Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 122237
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
437 PM MDT Tue Mar 12 2024

.SYNOPSIS...A storm system will settle into the eastern Great
Basin region through the next several days, bringing mixed
precipitation to valley areas and mountain snow throughout. A
period of easterly winds is expected to develop across northern
Utah and southwest Wyoming, bringing potentially strong
downsloping winds across portions of the Wasatch Front and Cache
Valley Thursday into Friday. Wrap around precipitation is
anticipated across southern Utah late Thursday through Friday,
bringing potentially impactful snow to portions of southern Utah.


&&

.SHORT TERM (Through 12Z/6AM Thursday)...Quite a few discussion
points to be touched upon this afternoon as a trough begins
dropping into the Great Basin region through the mid-to-late
week period. While a bulk of the impacts from this trough are
anticipated during the extended forecast period (Thursday onward),
there are some initial potential impacts that are expected as the
cold frontal boundary progresses through the forecast area.

Key Messages...

- A cold front progresses through the area through the evening
  hours, bringing potential for moderate to heavy precipitation
  across the western half of Utah. There is a low probability (25%
  chance or less) of higher rates allowing for rapid transition to
  snowfall.

- Lake enhanced snow showers are anticipated early Wednesday, with
  the most likely impacts and potential for snow accumulation
  expected in the eastern Tooele Valley and western Salt Lake
  Valley, especially for elevations above about 4,700ft.

- Snow showers continue tomorrow afternoon, bringing additional
  mountain snow accumulations. Showers begin to wrap up across
  much of the area Wednesday afternoon/ evening.

Discussion...

As of 330 PM MDT, radar and satellite data depict an area of
increasing convective shower activity across eastern Nevada,
likely tied to the cold frontal boundary that is expected to push
through the area through this evening and overnight. As the
boundary progresses through the area, the associated forcing along
the front will help maintain this area of enhanced showers,
expanding as it moves across western Utah later this afternoon.
Models have been flipping back and forth between potential
precipitation types as the frontal boundary progresses through the
western portion of Utah this evening, suggesting two very
different scenarios. On one hand, the most likely scenario, the
frontal band will progress through the western half of Utah with
the primary precipitation type being rain. A transition to snow is
anticipated on the back side of the frontal band, however, limited
snow accumulation is expected. It is very possible that snow mixes
in with rain in some of the higher intensity showers, but this
should be a fairly isolated occurrence.

On the other hand, the least likely scenario (about 25% chance or
less) brings potential for high precipitation rates along the
frontal band which would force snow levels to the valley floors
rapidly. While only a minority of the high resolution model
guidance is supportive of this occurring, each model produces
anywhere from 1 to 4 inches of accumulated snowfall. Much of this
snowfall potential is confined to the Salt Lake Valley, Utah
Valley, and I-15 corridor south of Nephi through Cedar City
(particularly the mountain passes) as the front moves through
after sunset. The thinking is that if this occurs, we could see
some slush like accumulations on paved surfaces. Again, this seems
to be the outlier solution, and models have been progressively
backing off with this potential... still worth a mention since it
could be a higher impact scenario to travelers on I-15 between
7PM and midnight.

This frontal boundary, and associated precipitation, looks to
progress through a bulk of the western half of Utah before 10PM,
reaching the Cedar City area by midnight. As the front converges
with the terrain following the I-15 corridor, the strength of the
precipitation is anticipated to weaken as the front loses its
structure... so areas east of the terrain should see less
potential for moderate to heavy precipitation (and associated
threat for snowfall), especially in the eastern Utah valleys.

With the cold front progression through northern Utah, overhead
temperatures will continue to decrease through the overnight
hours. As such, instability between the lower level environment
and the surface of the relatively warm Great Salt Lake will
increase and bring potential for lake enhanced snow showers
through the morning hours on Wednesday. Given that surface
temperatures will be much colder, thinking that there will be
potential for accumulating snowfall in the valley areas. In
particular, the eastern Tooele Valley and western Salt Lake Valley
fare the best chances for seeing accumulating snowfall, especially
for the upper elevation areas. It is likely that these snow
showers will persist through the commute hours, bringing potential
impacts to the morning commute. Current thinking from partners at
UDOT is that these showers may create brief road slush during the
morning commute on Wednesday. No headlines are in effect for these
valley areas at this time as impacts are anticipated to be quite
limited.

For the mountainous terrain, consistent snow is anticipated
through at least the passage of the cold front and the associated
precipitation band. A brief lull in activity is anticipated in the
post frontal environment, however, as we move into the daylight
hours on Wednesday, lingering moisture and increasing instability
will help lead to widely scattered snow showers over the
mountainous terrain of Utah through much of the heating hours.
Total snow accumulations at an individual location will heavily
depend on how many snow showers pass overhead. In general,
thinking that an additional 2 to 6 inches of snowfall will fall in
the higher terrain across Utah. As such, all existing Winter
Weather Advisories remain in effect.

Lastly, as the frontal boundary has progressed through the western
half of Utah tomorrow, this will promote thermal packing along
the Pine Valley mountains as well as a north-south oriented
pressure gradient between Cedar City and St. George. In short,
this setup will promote the development of gap winds along the
I-15 corridor between these two cities, where gusts could approach
40-45 mph during the overnight period on Wednesday into Thursday.
Current high resolution models show these winds to be mostly
confined to the canyon and higher terrain features surrounding
lower Washington County. No headlines are in effect for these
winds at this point in time given limited crosswind impacts, but continue
to monitor forecasts as we creep up toward this event.


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

Key Messages...

Winds: Easterly downslope winds along the northern Wasatch Front
are looking increasingly likely from the northern Salt Lake Valley
to perhaps eastern Cache Valley. Roughly 60% of ensemble
solutions supporting a moderate to strong downslope wind event,
with widespread gusts over 55 mph and over 70 mph in favored
locations from Bountiful to Ogden. Other locations that are likely
to experience gusty north-northeast winds of 40-55 mph include
Washington County I-15 from St George to Cedar City, across the
Castle Country/San Rafael Swell areas especially east of Highway
6, and terrain west-central and southwest Utah.

Precipitation/Snow: Wrap-around snow will impact the north slope of
the Uintas and parts of central/southern Utah east of Highway 89 on
Thursday and Friday. There is a 75% chance of more than 4 inches of
snow along with wind gusts to 50 mph in the Bridger Valley of Uinta
county WY, and a 75% chance of more than 5 inches of snow for parts
of Capitol Reef National Park.

Discussion...

A strong closed low will dive southward along the UT/NV border
early Thursday before stalling in Northern Arizona Friday, setting
up a unique weather pattern over our area with a variety of
impacts. Firstly, strong easterly flow develops on the north side
of the low along with a tight surface pressure gradient and cold
air advection along the north side of the Uintas, setting the
stage for a northern Wasatch Front downslope wind event. Looking
at deterministic model cross sections, ensemble- based comparisons
to model climatology, and early looks at high- res models that
extend that far in time, there is a strong signal for widespread
winds to 55 mph and gusts over 70 mph in the typical favored areas
(near Ogden Canyon, Weber Canyon, Farmington Canyon, Centerville,
and Bountiful). The same systems are also highlighting the Book
Cliffs region to US-6 between Price and Green River, as well as
mountain ranges in the west deserts near the UT/NV border. While
the winds are strong both Thursday and Friday mornings, the main
focus is honing in on Friday morning for the strongest winds.

Some lingering uncertainty can be found in the handful of high-res
models that extend this far out in time, for example the FV3 and
NAMNest. Specifically, there is uncertainty in the northern extent
of the mid-level jet and whether it would extend to the UT/ID border
to include the Cache Valley, and also in the aforementioned jet
potentially stretching southwestward which would bring unusually
strong northeast winds from the Great Salt Lake southwestward
(northeasterly is an abnormal direction for strong winds in the
corridor) and could be a crosswind impact. Opted to expand the
current High Wind Watch to the remainder of the northern Wasatch
Front to allow more high-res models to cover the time period of
interest for future shifts to decide on potential warnings.
Additional wind advisories may need to be added as well.

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.

The wrap-around precipitation portion of this closed low will favor
areas generally east of US-89. One wrap-around hotspot looks to the
north slope of the Uintas into Uinta County Wyoming. Opted for a
Winter Weather Advisory there given that the heaviest snow will be
away from populated areas (east of Evanston and south of I-80), but
will need to continue to monitor trends, especially given the
combination of snow and strong winds from the Bridger Valley
communities southward.

Another area that will be favored by wrap-around snow is the
central/southern mountains from roughly the Salina Summit vicinity
southward, especially in any areas favored by southeasterly flow.
Our probabilistic snow tools show eye-popping numbers for Boulder
Mountain, the Henry Mountains, and Fishlake areas. The upper Fremont
River Valley (e.g. Torrey to Loa) from Capitol Reef westward could
see the biggest snow event of the season, with US-12 from roughly
Escalante eastward also having the potential for a significant
event. One lingering uncertainty is snow levels wherein
precipitation intensities could drive snow levels lower than the
average of around 4,500`. Would not be surprised to see flakes down
to Lake Powell if a heavy band sets up in that area.

Ensembles show varying ideas of how quickly the low will finally
exit the area into the weekend and early next week, though with a
general theme of weakening/filling over that time. Thus, cooler
temperatures and unsettled conditions could linger over southeastern
Utah into early next week. Elsewhere in the forecast area,
temperatures should rebound to seasonal norms with generally quiet
conditions outside of potential additional rounds of easterly winds
(though not as strong). Looking further out in time, there are hints
that the low could evolve into a rex block pattern, continuing
similar conditions well into next week.


&&

.AVIATION...KSLC...Light erratic winds (<7kts) coupled with rain
showers anticipated through 01z this evening prior to deteriorating
conditions. Cold front with passage between 01-02z will drive
prevailing NW winds and MVFR rainfall with a 20% chance of switching
over to IFR snowfall for the timeframe through 03/04z prior to
improving conditions for the overnight hours. Said, a 25% chance
exists for snow showers to redevelop off the Great Salt Lake after
09z bringing temporary reductions, but most likely scenario keeps
any impactful snow showers west and south of the terminal.

.REST OF UTAH AND SOUTHWEST WYOMING...Widespread mountain
obscuration anticipated this TAF period with increasing rain with
snow over 5000 feet with a cold frontal passage this evening
(northern Utah) and overnight (southern Utah). Passage of front will
yield MVFR/IFR conditions with passage, isolated LIFR conditions for
the high elevations. Said, A 20% chance exists for IFR development
in the northern Utah valleys this evening under the heaviest showers.
General improvement in wake of frontal passage after 06z in the
north, and 08z in the south.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Watch from late Wednesday night through Friday morning
     for UTZ103>105-107.

     Winter Weather Advisory until 6 PM MDT Wednesday for UTZ110-111.

     Winter Weather Advisory until 6 PM MDT Thursday for UTZ112.

     Winter Weather Advisory until 6 PM MDT Wednesday for UTZ113-117-
     125.

     Winter Storm Watch from Thursday morning through late Friday
     night for UTZ117-125-127>129.

WY...Winter Weather Advisory from 6 PM Wednesday to 6 PM MDT Thursday
     for WYZ021.

&&

$$

SHORT TERM...Webber
LONG TERM...Van Cleave
AVIATION...Merrill

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


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