High Wind Watch 6, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
517 AM MST Tue Dec 20 2022

.SYNOPSIS...A storm system will move through the area Wednesday
through early Thursday, bringing snow and gusty winds to northern
and central Utah and southwest Wyoming, followed by cold
temperatures. A weaker system will follow for Friday.

&&

.SHORT TERM (THROUGH 12Z THURSDAY)...The large-scale pattern across
the western CONUS continues to feature a WNW flow aloft with
embedded weak disturbances crossing the area. One such disturbance
grazed northern Utah last night, with another expected this
afternoon. These disturbances have brought increased mid and high
level moisture to the area, and the associated cloud cover has
allowed overnight temperatures to remain warmer than the past few
nights. The warmer start to the day should also allow temperatures
to warm more in the afternoon, although cloud cover will limit the
extent of warming. Still expecting inversion conditions to persist
today, especially in northern Utah, but it is not expected to be
especially strong. Light snow across mainly the northern mountains
may accompany the weak disturbance this afternoon, generally
remaining less than 1 inch of accumulation.

The pattern will amplify tonight through tomorrow, with heights
building in the eastern Pacific which will allow a trough to drop
into the Rocky Mountain region and into the Great Plains. This
trough will bring an arctic airmass to the area. However, Utah will
be on the western periphery of this trough, so expect just a brief
blast of cold temperatures.

The storm will begin with increasing west to northwest flow aloft
tomorrow as a ~170kt H3 jet punches into northern Utah.
Precipitation will increase across the northern mountains late
tonight into tomorrow in the prefrontal environment. Some mixing
will occur in the valleys as south to southwest surface winds
increase, so temperatures are expected to be warmer tomorrow.
Northern valleys will also see precipitation develop during the day
tomorrow. Farther north, this will be all snow, but towards the
Ogden area and southward, there is a potential for some rain to mix
in during the afternoon.

A relatively shallow cold front will then push through northern Utah
during the late afternoon through evening hours tomorrow, behind
which the airmass will gradually stabilize and bring precipitation
to and end tomorrow night. While the surface cold front itself will
continue into southern Utah through the day Thursday, the boundary
will outrun the upper level support (the jet will cut through
northeast Utah), so precipitation chances decrease markedly for the
southern half of Utah. Overall, the northern mountains are still
expected to see around 6-12 inches of snow. Valley amounts will
generally remain in the 1-3 inch range. However, the timing of the
heaviest valley snow (near the cold front) looks to coincide with at
least part of the evening commute.

In addition to the snow, winds will be strong along exposed
ridgelines and in southwest Wyoming, with gusts over 50 mph a good
possibility tomorrow afternoon and evening. This will increase
travel impacts due to blowing and drifting snow. Downslope winds
over 40 mph will also be possible in the western Uinta Basin and
Castle Country behind the front.

Since the forecast snow amounts meet advisory criteria for the
northern mountains, combined with the added impacts with the winds,
have gone ahead and issued a Winter Weather Advisory for the Wasatch
and western Uinta Mountains. Valley snow headlines will depend on
impacts, and will punt decisions on those highlights, as well as
wind highlights, to the day shift to allow for coordination with
partners.

.LONG TERM (After 12Z/6AM Thursday)...As the longwave trough
continues to slide southward to our east, cold temperatures will
continue pushing through the forecast area, with highs near 10-15
degrees below normal on Thursday across northern and central Utah
and southwest Wyoming. Southern Utah temperatures will hover around
normal. There is some lingering uncertainty with exactly how cold we
get, depending on how closely that trough tracks to us. While most
ensemble members favor colder temperatures, there are still 19% of
members that favor surface temperatures 8-10 degrees above the
ensemble mean...which, granted, is still cold, just not as cold. As
for precipitation, any remaining orographic showers are expected to
have ended by Thursday morning at the start of the long-term.

A trailing shortwave trough with weak dynamics and incoming moisture
is expected to bring snow showers to mainly higher terrain overnight
into Friday. At most, a couple of inches are expected across the
northern mountains. Valleys may see occasional light snow showers,
but little to no accumulation is expected at this time.

Beyond Friday, a ridge will start to build in across the west coast,
leaving Utah and southwest Wyoming in overall northwesterly flow
with rebounding temperatures. This flow will transport moisture in
from the Pacific over the top of the ridge, increasing cloud cover
in our area over the weekend. The overall lack of dynamics will
substantively decrease precipitation chances, however there is a
slight chance of snow across the far northern mountains on Sunday
with increased flow and therefore orographic enhancement.

By Tuesday, 86% of ensemble members favor the ridge starting to
shift eastward. In the remaining 14%, northern mountain showers
could continue as long as flow direction is favorable. Roughly two-
thirds of these members favor the ridge maintaining its strength as
it moves over the Great Basin which could further increase
temperatures across the forecast area. The other third favors a
weakening ridge as it moves eastward.

We are still in an overall active pattern, with another potential
storm on the horizon at the very end of the long-term period, still
with plenty of uncertainty.

&&

.AVIATION...KSLC...Light, southeasterly winds will continue until
roughly 18z, when light, northwesterly winds are likely to prevail.
Still, there is a 25% likelihood that winds remain light out of the
southeast. Winds may also be variable at times. Hazy skies will
continue to produce VIS around 4-5SM for the valid TAF period.
Lowering CIGs to 7000ft are expected by ~02z. Light mountain snow
showers are possible after 02z, and will fill in more by 09-10z,
therefore have added VCSH.

.REST OF UTAH AND SOUTHWEST WYOMING...Largely light, diurnally-
driven flow will prevail, with increasing winds out of the south and
west after 12-14z Wednesday. Northern valleys will continue to see
hazy skies and some MVFR VIS. Lowering CIGs are expected after 18z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from midnight tonight to 4 AM MST
     Thursday for UTZ110>112.
WY...None.

&&

Cheng/Cunningham

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