Text Products for AFD Issued by SLC

Area Forecast Discussion
Issued by NWS Salt Lake City, UT
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FXUS65 KSLC 012257
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
457 PM MDT Sat Apr 1 2023

.SYNOPSIS...A significant winter storm will impact much of the
region through at least Tuesday.

&&

.SHORT TERM (Through 12Z Wednesday)...A significant late season
winter storm will impact much of the state with very heavy
mountain snow, significant valley snow and strong, gusty winds
across southeastern Utah over the short term forecast period.

Afternoon upper air and satellite analysis indicates a weak
shortwave trough is crossing the northern Intermountain Region. A
stronger and very cold upper level low is currently near the
British Columbia Coast. Near zonal flow is in place across the
Interior West.

The previously mentioned rather weak shortwave trough mentioned
earlier will actually be more impactful as a weak baroclinic zone
associated with this trough will settle into far northern Utah
tonight into Sunday morning. This will be a significant feature a
bit later in the forecast.

As the upper level low and associated trough continue to
strengthen and dig southward along the Pacific Coast Sunday, upper
level diffluence associated with the polar jet will shift into
northern Utah Sunday afternoon into Sunday evening. This will also
spread precipitation into far northern Utah as well as the
northern mountains of Utah. Current atmospheric profiles support
either rain/snow mix or all snow for the eastern Box Elder area as
well as Cache Valley and the Ogden Valley...which may see
accumulations pick up quickly near sunset Sunday.

As the best jet support moves over northern Utah, a period of
frontogenesis is expected with this baroclinic zone, with the
front gradually shifting south overnight Sunday into Monday
morning. This...at a time with IVT anomalies near the 99.5
percentile. This will likely bring a band of moderate to heavy
snow through the valleys of northern Utah, reaching Salt Lake
County near to before the Monday morning commute. Snow rates
combined with dynamic cooling should be sufficient to change any
rain quickly to snow as 700mb temperatures near -8 to -10C or so
shift into the area behind the front.

One of the main questions is how far south this front will get
during the Monday morning commute. For the Utah Valley, expect
impactful snow through Lehi...however there may be more of a
rain/snow mix for the lower portion of Utah Valley from Lehi to
Orem for a time Monday. Nearly all the guidance envelope supports
any rain changes to snow even for Utah Valley by Monday afternoon
into Monday evening. Note that the current winter storm warning
for Utah Valley indicates totals will be less for the Lehi to
Orem area for this reason.

Finally, the boundary will eventually orient nearly over the I-15
corridor from SLC to Cedar City through the remainder of Monday
into Tuesday, with moderate to heavy snow expected. Areas north of
Salt Lake City may see a temporary break behind the front from
later Monday morning into Monday afternoon...but this is unlikely
to last long.

As the best jet support shifts across eastern Utah, guidance has
consistently developed a period of cyclogenesis resulting in a
relatively deep surface low across eastern Utah. With this low in
place, the bulk of the model guidance suggests a combination of a
TROWAL (trough of warm air aloft) and wrap around precipitation
will likely increase snow rates across northern Utah by later
Monday afternoon into Monday evening, even in areas that see a
reduction in precipitation intensity behind the front.

With this low ejecting into the High Plains earlier Tuesday, a
gradual drying trend is expected across southern Utah. Northern
Utah however will remain in cyclonic, moist northwest flow...and
likely continue to see period of moderate to heavy snow into
Tuesday evening...and potentially Wednesday morning.

Right now, the highest confidence period of moderate to heavy
snow, particularly for the I-15 corridor is Monday morning through
Tuesday morning. Travel impacts are expected, with significant
mountain travel impacts.

Expect strong winds associated with the dry slot over
southeastern Utah associated with this developing low. This has
been depicted for days in the Extreme Forecast Index (EFI) as well
as EPS and NAEFS anomalies. Expect wind gusts to 65 mph as early
as late Monday morning continuing into Monday evening for the
Grand Staircase, western Canyonlands, Glen Canyon and the Swell. A
High Wind Watch remains in effect, though this is likely to be
upgraded to a warning with the next package.

Some guidance also indicates post-frontal winds may be sufficient
for at least wind advisory if not high wind warning criteria for a
time along the SR-10 corridor in Castle Country Monday night into
Tuesday morning. This is sufficiently far into the forecast
period to hold off on a high wind watch (and confidence/likelihood
isn`t sufficient yet for issuance). Something for the next shift
to consider.

As far as winter headlines, upgraded the winter storm watches for
the northern valleys and mountains to winter storm warnings for
all locations except the Great Salt Lake Desert and Mountains,
where amounts will be more limited. Note that Weber, Davis,
eastern Box Elder, Cache may have a period of lower precipitation
intensity later Monday morning through Monday afternoon but felt
the combination of heavy snow rates and impacts warranted
including these areas in the warning. Similarly as mentioned
earlier, lower Utah Valley from Lehi to Provo may see lower
amounts depending on precipitation- type in the frontal portion of
the storm.

The Wasatch and western Uinta Mountains have a high chance of
seeing storm totals exceeding 2 feet, with some portions of the
Wasatch particularly the upper Cottonwoods and ridgeline near
Bountiful and Farmington may see upwards of 4 feet to potentially
5 feet. The Wasatch Back is likely to see another 1 to 2 feet.
Snow loading weight may become an issue in some areas.

.LONG TERM (After 12Z Wednesday)...The axis of the trough
associated with our early week storm system will be located east
of the forecast area by Wednesday morning, however, below average
mid- level heights will stick around through at least Thursday
morning. What this means for the forecast area is temperatures
will be slow to warm, and colder than normal mid-level
temperatures will help maintain some instability over at least the
northern half of the CWA. With enough low-level moisture
remaining in place from the storm system early in the week, the
lingering lower level instability will help to bring snow showers
over the mountainous terrain from roughly Filmore northward. Areas
across northern Utah (i.e. Wasatch, Bear River, and Uinta
Mountains) will see the highest (50-65%) probability of seeing
afternoon snow showers, while the central mountains (including the
Bookcliffs) will see lower chances (~30-40%). For the northern
and central valley areas, there is generally anywhere from a 20 to
40% chance of seeing snow showers. Amounts from any additional
showers will be rather insignificant and there is a low to medium
chance (20-40%) that we see more than 1 inch of snow on the
Wasatch Front and in the Cache Valley.

With 500mb heights below normal and 700mb temperatures pushing
close to record lows (record: -14.26C, forecast: -12.75C),
afternoon max temperatures on Wednesday will still be running
about 15 to 20F below average for this time of year!

Past Wednesday, ensemble guidance begins to diverge a bit. Big
picture of the synoptic setup will involve the aforementioned
trough continuing eastward as an upstream ridge looks to build and
settle over the western U.S. into the weekend. In general, this
will bring a dry weather pattern with potentially rapidly
increasing temperatures. That said, ensemble means from the GEFS
and EPS show varying timing of how long below average 500mb
heights stick around, and how quickly above average heights move
in. The mean flow pattern supports troughing over the Great Basin
region morphing into a potentially cutoff low over the Southwest
with an upstream ridge over the PacNW region , but with
significantly varying amplitudes. A slower progression of this
setup would keep chances for precipitation in the forecast for a
longer period of time (potentially through the weekend) with a
slow warming trend. Models are very much split on this, with ~55%
favoring the introduction of above average heights by Friday
evening and the other 45% favoring below average. Have maintained
a slow warming trend in the forecast from Wednesday forward with
daily chances for showers over the higher terrain of northern Utah
through Saturday. Will hope for some increasing confidence
heading through the week, but given springtime trends... would
potentially hedge toward the development of a cutoff low sticking
around late in the week over the Southwest. Will see how newer
models trend.

&&

.AVIATION...KSLC...Gusty south to southwest winds, up to 35kts,
expected for the remainder of the afternoon and into the early
evening. These winds will switch to the north between 04Z and 06Z
and last until ~11Z. There is still a 40% chance that these winds
stay south all night. Regardless of wind direction, expect winds
to be light overnight and into tomorrow morning. North winds will
come back around 19Z. Mountain obscuration likely around 06Z as
CIGS drop to around 5kft and lasting through the TAF period.

.REST OF UTAH AND SOUTHWEST WYOMING...Southwest winds gusting up
to 30kts expected through the afternoon and into the evening will
transition to light and diurnal winds for the overnight period.
Winds will shift to the north for northern Utah and southern
Wyoming during the morning hours. Gusty southwest winds will come
back for southern Utah by the mid-morning. CIGS expected to lower
to ~5kft during the overnight hours for northern and central Utah.
Southern Utah can expect mostly clear skies. Some scattered
rain/snow showers possible for northern Utah and southwest Wyoming
overnight. There is a 40% chance that these showers could create
MVFR conditions.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from midnight Sunday night to 6 PM MDT
     Monday for UTZ101.

     Winter Storm Warning from 3 AM Monday to 6 PM MDT Tuesday for
     UTZ102-105-106.

     Winter Storm Warning from midnight Sunday night to 6 PM MDT
     Tuesday for UTZ103-104-107>109.

     Winter Storm Warning from noon Sunday to 6 PM MDT Tuesday for
     UTZ110>112.

     Winter Storm Watch from Monday morning through Tuesday afternoon
     for UTZ113-115>118.

     High Wind Watch from Monday morning through Monday evening for
     UTZ121-128-130-131.

     Winter Storm Watch from Monday afternoon through late Monday
     night for UTZ122-125.

WY...Winter Storm Warning from midnight Sunday night to 6 PM MDT
     Tuesday for WYZ021.

&&

$$

Kruse/Webber/Cecava

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