Spanish Fork, Utah 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

817
FXUS65 KSLC 221116
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
416 AM MST Sun Dec 22 2024

.SYNOPSIS...A series of weak weather disturbances will cross the
area through Monday. Potentially stronger systems will cross the
region over Christmas, as well as late in the upcoming week.

&&

.SHORT TERM (Through 12Z/5AM Tuesday)...A predominantly zonal flow
resides across the Great Basin early this morning, with a handful
of weak perturbations embedded within the mean flow. Of
consequence to the forecast area, one such wave is currently
spreading into the western Great Basin, while a somewhat better
organized feature is approaching the northern California coast
near 140W.

Ahead of these features, an unseasonably warm airmass remains in
place across the forecast area with RAP analyzed 700mb temps in
the 2C-4C range early this morning. This warm air aloft continues
to maintain valley inversion across northern and eastern Utah,
which will likely remain in place through the day today. Somewhat
cooler air upstream across Nevada will gradually spread into Utah
through the day as the weak eastern NV wave passes through the
region. This combined with increased cloud cover will trend max
temps down roughly 5F across well mixed areas, while inverted
valleys remain closer to or perhaps a bit below persistence.

The initial wave currently crossing eastern Nevada will slowly
translate eastward today, crossing the forecast area this
afternoon. Model guidance varies fairly widely with respect to the
coverage of precipitation with this feature, although current
radar trends across western Nevada would suggest some of the more
aggressive models such as the NAM Nest are at least initializing
and better handling the current situation moreso than the drier
solutions such as the HRRR and deterministic global models
(GFS/EC). That said, given the weak forcing and generally dry
boundary layer in place across the forecast area most northern
valleys will likely struggle to achieve measurable precipitation
with generally light snow accumulations above roughly 8000 feet in
the terrain through early this evening.

The secondary wave near 140W will translate inland this evening
before quickly propagating east across the Great Basin overnight
into Monday. Even 24-30 hours out there remains uncertainty
regarding how quickly this wave passes through the region, and is
largely dependent on how quickly upstream ridging is able to build
east. The consensus of model solutions has trended a little
faster/weaker with this wave, and have trended precip totals down
a bit accordingly. It`s worth noting however a few outliers
including the NamNest are much more aggressive maintaining more
continuous precipitation both ahead of and with the passage of
this feature across northern Utah late tonight into Monday
morning, as well as spreading measurable precip much further south
into central and southwest Utah. Low probability this idea
verifies, but certainly something to monitor as we go through the
day.

Lastly, this secondary wave looks like it`ll bring enough cooler
air aloft to dislodge most valley inversions by late Monday, with
the exception of the most sheltered/entrenched areas such as the
Salt Flats and perhaps the Cache Valley.

.LONG TERM (After 12Z/5AM Tuesday)...Active long-term portion of the
forecast remains on track to close out the year 2024. At the start
of the period, deterministic and ensemble forecast systems remain in
good agreement with respect to the large-scale pattern evolution,
with a progressive trough entering Utah Tuesday evening before
exiting late in the day Wednesday. Despite the good large-scale
agreement, there are some relevant elements of spread among the
model solution space that would result in various forecast outcomes
that have yet to by fully resolved. Such spread surrounds the speed,
location, timing and certainty of the trough closing off into a
potential closed low over some portion of the forecast area. These
differences have important implications on the forecast.

Starting with the ensemble mean outcome, it resembles largely the
same forecast that we`ve been discussing for several days, namely a
state-wide system that brings beneficial, sub-advisory level
mountain snows and mainly rain in northern and central valleys with
a brief period of snow at the outset Wednesday morning. However, a
faster solution (such as represented by the deterministic GFS) would
result in not only an earlier changeover to snow in valleys, but
also more snow in mountains. This model offers a good
representation of the 90th percentile (or 10% chance outcome) with
1-2" of snow in valleys, slightly higher than amounts offered by
previous runs. 90th percentile snowfall amounts in mountains have
nudged upwards as well, generally in the 1-2 foot range, with the
Bear River Range, Upper Cottonwoods and Tushars favored. NBM
precipitation type percentages have ever so slightly nudged in the
colder direction as well at SLC, with snowfall chances now
approaching 20%. Although largely rain remains favored in valleys,
odds are gradually nudging higher for snow at the outset. Either
way, most likely scenario paints 1" to perhaps up to 3" of snow for
benches. Those planning to travel over mountain passes should be
prepared for periods of winter driving conditions.

Have nudged winds and gusts slightly upwards in advance of the
system, especially Tuesday afternoon/early evening across southwest
Utah and Western Millard/Juab Counties, with southwest gusts in the
20-30 mph range, with locally higher gusts for exposed ridgelines
and locations on the lee of terrain features perpendicular to the
mean flow. In the wake of the system, W-NW gusts of similar
magnitudes will develop in the lee of the Raft River Range and the
central mountains.

After a brief respite from the precipitation Wednesday night and the
first half of Thursday, mid-level moist and warm advection will
allow for another wave of precipitation to develop across northern
Utah and SW Wyoming in particular by Thursday afternoon, with this
activity persisting through Friday morning. There remain some
amplitude differences with this system. Most likely outcome brings
precipitation to the above-mentioned areas, however more amplified
solutions would nudge areal extent of precipitation towards a
statewide outcome. Additionally, lingering valley cold pools that
are deeper than forecast could result in a longer period of snow at
the surface, before warm advection allows for a gradual changeover
to rain. Rain and snow are equally favored across the Wasatch Front
for the onset of this wave of precipitation late Thursday
morning/early afternoon, with rain gradually favored from late
Thursday afternoon into Friday. Trace to 1" of snow favored in
valleys, with up to 3" for benches. Snowfall amounts on the low end
for northern mountains (10th percentile) is up to 6" of snow, with
reasonable ceiling (90th percentile) in the 1 to 2 foot range for
northern mountains, and 6-12 inches of snow for central and southern
mountains.

Very brief break later Friday before a decaying atmospheric river
with a ribbon of enhanced IVT/IWV sneaks around the Sierras and
across northern Utah late Friday through much of Saturday,
maintaining valley rain and mountain snow. This is the system most
likely to push northern mountains into consideration for headlines,
with a solid 1-2 feet of snow, highest across the Bear River Range,
with 90th percentile amounts slightly exceeding 2 feet. Adding it
all up through the next 7 days, 50th percentile SWE amounts in the
northern mountains ranges from approximately 1.50"-2.50", and
between 0.50"-1.50" for the central mountains.

&&

.AVIATION...KSLC...Largest source of uncertainty through the period
is wind direction. Most likely scenario starts off with south winds
at 12Z Sunday, with winds then transitioning to NW by 18Z, then back
to south around 03Z this evening. However, periods of variable winds
are likely this morning, and again from ~21Z-03Z when rain showers
will be in the area. Mountain obscuration is certain (90-100% chance
for CIGs to drop to 6kft by 21Z), with a 50-60% chance for MVFR CIGs
with rain showers from 21Z to 12Z.

.REST OF UTAH AND SOUTHWEST WYOMING...Mid and upper deck will
gradually thicken/lower across northern Utah and SW Wyoming this
morning, with mountain obscuration developing as early as 18Z.
Northern Utah valley terminals have a 50-60% for MVFR CIGs from
about 21Z-12Z, with higher probabilities towards the Idaho border.
VFR conditions are likely across southern Utah through the period.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Seaman/ADeSmet

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http://weather.gov/saltlakecity