High Wind Watch 4, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
459 PM MST Sun Feb 19 2023

.SYNOPSIS...A very significant winter storm is expected to impact
the region Tuesday into Thursday, with heavy snow and strong winds.
Another winter storm will impact the area Thursday into Friday.

&&

.SHORT TERM (THROUGH 00Z THURSDAY)...Utah remains under a
northwesterly flow aloft this afternoon with last night's trough
having already exited the area and high pressure upstream over the
central/eastern Pacific. Clouds will be increasing into the area
from the north this evening and overnight while H7 warm advection
strengthens. This will allow generally light snow to develop again
over the northern mountains and potentially the far northern valleys
late tonight into tomorrow. The increasing cloud cover will maintain
min temperatures of near to slightly above Climo tonight and near to
slightly below Climo tomorrow. The exception is the lower portions
of the western Uinta Basin where a strong inversion remains and is
unlikely to break out tomorrow.

Of greater interest is the upper trough currently noted on satellite
imagery near 45N 160W. This trough will dive into the PacNW area on
Tuesday as the upstream ridge retrogrades, and carve out a longwave
trough across the western CONUS Tuesday night through midweek. There
is very good model agreement on the large scale pattern depicting a
broad and anomalously deep trough across the West.

Going a bit more into the details, precipitation will increase
across the northern mountains and far northern valleys late tomorrow
night into Tuesday morning aided by increasing upper diffluence. The
associated cold front will push into far northern Utah Tuesday
afternoon, then continue through the northwestern third of Utah by
late Tuesday evening. Also on Tuesday evening, lee Cyclogenesis will
occur across southern NV, with the low then proceeding to track
across southwest through east-central Utah through Wednesday
morning. As it does so, the thermal gradient/baroclinic zone will
pivot counter-clockwise from an H7 circulation developing over
northeast Utah. Thereafter, a moist northwesterly flow will continue
through Wednesday night.

Several factors make this a very potent storm. First, this storm
will be associated with an anomalous plume of moisture (above the
95th percentile of Climo). Second, a cold airmass will spread into
the area, considering the far northern trajectory of the trough.
Third, there will be very favorable dynamics owing to the strong
cold front and rapid cyclogenesis. All this will result in
widespread snow, heavy in many areas, with strong winds especially
across the south as the surface low deepens.

Some valleys may see a bit of rain ahead of the cold front, but a
changeover to snow will likely occur rather quickly given the strong
dynamics. This changeover may occur early enough to impact the
Tuesday evening commute along the Wasatch Front. Even though the NBM
maintains higher snow levels during this time, strong lift indicated
by BUFR soundings and associated time-heights suggest a good
potential for snow by mid-afternoon Tuesday. This snow could be
heavy owing to strong lift through the DGZ.

Added a Winter Storm Watch to a number of zones with the afternoon
package. Other zones not in the Watch area will potentially need a
Winter Weather Advisory at some point; just highlighting the biggest
threat areas right now with the watches. Also went for a High Wind
Watch for southern valleys. Thinking the NBM may be overdone with
what is on first glance outrageous speeds in some areas, but with
strong pressure falls expected to occur across southern/eastern Utah
with the cyclogenesis, it may be difficult to completely dismiss.

.LONG TERM (After 00Z/5PM Thursday)...Our weather pattern will
remain active through the latter half of the week. A very broad area
of low pressure will continue having influence over the pattern
across Utah and southwest Utah and across much of the Intermountain
West. By Friday, a trough of low pressure is forecast to dig off the
west coast and progress into a negative tilt before cutting off from
the main flow Saturday and then tracking eastward across Utah
sometime Sunday/Monday. All lot of moving parts are involved in that
synoptic set-up and thus makes for a complicated weather forecast.
Interestingly, cluster models are in quite good agreement with each
other through Friday regarding the above set-up. They begin to
divulge a bit when the low cuts off and tracks east. The timing to
which this will happen is in flux (faster in about 30% of the
ensembles, slower in about 40% of the members, the other 30%
somewhere in the middle). The takeaway here is the forecast
confidence decreases over the weekend.

Now lets go back and discuss some details on Thursday and Friday. A
frontal passage will have already passed by the start of this long
term period. Strong west to northwest winds are expected across
eastern Utah Wednesday evening, in particular across Castle county
and in gaps areas on the lee side of the Central Mountains. NBM
indicates anywhere from a 90% to 95% chance of exceeding 55 mph in
these areas with a 55% chance of exceeding 55 mph in Price. High
Wind Warnings may be considered if trends persist. Similarly
impressive chances (90%) exists for chances for exceeding 60 mph in
the gap areas. A similar set up is noted for much of Grand Staircase
and Glen Canyon/Lake Powell. Wind gusts upwards of 50 mph are
expected Wednesday evening with a decreasing trend forecast after
sunset.

Snow Wednesday evening is expected to persist across much of the
state though this will likely be lighter snow and not widespread
higher intensity snowfall. That stated, we can't rule out any
mesoscale banding at this point. Snowfall accumulations through
Thursday morning are expected to be relatively light across the
mountains. Our focus will shift to the southern mountains Thursday
morning through Saturday as southwest flow takes over and this
becomes the impetus for localized heavier snowfall. In the Brian
Head/Cedar Breaks area, an 80% chance for a foot of snow exists
during this period with 30% chance of 2 feet.

Behind the frontal passage very cold air will most certainly filter
in. Thursday morning is expected to be the coldest morning with the
potential for extreme cold conditions. Areas that will be most
vulnerable include the Sanpete Valley, Bryce Canyon and Zion NP. Our
mean flow will quickly shift back out of the southwest and south and
allow for a warming trend thereafter, though still a good 10 degrees
below climate normals. Therefore, the threat for extreme cold will
be very short lived.

&&

.AVIATION...KSLC...Fairly benign conditions for first portion of TAF
period, with fairly light diurnal winds and VFR conditions
prevailing. By Monday morning, approaching system results in
increasing moisture and cloud cover especially between around 4-10
kft. These conditions are then expected to largely persist
thereafter. Uncertainty noted on potential south to north wind shift
Monday afternoon, with guidance split on maintaining southerly
through the day, or switching at a delayed time in the afternoon.

.REST OF UTAH AND SOUTHWEST WYOMING...For southern terminals, quiet
conditions with clear to mostly clear skies and fairly light winds
are expected. For northern terminals, most locations see period of
partly cloudy conditions, with increasing clouds/moisture between ~4-
10 kft by Monday morning as a system approaches. At LGU/EVW/OGD,
will also see the system bring in some low end chances of
precipitation by mid morning Monday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Storm Watch from Tuesday afternoon through late Wednesday
     night for UTZ102-104>106.

     Winter Storm Watch from late Monday night through late Wednesday
     night for UTZ103-107-109.

     Winter Storm Watch from Tuesday afternoon through Thursday
     morning for UTZ108-111-112.

     Winter Storm Watch from Monday evening through Thursday morning
     for UTZ110.

     Winter Storm Watch from Tuesday afternoon through Wednesday
     evening for UTZ113>117-125.

     High Wind Watch from Tuesday afternoon through Wednesday morning
     for UTZ122-128-130-131.

     Winter Storm Watch from late Tuesday night through Wednesday
     evening for UTZ122.

WY...Winter Storm Watch from Tuesday afternoon through late Wednesday
     night for WYZ021.
&&

Cheng/NDeSmet/Warthen

For more information from NOAA's National Weather Service visit...
http://weather.gov/saltlakecity