057
FXUS65 KSLC 042301 AAB
AFDSLC

Area Forecast Discussion...UPDATED
National Weather Service Salt Lake City UT
401 PM MST Wed Dec 4 2024

.SYNOPSIS...Persistent valley inversions will remain in place
under the influence of broad high pressure through at least
Saturday. A trough looks to sweep through the region late in the
weekend, bringing relief from valley inversions and a shot of
precipitation to northern Utah and southwest Wyoming.

&&

.DISCUSSION...A broad ridge of high pressure remains in control of
the western U.S., bringing relatively unchanged conditions day
after day across Utah and southwest Wyoming. Stable conditions
under the influence of the high has brought, and will continue to
bring, stagnant air across Utah`s western valley areas. Strong
inversion conditions are expected to be prevalent through all of
Utah`s valleys through the period of high pressure, however, the
most notable valley haze/ pollution will be confined to the
Wasatch Front (including the Tooele Valley). Up until Saturday/
Sunday, expect daily temperatures to fluctuate within a few
degrees as temperatures warm overhead but cannot mix to the valley
floors. A weak shortwave trough will introduce some cooler
temperatures that will allow temperatures in the mid- and upper-
elevations to cool off by around 5 degrees, but this shortwave and
cooler temperatures are not expected to help clear valley
inversions.

The focus of the forecast period then shifts to Sunday as there is
high confidence in a trough dropping into the western U.S.,
bringing us a shot of precipitation and, more importantly, a cold
front that will help to mix out valley inversions. While a shot of
precipitation looks more likely for the northern half of the
forecast area, the airmass associated with the trough will be
sourced from interior Canada, meaning that it`s likely to be dry.
How dry? For the mountainous terrain across northern Utah, the
25th to 75th percentile (the most likely range) 48-hour QPF ending
Tuesday morning ranges from 0.01" to 0.16" for less favored
terrain and 0.07" to 0.4" for the more favored terrain (i.e.
Cottonwoods, Wellsvilles, and northern Bear Rivers). By no means
is this next storm expected to be a significant storm (worthy of
advisory/ warnings). There is still some uncertainty to work
through with the precise intensity and moisture content associated
with this storm (as is suggested by the range in the mostly
likely QPF), but at this point it appears this system will bring
generally low impact precipitation to northern Utah and southwest
Wyoming.

The portion of the forecast that looks more certain at this point
in time is the fact that we`ll see a potent cold front sweep
across the region on Sunday. As such, temperatures will trend
downward Sunday into Monday. Will need to maintain awareness in
trends of the speed of the cold front, as an earlier arrival in
northern Utah/ southwest Wyoming may keep temperatures cooler than
forecast. Current model consensus points toward a cold frontal
passage on the Wasatch Front by late Sunday evening, which would
give the area at least one more day of stagnant air. Heading into
Monday, however, temperatures should drop by 5-7 degrees in the
wake of the cold front (excluding lower Washington County).

By Tuesday, ensemble guidance maintains solid agreement on
building mid-level heights back over the region with surface high
pressure reestablishing through at least the end of the week. On a
positive note, modeled indices of the Pacific-North American
pattern (essentially an index detailing the ridge-trough patterns
over North America and the eastern Pacific) are trending solidly
negative heading into next weekend (favoring longwave troughing
over western North America/ eastern Pacific). So it appears that a
pattern flip will be underway by the second week of December...
things may begin to shape up for the final weeks of the year.

&&

.AVIATION...KSLC...Inversion conditions will persist with haze
lingering over the terminal through the period creating borderline
MVFR-VFR conditions as VIS remains varied between 3-5SM. Slantwise
visibility will be heavily restricted around sunset and sunrise. Fog
likely develops around the Great Salt Lake overnight, but overnight
light southerly winds will likely keep this pushed just to the north
of the terminal.

.REST OF UTAH AND SOUTHWEST WYOMING...Inversions remain in place
which will keep haze and reduced VIS for urban valleys in northern
Utah creating high end MVFR conditions. Slantwise VIS will be
heavily restricted around sunset and sunrise. Fog development is
likely again tonight near the Great Salt Lake and near larger bodies
of water that could drop VIS to IFR through mid morning. Winds will
be light and variable.

&&

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

$$

Webber/Mahan

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