type overnight,
maintaining low ceilings. Improving visibilities are expected during
the evening.

.REST OF UTAH AND SOUTHWEST WYOMING...A frontal passage this
afternoon will have a plethora of impacts this period. Strong south
to southwest flow ahead of the boundary will give way in the north
this morning and afternoon but will be maintained or even strength
for southern Utah this morning. Gusts of 50+kts are forecast with
blowing dusts and lowered visibility impacts in these areas. Rain
and snow showers will bring decreasing CIG and VIS to MVFR/IFR
status expected this afternoon through Sunday morning from northwest
to southeast behind the frontal boundary. Look for improving winds
in the south by early this evening.

&&

.FIRE WEATHER...A strong cold front will slide across northern Utah
this morning, then continue southward through this evening, exiting
the state to the south and east tonight. Ahead of the front, strong
gusty winds will develop across southern Utah, with southwesterly gusts
as high as 60 mph expected, strongest late afternoon through this
evening across southern Utah. While humidity will increase today,
pockets of near-critical winds and humidity will be in place
across the south today. Widespead valley rain and mountain snow
will accompany the front, and temperatures will even be cold enough
for valley snow across northern and central Utah by tonight. This
system will provide widespread wetting rains to the state, with 1 to
2 feet of snow falling across the mountains.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 9 PM this evening to midnight MDT
     Sunday night for UTZ108.

     Winter Storm Warning from noon today to midnight MDT Sunday
     night for UTZ110>113.

     Wind Advisory from noon today to midnight MDT tonight for UTZ115-
     116-119-121-123-124-126>131.

     Winter Storm Warning from 6 PM this evening to 6 AM MDT Monday
     for UTZ117-125.

     High Wind Warning until midnight MDT tonight for UTZ122.

WY...Winter Weather Advisory from 9 PM this evening to midnight MDT
     Sunday night for WYZ021.

&&

ADeSmet/NDeSmet

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