Winter Storm Warning 5, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
602 AM MDT Sun Apr 10 2022

.SYNOPSIS...A significant winter storm is expected Monday night into
Wednesday, bringing cold temperatures and snow to the valley floors.
Unsettled and cold conditions are expected to continue through the
week.

&&

.SHORT TERM (THROUGH 00Z THURSDAY)...The axis of the upper trough
that pushed a cold front through Utah yesterday continues to shift
east of the area this morning. While a cyclonic northwest flow aloft
currently remains in place, it will trend slightly anticyclonic this
afternoon, with more stable conditions settling into the area. That
said, a few weak showers are not out of the question across the
northern Utah mountains this morning with a modest jet streak nosing
into the area. Recent satellite trends indicate developmental cloud
cover, which supports this possibility.

With the frontal boundary now over northern Arizona, a northwesterly
pressure gradient is in place over Utah, strongest across central
and southern Utah. Gusty winds remain in place this morning in this
postfrontal environment, particularly in favored downslope areas.
This includes the western Uinta Basin, Castle Country, and the
canyons of Washington County. Wind Advisories and Warnings remain
in place in these areas, but expect winds to trend weaker after mid-
morning when the gradient relaxes and H7 flow decreases.

Cooler air will settle across the area today. Max temperatures will
be below Climatological normals across the forecast area, but
especially across northern and central Utah and southwest Wyoming
where departures from normal are expected to be in the 10-15 degree
range. Temperatures will rebound tomorrow as H7 flow turns more
southwesterly while warm advection takes hold, this as the next
storm system comes onshore over the PacNW. Even so, northern Utah is
still expected to continue to see afternoon max temperatures
remaining slightly below normal.

As the aforementioned storm system and associated cold front
approaches the area, winds will increase tomorrow, especially across
southern Utah. A favorable southwesterly pressure gradient and H7
flow to 55kt will bring strong gusty winds to the southern tier once
again tomorrow afternoon into tomorrow night. These winds should be
a bit stronger than with the last cold front, with potential for
widespread 45-55 mph gusts and localized areas over 55 mph. This is
supported by the HREF ensemble means, while the NAEFS and EPS
indicate a relatively anomalous wind potential with H7 wind speeds
in the 99th percentile of climatology. Wind highlights will likely
be needed, but prefer to wait until most of the existing ones have
been expired or cancelled to make for a less-confusing product.

So what will this storm and cold front bring? There will be more
moisture available compared to yesterday's storm owing to the
presence of an atmospheric river that will impinge onto the NorCal
coast tomorrow morning. This moisture, coupled with another strong
frontal boundary, will bring fairly widespread and signficiant
precipitation to the area. Precipitation is expected to begin
tomorrow afternoon across northern Utah, particularly the northern
mountains, ahead of the cold front. As the front moves into northern
Utah tomorrow early evening and then into central and southwest Utah
by midnight, expect precipitation intensities to increase with snow
levels quickly falling to valley floors. As the front moves out of
Utah Tuesday morning, northwesterly flow aloft and H7 cold advection
will maintain favorable conditions for orographic precipitation in
favored northwesterly aspects through Tuesday evening, while
embedded disturbances rotating through the mean trough will also
bring periodic snow showers to northern Utah.

Accumulating snow could impact travel along the Wasatch Front and I-
15 corridor through Cedar City tomorrow night into Tuesday, although
the extent of the impacts are still a bit tricky at this point.
However, given that the greatest snow intensities will occur during
the colder nighttime hours, there seems to at least be a reasonable
chance for some road accumulations and travel impacts through the
Tuesday morning commute, with perhaps lesser impacts Tuesday
afternoon owing to above-freezing temperatures and the April sun
angle. Mountains will see significant accumulations regardless,
especially across the north, and a Winter Storm Watch has been
issued. Snow will decrease across the area Tuesday night. However,
snow showers are expected to develop again across the north for
Wednesday afternoon as a trailing shortwave disturbance along the
back edge of the trough crosses the area.

Temperatures will fall back to well-below Climo values behind the
cold front, given the cold winter-like airmass (-14 to -15C at H7
across northern Utah by Wednesday morning). While afternoon
temperatures will be moderated by the sun angle, overnight mins will
fall below freezing in many valleys for both Tuesday and Wednesday
mornings. Some of the colder valleys will likely see hard freezes,
which would impact agricultural interests where the growing season
has commenced.

.LONG TERM (After 00Z Thursday/6PM Wednesday)...As the departing
trough swings eastward and deepens, a brief period of quiet weather
will return for Thursday as a ridge builds into the mid-level height
field in its wake. Strong warm advection from the south is likely to
help drive a nearly +10F rebound in high temperatures for Thursday
afternoon, though these will still run 5-10F below seasonal normals.
However, substantial uncertainty remains in the extent of cloud
cover over northern and central Utah for Thursday afternoon, which
could produce drastic deviations from the current forecast maxes.
Continuing to advertise potential for hard freeze conditions across
much of northern and central Utah with minimum temperatures falling
to or below 28F in valley locations.

By Thursday night, nearly all model solutions agree that the short
lived period of ridging ends with the axis shifting off to our east
and another trough developing to the northwest. There is a high
degree of uncertainty associated with the progression of the pattern
from Thursday night through the end of the weekend. Each of the
deterministic solutions handle the timing, amplitude, and overall
organization differently, though appear to share a common thread in
producing a quick hitting shortwave trough Friday and a second,
higher amplitude shortwave trough moving through late Saturday/early
Sunday. Additionally, each solution features a well-defined cold
front, which will improve overall potential for more widespread
precipitation and likely drive yet another period of enhanced wind
gusts for southern Utah.

Hard to pin down PoPs and QPFs for the Friday-Sunday period and have
chosen to maintain NBM/WPC guidance for more widespread low amounts.
Noting the cluster analysis highlights 14% of members produce a
strong ridge centered over the west coast at this time, some bust
potential exists in a non-event outcome. However, 47% favor a high
amplitude solution bringing widespread precipitation to the forecast
area and the remaining 39% a low-amplitude grazer limiting
precipitation to the northern mountains at best. Thus, confidence is
highest that the northern mountains and valleys will see some amount
of accumulating precipitation with snow levels initially around 6kft
and falling, whereas confidence is substantially lower for central
Utah.

&&

.AVIATION...KSLC...VFR conditions and non-impactful weather are
expected to continue through Sunday night. Winds are expected to
remain out of the northwest throughout the day, with gusts in excess
of 20kts possible. Winds will eventually turn out of the south in
advance of the next storm system around 12Z Monday, and with it,
expecting increasing clouds and lowering CIGs with a minimal chance
of dropping to IFR conditions in the valid TAF period at this time.
Precipitation on Monday will likely remain as rain, with a
changeover to snow early Tuesday morning.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions expected across
the forecast area for much of Sunday, with increasing cloud cover
through the afternoon, especially across northern Utah and southwest
Wyoming. Winds will be out of the north through northwest at most
terminals, and may gust in the 20-30kt range. By Monday evening,
CIGs may drop into MVFR across northern Utah and IFR for the Cache
Valley as showers begin to spread into the region with the next
storm system moving in. Monday night will also bring a shift to more
steady southerly winds across much of the forecast area.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Warning until 10 AM MDT this morning for UTZ120.

     Wind Advisory until 1 PM MDT this afternoon for UTZ114.

     Wind Advisory until 9 AM MDT this morning for UTZ123.

     Winter Storm Watch from Monday afternoon through late Tuesday
     night for UTZ110>112.

WY...None.
&&

Cheng/Wessler

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