Winter Storm Warning 6, NWS Salt Lake City, UT

Beaver County, UT, Box Elder County, UT, Cache County, UT, Carbon County, UT, Davis County, UT, Duchesne County, UT, Emery County, UT, Garfield County, UT, Iron County, UT, Juab County, UT, Kane County, UT, Millard County, UT, Morgan County, UT, Piute County, UT, Rich County, UT, Salt Lake County, UT, Sanpete County, UT, Sevier County, UT, Summit County, UT, Utah County, UT, Wasatch County, UT, Washington County, UT, Wayne County, UT, Weber County, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
807 PM MDT Thu Oct 20 2022

.UPDATE...In consultation with surrounding offices, have added a
Wind Advisory for south central Utah for Saturday afternoon and
evening with the majority of guidance solidly showing wind gusts
in excess of 50 mph for that area in addition to southwest Utah
ahead of the front. Given the tranquil conditions in the short
term, no additional updates planned. Previous discussion follows.

&&

.SYNOPSIS...One more day of warm and dry conditions before a
potent cold front brings widespread precipitation, much Ahead of
a strong storm system, southwest winds will increase Friday and
Saturday. Much cooler, wetter conditions will arrive Saturday,
then persist through next week.

&&

.SHORT TERM (Through 12Z/6AM Monday)...Regional satellite imagery
shows a closed low spinning offshore of SoCal, pushing high
clouds towards southwest Utah at the moment, while the first signs
are appearing of a developing trough along the British Columbia
coast which will bring major changes for us in a few days. Some
good news, our northern Utah radar has returned to service this
afternoon after a multi week outage to fix panels damaged by a
lightning strike earlier this year. Just in time!

Heights will be decreasing across the Great Basin tomorrow in
advance of the approaching weekend trough. Increasing westerly flow
at mid levels will lead to increasing winds, stronger near the
UT/ID border. Mixing with these winds will bring a slightly warmer
day vs today, with temperatures in the north pushing 10 to 15
degrees above normal - making the upcoming change even more
dramatic. A relatively warm night can be expected Friday night as
the elevated mid level flow backs to southwesterly and spreads
south through the night.

Showers and generally light precipitation will edge into northern
Utah by Saturday morning as the trough carves out upstream. While
the moisture trajectory is not quite the ideal atmospheric river
route around the Northern Sierra, AR diagnosis tools indicate that
an appreciable moisture tap will be routed across the northern
Great Basin early Saturday. Meanwhile, stronger southwest winds
at the surface will develop during the day, particularly over
southwest Utah where wind gusts to 50 mph are possible and
consequently a wind advisory has been issued. While not a
crosswind for most major roads in that area, blowing dust is a
possibility and holiday decorations will need to be secured.

The much-anticipated cold front will push through the region
during the afternoon and evening, with a quick burst of northwest
winds aligning with the surface front and widespread precipitation
and rapidly lowering snow levels associated with the mid level
front. Given that this front is near forecast hour 60 and thus a
bit outside of most high-res models' timeframes, we eyeballed all
50 members of the Ecmwf ensemble to gauge the current frontal
passage time, which is currently average around mid afternoon for
the far north, 6pm for the Wasatch Front metro, and 9pm for the
I-15 corridor to Cedar City. Strongly suspect that precipitation
rates will be able to drag down snow levels lower than the 0.5C
wetbulb height method we use for snow level, which could bring
flakes to valley floors Saturday night, and certainly a chance of
accumulating snow for benches. For higher-elevation mountain
valleys and all mountain passes, expect a burst of heavy snow
Saturday evening and overnight. Per coordination with our local
DOT partners, winter driving conditions are likely for many high-
elevation routes, especially considering that seasonal routes
such as Monte Cristo summit are still open. In addition, a sudden
change to harsh conditions between Saturday and Sunday can be
expected for those in the backcountry, for example hunters. As
such, a Winter Storm Watch has been issued for all Utah mountains,
with the great impacts currently expected from roughly I-70
northward. Higher points on I-15 and I-70 may be impacted as well.
A watch is in effect for the Wasatch Back as well, namely for the
Park City vicinity.

While details are still coming into focus, Sunday may be an
impactful day as well. The backside of the parent trough looks to
slide through northern Utah and southwest Wyoming under moist
northwesterly flow, setting up favorable orographics for multiple
mountains. Meanwhile, the Great Salt Lake surface temperature
difference with 700MB projected temperatures well exceeds locally-
studied criteria for lake enhancement; in fact, it exceeds by so
much that organized convection may be less likely with numerous
showers developing instead. Such lake enhanced snow and graupel
showers will be possible anywhere from Tooele to Bountiful
depending on how the flow direction ultimately sets up. Meanwhile,
orographically-enhanced snow showers are possible in the higher
terrain continuing into the overnight hours, depending on details
which are still uncertain at this time. However, given the
potential, have continued the mountain Winter Storm Watch through
Sunday night.

.LONG TERM (After 12Z/6AM Monday)...An active pattern continues for
the long term period. As the main system exits out of the area on
Monday, another low pressure system begins to form off the PacNW
Coast. This system will be influencing the area later in the long
term. Between the two systems, a couple of shortwaves are expected
to move through the area.

Beginning on Monday the main storm system looks to be exiting the
region, however there are still 40% of all ensemble members that
keep the system closer to our area. This could continue the chance
for terrain induced showers throughout the day on Monday. Something
to also note is that there is a potential for some enhanced snow
showers Monday morning as there will be good north to northwest flow
across the lake. Depending on what the flow regime is and how much
instability there is, the location of the snow showers is uncertain
at this time. If there is a more northerly flow, than the Tooele
Valley and Oquirrh mountains would be favored. On a more northwest
to west-northwest flow, mountainous areas along the eastern Salt
Lake Valley, such as the Cottonwoods, would be favored.

Heading into Tuesday the first shortwave moves into the area. This
shortwave is expected to be weak but some terrain induced showers
can not be ruled out. Other than this shortwave, expect mainly dry
conditions across the CWA. Overnight Tuesday into Wednesday is
approximately when the other shortwave will move into the area. This
second shortwave will ultimately have the same result as the first,
with the possibility of higher terrain/mountainous areas having the
greatest potential, albeit small, for some snow showers. Both
shortwaves look to be dissipating in our CWA when they move through.

The last part of the long term is when the second low pressure
system is expected to effect the area. Due to how far out this
second system is, there are still uncertainties with this storm. All
ensemble members show that there is a trough that will be entering
the area Wednesday night into Thursday morning. There is uncertainty
in how strong this system is, as well as the positioning of the
trough and how long it will influence our area. Almost all of the
ensembles show precipitation will be possible with this storm. Among
the ensembles, the ECMWF ensembles has the trough moving faster and
not as amplified as the GEFS which as the trough more amplified and
keeps it over our area for a longer period. The matter of how
influential the system will be in our area is where the uncertainty
lies.

&&

.AVIATION...KSLC...VFR conditions will exist for the entire period
for the KSLC terminal. High pressure remains in place which will
result in mostly clear skies and light diurnally driven winds.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected
for the valid TAF period. High pressure remains in place which will
result in mostly clear skies and light diurnally driven winds with
the exception of Uinta County, Wyoming where afternoon winds will be
gusting near 24 kts out of the west.

&&

.FIRE WEATHER...Major changes are in store for the region as an
unseasonably cold storm swings in from the northwest this weekend
and a cooler and more unsettled pattern settles in. Before the
storm arrives, southwest flow will increase Friday and especially
Saturday. Meanwhile, with continued warm temperatures Friday
minimum RHs will reach the low teens in a few low elevation areas,
especially the deserts of western Utah. RHs should begin to climb
by the time the stronger winds arrive Saturday, focused especially
over southwest Utah. That said, there is still a chance of
isolated areas of critical fire weather conditions. Fuels are not
expected to be receptive at this time, so no warnings are in
effective.

By late Saturday, a potent cold front will slide through the
entire area, bring a burst of gusty north winds and widespread
precipitation with rapidly falling snow levels. Wetting
precipitation can be expected for nearly all districts. Cooler and
unsettled conditions will then linger into next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Saturday evening through late Sunday
     night for UTZ108-110>113-117-125.

     Wind Advisory from noon Saturday to midnight MDT Saturday night
     for UTZ122>124-128.

WY...Red Flag Warning until 8 PM MDT Friday for WYZ277.

&&

Traphagan/Van Cleave/Wessler/Mahan/Cecava

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