Dense Fog in Utah, December 16, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
416 AM MST Fri Dec 16 2022

.SYNOPSIS...Cold and dry conditions are expected Friday through
the weekend, with the potential for more active weather returning
by the middle of next week.

&&

.SHORT TERM (Through 12Z Sunday)...As a trough exits the area this
morning, high pressure along the Pacific coast is building into
Utah and southwest Wyoming. Lingering moisture has allowed an area
of stratus to continue over the valleys of west central Utah, with
some fog being observed in portions of the Cache Valley. These
areas of low clouds will tend to diminish during the morning. In
northerly flow, have seen some canyon winds develop over prone
areas of Washington county, but peak gusts have been below 40 mph,
limiting impacts.

In the wake of the exiting trough, an unseasonably cold airmass
will settle over the area through the upcoming weekend into early
next week. Maxes today will generally run around 15F below
seasonal normals with mins near zero down to some mountain valley
locations. Developing zonal flow will bring a bit of warm
advection, but developing valley inversions will limit warming in
those areas, and ample snow cover increases fog concerns.

.LONG TERM (After 12Z Sunday)...Long term forecast period begins
with an inconsequential shortwave sliding eastward through the
Desert Southwest. Following its departure, west to northwesterly mid
to upper level flow already in place across the northern half of the
forecast region develops throughout the remainder. Overall, not too
much in the way of impacts anticipated, but rather just a
continuation of a very gradual warmup into the upcoming week. That
said, as mentioned in previous discussions, combination of the
warmup along with lingering snow pack may result in low level
temperature inversions in some valley locations, and will be
something worth monitoring going forward.

By Tuesday into early Wednesday, an amplifying Pacific ridge will in
turn help drive stronger northwesterly flow over the region. Given
the origins of the airmass, will actually see a continuation of the
gradual warmup back nearer to climatological normal across the
forecast region, as well as a slight uptick in available moisture,
particularly across the northern half of the forecast region. As a
result, will see low end PoPs creep back into the forecast for
northern Utah and southwestern Wyoming.

For the remainder of Wednesday onward through the rest of the long
term period, forecast uncertainty increases modestly. There is
reasonable agreement in deterministic and ensemble solutions that
the continued amplification of the Pacific ridge will yield a strong
northwesterly jet digging in from the PacNW. Most guidance currently
suggests this will at least provide a glancing blow, and support
isolated to scattered PoPs for the northern portion of the forecast
region. By Thursday however, guidance appears to split into two
camps, with roughly 30% of ensemble members supporting a sharper
digging trough into the central Plains and more lingering northerly
flow (likely ending the gradual warmup), and 70% of ensemble members
supporting a quicker transition to ridging with the ridge axis
shifting into the Intermountain West (likely continuing the gradual
warmup). Unsurprisingly this also results in a bit of a mixed signal
for PoPs, but given continued ensemble mean PWATs in excess of 100%
of climatological normal and potential for modest upper level
support, will maintain the isolated to scattered NBM PoPs for the
northern half of the forecast region.

&&

.AVIATION...KSLC...Lingering low level moisture to result in the
potential for some periods of low level clouds and associated MVFR
CIGS/VIS during the morning. Brief window of IFR possible, but low
confidence in this occurring given ongoing trends. Expecting
daylight to begin to scatter out any remaining BR and bulk of low
clouds by mid to late morning, with only SCT high clouds thereafter.
Winds anticipated to remain light, with diurnal directional shifts
near typical times.

.REST OF UTAH AND SOUTHWEST WYOMING...Weak disturbance will continue
to push out of the region early Friday. With the exception of LGU
where BR/FZFG will yield significantly reduced VIS/CIGS through late
morning, generally expecting at worst MVFR CIGS, which will also
begin to scatter out by mid to late morning. Thereafter,
anticipating primarily just SCT high clouds, once again with the
exception of LGU where abundant lingering low level moisture will
result in diminishing VIS/CIGS by Friday evening. Generally
expecting fairly light and diurnally normal winds through the
region, though SGU will see some isolated potential for canyon
enhancement and stronger associated winds/gusts through late Friday
morning.

&&

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

SHORT TERM...Traphagan
LONG TERM/AVIATION...Warthen

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