Winter Storm and Heavy Snow in Utah, November 7 to 10, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
433 AM MST Mon Nov 7 2022

.SYNOPSIS...A significant winter storm will bring strong winds
today, developing mountain snow Monday night through Thursday,
valley rain Tuesday, then valley snow on Wednesday/ Wednesday
night. Well below average temperatures will follow the passing of
this storm late in the week.

&&

.SHORT TERM (Through 12Z/6AM Thursday)...This morning's GOES-W
water vapor imagery depicts a deep, well defined trough
encroaching on the PacNW coast, with a cloud shield blanketing the
northern two-thirds of the forecast area. This trough will
continue pushing inland, and over the forecast area, through the
week ahead, bringing prolonged impacts across the region.

Already, decreasing surface pressure to the northwest of the area
has aided in the development of a solid north-south oriented
surface pressure gradient, helping to drive modest southerly flow
across the western half of Utah. A strong low-level jet has
developed overnight across this similar area, which has helped
produce localized strong surface wind gusts in excess of 50 mph
primarily right near terrain features (likely a result from
terrain induced rotors). Isolated peak wind gusts to, or above, 58
mph have been observed on the northern end of the Oquirrh
mountains. Have opted to have the existing High Wind Warning to go
into effect at 8AM, mainly because these isolated high winds will
become much more widespread as we begin to break the weakening
stable layer. Model guidance shows the aforementioned low-level
jet weakening as a deeper mixed layer sets up this afternoon, but
given the strong surface gradient and 45-60 knot forecast winds at
700mb, high winds will very much remain in place across much of
western Utah. For the Salt Lake Valley, eastern Juab and Millard
Counties, and Sevier Valley, wind advisories will go into effect
starting at 11AM, but isolated strong winds will be possible
before the headline goes into effect. In fact, localized
downsloping winds in the southeastern Salt Lake Valley have
already been observed.

The frontal boundary associated with this deep trough will be
draped across northwestern Utah this afternoon. Ahead of this
boundary, where the strongest winds will be observed, moisture
will be readily available in the lower and upper levels to help
kick off areas of light precipitation from roughly Salt Lake
County northwestward. Given the lack of mid-level moisture ahead
of the front, it is unlikely that these showers will produce much
precipitation. Deeper moisture will, however, be available along
and behind the boundary, so rural areas of far northwestern Utah
will likely see the best shot of rain/ snow. Model guidance
continues to show a very slow progression of this boundary and the
precipitation associated with it. Current guidance begins to show
a significant increase in precipitation chances across the
western half of the area overnight Monday into Tuesday as the
boundary is forecast to begin pushing into the remainder of the
area, but models still have a strong disagreement on when this
progression will actually take place, and how long it will take.
What we do know is that precipitation forming along this boundary
will be generally moderate to heavy in nature, likely favoring
south to southwesterly upslope regions of terrain. Valley areas
are also likely to perform very well given the strong
frontogenetic forcing associated with the boundary progression.
What we don't know, and what could very much mess up the forecast,
is how quickly valley areas transition to snowfall. If the
boundary is slower to progress, more cooling of the layer due to
moderate to heavy precipitation will force snow levels to drop to
valley floors at a more rapid rate. For now, have trended valley
areas to a rain- snow mix during the mid-afternoon on Wednesday,
but this could certainly shift earlier if the front picks up
speed. Bottom-line, there is still a solid amount of short term
uncertainty. All existing Winter Storm Watches remain in effect,
but are likely to be upgraded this afternoon, or shortly
thereafter.

.LONG TERM (After 12Z/6AM Thursday)...Long term forecast period
begins with an exiting trough that will keep some lingering snow
showers possible in northern UT and southwest WY through the morning
and into the early afternoon period. After this system exits, flow
aloft remains westerly to northwesterly which means that subsidence
will likely be in place for awhile. This can be seen with PWAT
values as they will drop to near a tenth of an inch. Ridging will
follow this system and is expected to remain for the forecast
period.

Behind this trough, some of the coldest temperatures of the season
are currently forecasted as there is a deep cold pool moving into
the area, so get those winter clothes out. Temperatures are
forecasted to be 10 to 20 degrees below normal for the forecast
period. High temperatures for most places are expected to range from
the mid 20s to near 40 degrees, while lower Washington County will
be in the low 50s. Low temperatures will be coldest for Friday
morning and Saturday morning. Low temperatures for higher elevation
areas are expected to range from the single digits to the middle
teens. Most valley locations can expect low temperatures between the
middle teens to mid 20s, while lower Washington County will be near
freezing.

There is some uncertainty towards the end of this forecast period.
Deterministic GFS and EC models have another trough affecting our
area, while ensemble guidance has mixed solutions. By Sunday all
ensemble guidance is pointing to three different possibilities. The
first solution that represents 60% of all ensembles is the ridging
continues and we remain cool and dry, but begin to warm up slightly.
The second solution that represents 30% of ensembles is what the
deterministic models are showing and that is another system affecting
the area. This would mean cooler temperatures and potential for
precipitation chances to increase. The last remaining solution that
accounts for 10% of ensembles is that we stay under the influence of
the longwave trough that will impact us throughout the week.

&&

.AVIATION...KSLC...Gusty southerly winds expected throughout the
TAF period. There is potential for wind shear to occur, especially
before we break the nocturnal boundary layer. Can expect winds near
2kft to be around 50kts until ~18Z. Mountain obscuration will be
possible as CIGs remain near 7kft for most of TAF period. Towards
the end of the period could see lower CIGS, wind shift towards
northwest, and precipitation chances increase as cold front moves
in.

.REST OF UTAH AND SOUTHWEST WYOMING...Gusty south to southwest winds
expected for most of TAF period. Cold front begins to move through
northern UT around 22Z and move southwesterly. This front would
change winds to the northwest. When the front moves through it will
also lower CIGS and increase precipitation chances. CIGS before this
front moves through are expected to be between 5kft and 10kft. Lower
flight categories possible as front moves through an area. Wind
shear will be possible throughout the CWA, especially between now
and before 18Z as there are strong winds between 40kts and 50kts at
2kft.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Warning from 8 AM this morning to 8 PM MST this
     evening for UTZ101-102-115-122.

     Wind Advisory from 11 AM this morning to 8 PM MST this evening
     for UTZ105-116-119.

     Winter Storm Watch from late tonight through Thursday morning
     for UTZ110>112.

     Winter Storm Watch from Tuesday afternoon through late Wednesday
     night for UTZ113-117-125.

     Wind Advisory from 8 PM this evening to 5 AM MST Wednesday for
     UTZ115-122.

WY...None.

&&

Webber / Cecava

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