867
FXUS65 KSLC 222235
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
335 PM MST Fri Nov 22 2024

.SYNOPSIS...A dry and mild southwesterly flow aloft will reside
across the region today. The first in a series of Pacific storm
systems will cross Utah and southwest Wyoming late Saturday
through Sunday, with another potential winter storm Tuesday into
Wednesday.

&&

.SHORT TERM (Through 12Z Monday)...Early afternoon upper air and
satellite analysis indicates a landfalling atmospheric river
continues to impact portions of northern and central California.
The associated upper level trough is nearing the Pacific Coast.
A 140+kt jet max is now approaching the California Coast, with
broad upper level diffluence shifting into the western Great
Basin.

Expect this trough and the associated speed max to shift east
through Saturday. Ahead of this, strong southwest winds are likely
to impact southwestern Utah in particular, though will need to
monitor the Tooele Valley and the Sevier Valley as well. 700mb
flow is highest from around 6 AM to 2 PM, near or above 50 kts.
Expect once the cloud shield moves into the area, winds may
decrease by late afternoon. For now, stuck with the higher
probability area (greater than 60 percent chance), southwest Utah,
and issued a wind advisory from Saturday morning into Saturday
afternoon. Expect gusts to 50 mph in the Cedar City and Milford
areas, up to 55 mph for exposed terrain across Beaver and Iron
Counties. Typically in this pattern, the Cedar City area will see
downslope winds materialize earlier than other areas...but if
Cedar City manages to remain stable longer, this will push the
wind threat later.

Pre-frontal precipitation is expected to develop across portions
of the northern mountains, though will remain light to moderate,
particularly north of Utah County. As the associated cold front
crosses northern Utah late Saturday night into early Sunday
morning, a band of moderate to heavy precipitation will cross
northern Utah. This is in response to upper level divergence
shifting across the frontal boundary, inducing a period for
frontogenesis. Given 700mb temperatures near -9C, expect valleys
to see snow to valley floors, especially in heavier precipitation
Sunday morning. Snow rates may approach or exceed 2 inches per
hour for mountain routes across northern and central Utah after
about 3 AM Sunday morning to near noon Sunday afternoon.

The front will gradually weaken as the enhanced upper level
support shifts away from the region...with snow totals quickly
decreasing across the southern mountains.

Expect a general 5 to 10 inches across the northern mountains
(locally up to 15 inches in the upper Cottonwoods) and 4 to 8
inches for the central mountains. Issued a winter weather advisory
for the locations based on snow rate intensity near/behind the
front and impact to the mountain routes in coordination with UDOT.
Will need to monitor adding the Wasatch Back and Uinta County, WY
depending on the residence time of the frontal band in these
locations.

What could go wrong? The frontal band could stall or remain in
place longer, especially across Salt Lake and Utah Counties,
resulting in measurable snow impacting surfaces Sunday morning.
Temperatures will likely be cold enough to snow, however surfaces
are quite warm. That said, a longer residence time will mean more
potential for accumulation. The front could also weaken earlier
and/or see much weaker frontogenesis, resulting in less snow for
the northern and central mountains.

Any snow will come to an end by evening across the region...with
upper level ridging moving into the area by Monday morning.

.LONG TERM (After 12Z Monday)...Brief shortwave ridging on Monday
will bring a lull in activity across the region before a trough
moves into the region that has the potential to bring a
significant amount of precipitation Tuesday and Wednesday. This
trough will shift to the east on Thursday leaving Utah and SW
Wyoming in a cold and dry period of northwest flow.

Guidance has converged more towards a solution for the trough
next week. Previously, there was still a significant portion of
membership keeping this trough offshore. But now that scenario
seems to have mostly disappeared amongst the clusters. The main
questions upcoming is how far south the storm tracks and how
quickly it progresses east.

By late Monday southerly flow will return that will bring a surge
of moisture advection to the entire CWA. Ensemble mean PWATs
approach 0.5" across a broad swath of Utah. These values are
amongst the 90th percentile for this time of year, so moisture
availability will be plentiful for the upcoming trough.

This moisture will interact with a stalled frontal boundary over
central Utah on Tuesday paired with an east/west 125+kt jet. This
will provide for widespread ascent that will help valley rain and
mountain snow to spread into much of the region on Tuesday ahead
of the approaching trough. Given southwest flow ahead of the
trough, terrain favored in this flow regime will likely receive
the highest QPF amounts during this window. Timing of the main
trough and associated cold front is still a bit uncertain. A
majority of guidance (~63%) bring this frontal passage through
overnight Tuesday while the remaining guidance hold off on a
frontal passage until during the day on Wednesday. There is also a
large spread in H7 temperatures behind the front with the
25th-75th percentile ranging from -7C to -11C. The colder scenario
would result in snow down to valley floors, with more of a rain
snow mix for the warmer scenario. As flow transitions to northwest
behind the front expect a lower density snow along with favoring
areas that do better in a NW flow regime.

Another recent trend has been a storm track that brings the
heaviest precipitation to central Utah. 25th-75th percentile
ranges for QPF in valley locations are generally ~0.25"-0.75" with
slightly lesser amounts closer to the Idaho border and lower
Washington county. Mountain QPF totals are generally ~0.5"-1.25".
QPF ceilings are pretty high with totals over 2" in the 90th
percentile range. The large range in potential values reflects the
uncertainty in how quickly the trough moves through the region.

By Thursday the trough will be pushing to the east with much
drier air moving in. This will result in temperatures ~5 degrees
below average areawide Thursday and Friday.

&&

.AVIATION...KSLC...Dry conditions with light winds will last
through the evening, with a transition from northwest winds to
southeast winds around 03Z. Southeast winds will gust around 20
knots after 06Z. Winds will increase, with gusts around 25 knots
from around 12Z through 18Z as overcast conditions build in. As
winds aloft increase from the southwest, low level wind shear is
likely much of the morning. Winds will decrease and transition to
northwest around 21Z. Dry conditions will last through most of the
day with rain showers after 00Z.

.REST OF UTAH AND SOUTHWEST WYOMING...Southeast to southwest
winds will increase after 03Z. Peak gusts through 19Z will be for
southwest Utah, where gusts will be around 40 knots. Throughout
southwest Wyoming and northern Utah, gusts exceeding 30 knots are
likely. Winds aloft will increase early Saturday from the
southwest. The faster speeds will bring low level wind shear
throughout southwest Wyoming and Utah much of the morning. Winds
will diminish through the afternoon. Clouds will increase from
north to south, with precipitation tracking into southwest Wyoming
and northern Utah around 12-18Z and into southern Utah around
00-06Z. Rain and snow will lower conditions into MVFR and lower
ranges Saturday through most of Sunday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 5 PM Saturday to 5 PM MST Sunday
     for UTZ110>113-117.

     Wind Advisory from 5 AM to 5 PM MST Saturday for UTZ122.

WY...None.
&&

$$

Kruse/Mahan/Wilson

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http://weather.gov/saltlakecity