Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 120946
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
346 AM MDT Tue Mar 12 2024

.SYNOPSIS...A Pacific Northwest trough will make its way across
the area Tuesday through early Thursday before becoming a closed
low over the Desert Southwest. Gusty downslope/canyon winds are
forecast to develop behind the low, impacting portions of the
Wasatch Front Thursday and Friday, with significant precipitation
possible for southeastern Utah during this time.

&&

.SHORT TERM (Through 12Z.6AM Thursday)...The front edge of a broad
Pacific Northwest storm system is approaching the forecast area
this morning, with an initial frontal boundary moving into
northwest Utah. This first boundary will cross northern Utah this
morning, bringing slight cooling and some light valley rain and
mountain snow with it. The precipitation will increase in
intensity and areal coverage with the arrival of the front edge of
the main trough and a second frontal boundary during the
afternoon hours, with an outside chance of lightning with the
boundary in a frontogenetic environment. Though the airmass cools
somewhat, will see precipitation stay as rain in the valleys
during this time. Overall, guidance this morning appears to be
just a bit slower with the arrival of the front, bringing the most
intense frontal precipitation to northern Utah from about mid
afternoon to the early evening. Expected later onset of
precipitation and slightly earlier end of the heaviest
precipitation has brought snow totals down ever so slightly.

For Tuesday evening into Wednesday morning, will see the main
frontal boundary shift into central and southern Utah, with
associated fgen forcing decreasing as Wednesday morning
approaches. As a result, best totals for the southern half of the
state should be over the southwest mountains, with anticipated
amounts generally decreasing farther east. Northern Utah and
southwest Wyoming will see a relative lull in precipitation late
Tuesday into early Wednesday, with precip becoming more widespread
as the center of the back edge of the trough closes off and slides
south along the Utah/Nevada border, increasing instability as it
does so. As cold pool instability will be the primary force in
play here, precipitation during this period will be primarily
focused over the higher terrain, though valley precip for
Wednesday will fall as snow, given the reinforcing shot of cold
air accompanying this part of the low. Going Winter Weather
Advisories cover the expected impacts with the snow well and have
made no changes with this package. By Thursday morning, the center
of the low will find itself over the Desert Southwest, effectively
ending the precipitation threat over the northwestern half of the
forecast area.

.LONG TERM (After 12Z/6AM Thursday)...

Key Messages:

-Winds: Easterly downslope winds along the Wasatch Front (especially
Northern Wasatch Front), continue to appear likely, with roughly 60%
of ensemble solutions supporting a moderate to strong downslope wind
event (with gusts over 60 mph). Other locations that may experience
gusty north-northeast winds of 40-55 mph include northerly gap winds
in Washington County, and across the Castle Country/San Rafael Swell
areas, and ridge portions of west-central and southwest Utah.

-Precipitation/Snow: There is greater than 50% chance of snow
exceeding warning criteria (6 inches) in southeastern Uinta County,
as well as near Capitol Reef National Park. A prolonged area of
heavier precipitation is becoming increasingly likely across
southern Utah late Thursday into Friday. Depending on the track of
the low and intensity of precipitation, snow may even be possible
down to Lake Powell. Snowfall amounts will likely reach 12 inches
across Boulder Mountain, the Henrys and the Manti-Skyline. There is
a low, or 10% chance for amounts up to 24 inches across these areas,
and up to 12 inches across higher elevations of Capitol Reef.

Early Thursday morning, model consensus places a closed low in the
UT/NV/AZ border vicinity, with nearly all of the model solution
space now on board with this idea. The low is likely to linger into
at least Saturday, leading to a prolonged rain/snow event across
southern and central Utah. Increasing easterly flow on the northern
periphery of the low`s circulation will be the primary impact/hazard
driver early in the long term period, with snow across portions of
eastern, central and southeastern Utah a secondary impact/hazard
through at least early Saturday. First, we`ll start with the
downslope wind potential.

With respect to winds, the main uncertainty continues to lie with
how strong the event will be and exactly when the peak winds will
occur. Ultimately, the orientation and strength of the 700mb winds,
stability profile from the surface to the ridgeline and temperature
and pressure gradient between the Wasatch Front and SW Wyoming will
drive how strong winds are eventually realized at the surface. All
of the ingredients are in place, but it`s the magnitude of the winds
where the greatest uncertainty lies. There is good clustering and
relatively high confidence in timing in terms of the start and end
of the event, with the event expected to get underway Thursday
morning around or just prior sunrise and persist through Friday,
strongest Thursday night. It`s the peak of intensity of winds where
there is some lingering uncertainty.

Ensemble situational awareness tables and EFI highlight that
easterly 700 mb wind speeds during this time period are forecast to
be the strongest in the 30 year model climatology for this time of
year, certainly a robust signal for easterly cross barrier winds
along the Wasatch. The question then becomes exactly how strong
will these winds be, and there is about a 60% chance of having
easterly 700mb winds around 45 mph or higher, and nearly 100% of
ensembles have easterly 700mb winds exceeding 25 mph. Combine that
with easterly cold air advection out of Wyoming and the surface
pressure gradient becoming more east-west oriented across the
Wasatch Front during the day Thursday into Thursday night - all of
the solutions should materialize at least a canyon wind event,
with the majority supporting a downslope wind event. Some
additional questions, include just how strong will the downslope
winds get - and at this time range its just a bit too early to
say, however worst-case scenario wind gusts (representing a 10%
chance of occurrence) are now in the 70 to 80 mph range. The
question is how widespread do the winds become, especially
Thursday during daytime heating. The lack of snow in the valleys
combined with a more powerful mid-March sun angle should support
deeper boundary layer mixing beneath the ridge top critical layer
(which is becoming increasingly well-defined in model forecast
soundings). This could cause elevated wind gusts to spread farther
off the terrain during the afternoon hours. Thursday night into
Friday morning, however, when the critical/stable layer is
forecast to be strongest, would expect the stronger gusts to hug
closer to the terrain, with particular emphasis in the Tremonton
area, Layton, Kaysville bench, Fruit Heights &
Farmington/Centerville areas.

Those residing within the typical easterly downslope wind areas from
Tremonton to Parleys should consider planning for a wind event now
by securing loose/lightweight objects and considering the potential
for some tree and powerline damage. One plus is that with deciduous
trees in the pre-leaf stage, they have less weight on their limbs
and less prone to damage than a comparable event during the leafy
season. A High Wind Watch has continues for the Northern Wasatch
Front where confidence is highest in exceeding 60 mph wind gusts,
but locations north and south of the watch area are not in the clear
and should continue to monitor the latest forecast as well.

A secondary area of strong winds includes ridgetops across the west
desert and SW Utah as well as gap/downslope wind areas of Lower
Washington County, where N-NE gusts in the 35-45 mph range are
likely behind a cold frontal passage from Wednesday night through
Thursday evening. There is also nearly a 50% chance for gusts to
reach 45 mph across low elevation areas of the west desert and SW
Utah as well, with the signal strongest nearest the Nevada border.

Regarding central, southern and southeastern Utah rain/snow
potential, a prolonged period of productive precipitation is looking
increasingly likely from Thursday through at least early Saturday as
a nearly stationary area of low pressure parks over Arizona. From a
dynamical perspective, 00Z model suite is now looking rather
impressive, with a prolonged area of impressive upper divergence
from central Utah southward. Additionally, the presence of strong
deformation/TROWAL structure within a region of weak static
stability is now seen pivoting into southern, southeast and central
Utah early Friday morning and persists at least into Friday evening.
When combined with NE-SE flow through a deep layer, orographic
enhancement will be in play for terrain features perpendicular to
the flow. There remains some uncertainty regarding snow levels
(dynamic cooling could bring snow levels down to the surface as low
as Lake Powell) but for now most likely outcome suggests an all snow
event for elevations above 5500 feet until late Friday when warmer
air wraps in from the southeast and snow levels briefly rise closer
to 6000 feet, again barring rate-driven reductions. Most likely
snowfall totals range up to 1 foot above 5500 feet (including the
Henrys, Fiftymile Mountain, Boulder Mountain) with 2-6 inches for
higher elevations of Capitol Reef. 90th percentile snowfall amounts
are double these values. When we look at the 90th percentile plots,
4-8 inches appear along the I-70 corridor from Castle Country
through higher elevations west of Green River. EFI values continue
to increase in this area, with SoT values approaching 2 here. This
indicates both an increasing potential of an unusual snowfall event
with an increasing subset of the ensemble solution space out on the
high end of the tail of the distribution. Should this event verify
close to or within the tail of the distribution, this could be a
once in every 10-20 year snowfall event for the time of year over
southeast Utah.

After the upper low moves out, high pressure ridging courtesy of an
Omega Block will build across the Great Basin. This will lead to
dry, stable and increasingly warm conditions building into the area
from the second half of the weekend into the middle of next week,
when temperatures are forecast to rise to 5-15F above mid-March
normals.

&&

.AVIATION...KSLC...Incoming low pressure trough will result in
gradual thickening cloud deck and lowering CIGs through the morning.
Rain showers develop between 16Z-17Z, and this is when the wind
direction becomes less certain. High confidence in S winds through
16Z, then latest guidance suggests variable winds 16-22Z before
briefly going back south. Just over 50% chance for MVFR CIGs 16Z-
23Z. Latest high-res models bring a convective line through the
terminal between 01-03Z with gusty NW winds, graupel and a 15%
chance for thunder. Winds then mainly remain NW-N through 12Z Wed
with brief switch to the E-SE 04-06Z in the wake of the convective
line. Increasing likelihood of at least vicinity lake effect snow
showers developing by 12Z Wed.

.REST OF UTAH AND SOUTHWEST WYOMING...A low pressure trough will
cross SW Wyoming and northern and central Utah through tonight.
Batch of widespread rain with snow above 6000 feet elevation will
overspread northern Utah after 12Z this morning then shift into
central Utah this afternoon with mountain obscuration and widespread
MVFR CIGs with occasional IFR CIGs. CIGs and flight categories then
briefly improve late afternoon. Convective line with gusty NW winds,
graupel and a low chance for lightning will cross northern Utah and
SW Wyoming 22Z-05Z and cross central Utah 00Z-07Z. IFR and brief
LIFR CIGs/VIS with this line. Worst case scenario wind gusts up to
40kts with this line.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Watch from late Wednesday night through Friday morning
     for UTZ104.

     Winter Weather Advisory until 6 PM MDT Wednesday for UTZ110>112.

     Winter Weather Advisory from 3 PM this afternoon to 6 PM MDT
     Wednesday for UTZ113-117-125.

WY...None.
&&

$$

SHORT TERM...Traphagan
LONG TERM/AVIATION...ADeSmet

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity


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