Heavy Snow and High Wind in Utah, April 10 to 13, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
400 PM MDT Sun Apr 10 2022

.SYNOPSIS...A significant winter storm is expected Monday
afternoon into Wednesday, bringing winds along with cold
temperatures and snow to the valley floors. Unsettled and cold
conditions are expected to continue through the week.

&&

.SHORT TERM...(Through 12Z/6AM Tuesday)...

A dynamic system will move through Utah and SW Wyoming from
Monday afternoon through Monday night and beyond, bringing a
myriad of impacts to the area. For the short term period, winds
and heavy snow are primary concerns. So, what's behind all of
this? A strong area of low pressure with very cold air for this
time of year will drop into the PacNW Monday morning before
ultimately tracking across central and eastern Idaho Monday
evening and Monday night. This will drag a strong cold front
across the area Monday afternoon and evening. Ahead of the front,
a combination of increasingly moist westerly flow and modest warm
advection will lead to occasional valley rain and mountain snow
showers near/along the Idaho border tonight and Monday morning.
Signficiant low-level dry air will serve to significantly limit
accumulations.

As the low and cold front approach late Monday morning, deep-layer
southwest flow will develop across Utah and SW Wyoming. A
combination of steep low to mid-level lapse rates, deep boundary
layer mixing and increasing winds aloft (700mb winds increase from
35kts at Noon Monday to 50-55kts Monday evening) strong, gusty SW
winds will reach the surface across much of central and southern
Utah. Gusts to 55 mph are expected across the west deserts Monday
afternoon and evening, with gusts reaching 50 mph in lower
Washington Co and Zion late Monday afternoon and evening. Farther
east across south-central Utah, gusts in excess of 58 mph are
expected, especially from late afternoon through the evening when
the strongest winds aloft will be juxtaposed with the tightest
pressure gradient. Impacts will include strong crosswinds, a few
power outages, rough chop on area lakes, blowing dust and even
some property damage can't be ruled out. The specific location(s)
and timing of blowing dust is difficult to pin down. Best chances
will be near and downwind of areas where soils have been recently
disturbed by agricultural and/or recreational activity, such as
near ATV trails.

Locally strong winds cannot be ruled out across northern Utah Monday
afternoon and early evening as showers and isolated thunderstorms
will be capable of producing strong, gusty outflow winds. The
combination of instability/buoyancy and wind shear will be
sufficient for at least loosely organized, strong convection.

Shifting gears to precipitation, showers will gradually develop
across northern UT and SW Wyoming during the afternoon as large-
scale ascent gradually increases. Low to mid level frontogenesis
will gradually increase through the evening, signaling a gradually
strengthening cold front/baroclinic zone. Models (both high-
resolution and deterministic) represent this idea by depicting a
gradual transition to a SW-NE oriented banded frontal zone with
embedded convective elements. This process is expected to begin
across northern UT and SW Wyoming early Monday evening.
Precipitation will become moderate to locally heavy, with 1-2"/hr
snowfall rates indicated by the HREF across northern UT mountains
during the evening. Based on the dry sub-cloud layer and wet-bulbing
effects, remained concern that an earlier than currently forecast
changeover to snow will occur, particularly in valleys. This is most
likely to occur between 9-11PM Monday evening, but could occur as
soon as 6PM, especially across bench areas. An earlier transition
would imply higher snowfall totals than currently anticipated (at
least 1" appears likely on grassy surfaces across the Wastach
Front), along with a greater potential for evening commute
impacts.

As we continue through the evening, snow will expand southward
along I-15, and the central mountains and adjacent valleys. Since
precipitation will reach these areas later in the evening,
precipitation will begin either briefly as rain before changing
over to snow, or begin as snow right from the onset. HREF suggests
a 40- 50% chance of 1"/hr snowfall rates along I-15 in the Cove
Fort area by midnight, so winter driving conditions are appearing
increasingly likely here.

Organized, banded snowfall will gradually transition to a lesser
organized, upslope-induced characterization between midnight and 6AM
Tuesday morning. Higher elevation areas that tend to perform well
within a westerly flow regime will be favored by this time.

.LONG TERM (After 12Z Tuesday/6AM Wednesday)...

The recent model trends towards a sped-up cold front and the bulk
of the upper-level energy remaining to our north result in the
primary period of heavier precipitation having already moved out
of our area by 12Z Tuesday. A lull in precipitation is now
expected Tuesday morning for most areas as the trough axis passes
and general subsidence moves in along with a dry-slot-like feature
causing a quick end to precip post-frontal. However, with an
unseasonably cold and unstable air mass settling in during the day
(e.g. -15C at 700MB for much of the area), instability showers
will quickly develop by the afternoon hours. An instability axis
roughly along the I-15 corridor could bring a few rumbles of
afternoon/evening thunder along with snow and likely graupel
showers. Any backside shortwave trough would serve as a focus
point to organize such activity, but this period is a bit too far
out in time to have much certainty in the location. Meanwhile,
locations favored orographically in northwest flow should see
stronger showers through this period owing to the favorable flow
direction. Some level of lake enhancement is also possible Tuesday
evening, favoring the Bountiful Bench vicinity should it develop.

Persistent northwest flow aloft will develop late Tuesday and
persist through at least Wednesday night, along with an unstable
and cold atmospheric profile and lingering low-level moisture.
Temperatures will remain quite cold for mid April, with 700MB
temps hovering around -15C resulting in surface temperatures 15-25
degrees below normal. Will all these ingredients combined,
expecting areas favored in such flow such as the Cottonwoods to do
well, with convective showers making the forecast less certain
elsewhere. Valleys (other than Washington County which would be
rain/graupel if any and areas east of the Wasatch Plateau which
will be shadowed) could see anywhere from a trace to a few inches,
while area mountains (especially Soldier Summit northward) could
easily pick up another 6" to a foot.

Following the passage of the Monday-Wednesday storm, models are in
good agreement of a brief period of high pressure. During this
period, strong warm-air advection at H7 will increase temperatures
by 10-15 degrees over Wednesday's max temperatures, though
increased cloud cover will likely prevent additional warming
beyond that. Many GEFS members suggest a solution of nearly
overcast skies, which could further limit the max temperatures on
Thursday. In addition, this could help keep overnight minimum
temperatures warmer on Thursday and Friday nights, potentially
limiting areas that may receive a hard freeze.

Overnight into Friday, mountain showers across northern UT and
southwest WY are possible with the arrival of the first weak
shortwave trough just to the north of our area. This is the most
likely solution, however, the EPS mean and deterministic EC do not
favor this trough; instead, ridging continues in these solutions.
Whether or not this shortwave trough occurs will likely impact our
max temperatures on Friday, as well as QPF across the northern
mountains.

By 00z Saturday, about two-thirds of ensemble members favor the
presence of a longwave trough upstream, with some suggesting a PacNW
location and some suggesting a northern Great Basin location. The
northern Great Basin location solution (16% of members) produces the
highest amount of precipitation, with precipitation across more of
northern UT and southwest WY. The rest of the solutions produce
precipitation limited to just the northern mountains. This includes
the last third of members that suggest a very zonal, broad trough,
which might produce less precipitation overall due to lack of
dynamics. Overall, this feature is expected to limit any
precipitation to the northern half of our area, due to a lack
of moisture across southern UT.

Beyond Saturday morning, uncertainty in the pattern increases
greatly. Eighty-eight percent of members depict our area under some
sort of trough by evening, but in various trough patterns such as a
broad longwave trough or a more amplified trough either upstream or
overhead. By Sunday, solutions vary even more and have a lot of
uncertainty for temperatures and precipitation potential.

&&

.AVIATION...

KSLC...Light prevailing northwest flow will transition to
southeast around 03Z. Winds will increase late morning, from the
south and become more southwest into the afternoon. Gusts during the
afternoon will reach around 30 knots. Isolated rain showers are
expected during the afternoon, especially later afternoon. Nearby
showers have the chance to enhance winds or change directions.
Better chances for rain will build in after 00Z, with precipitation
changing to snow from 03-06Z. MVFR or IFR conditions are likely with
snow through much of the following morning.

REST OF UTAH AND SOUTHWEST WYOMING...Light winds are expected
through much of Monday morning. Winds will increase late morning
throughout southwest Wyoming and Utah. Southwest gusts reaching
around 50 knots are expected for southern Utah Monday afternoon.
Speeds will be lighter further north, but gusts around 30 knots are
expected. Showers are expected for southwest Wyoming and northern
Utah in the afternoon, with gusty outflow winds possible. Better
chances for precipitation will build in later Monday. Any rain will
transition to snow late Monday. This will bring MVFR or IFR
conditions.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 3 PM Monday to 6 AM MDT Tuesday for UTZ120-
     121-131.

     Wind Advisory from noon Monday to 4 AM MDT Tuesday for UTZ115-
     116-122.

     Wind Advisory from 3 PM Monday to 4 AM MDT Tuesday for UTZ123-
     124.

     High Wind Warning from 3 PM Monday to 6 AM MDT Tuesday for
     UTZ128-130.

     Winter Storm Warning from noon Monday to 3 AM MDT Wednesday for
     UTZ110>112.

     Winter Weather Advisory from 6 PM Monday to 3 AM MDT Wednesday
     for UTZ113-117.

WY...Winter Weather Advisory from noon Monday to 3 AM MDT Wednesday
     for WYZ021.

&&

ADeSmet/Van Cleave/Cunningham/Wilson

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