Severe Thunderstorm Warning 62, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
401 PM MDT Thu Sep 15 2022

.SYNOPSIS...A potent shortwave trough is expected to move through
northern Utah this evening and overnight into Friday morning,
bringing periods of heavy precipitation to portions of the state.
As the longwave trough to the northwest amplifies, drier southwest
flow will set up and usher in a drying trend into early next week
with increasing southwesterly winds.

&&

.SHORT TERM (Through 12Z Sunday)...The longwave trough situated to
the west will continue its slow progress through the forecast
area this evening. Overnight tonight, a potent shortwave feature
and associated jet maximum are expected to swing through northern
Utah, providing substantial synoptic-scale lift and sufficient
mid-level instability to allow for nocturnal convection through
at least 2AM. PWATs around 0.5-0.8 inches this evening will climb
to or above 1 inch ahead of the shortwave trough axis as it brings
with it a slug of moisture. While storm motions will be fairly
fast from the southwest to northeast, high rain rates combined
with the potential for an orographic component that could drive
training storms will increase the potential for flash flooding and
debris flows on the northern burn scar areas overnight.

With the shortwave trough clear of the area by mid-morning Friday,
models highlight the longwave trough digging further into the
western Great Basin. As southwesterly flow aloft increases,
expecting continued dry advection throughout the column across
southern Utah, with PWATs falling below 0.5" through all but the
northern third of the forecast area. While Friday is lacking any
sort of broad forcing for precipitation, the lingering moisture
and modest mid-level instability along the Utah/Idaho border will
likely keep showers and isolated thunderstorms around through
Friday evening. Drier air in the low-levels will see the primary
threat transition from one of heavy rainfall to gusty winds in the
valleys, though heavy rainfall is still anticipated with
thunderstorms east of the Wasatch and in the high terrain of the
Uintas and northern Skyline/Wasatch Plateau.

Little change in the pattern is expected from Friday to Saturday,
and as a result, the threat for showers and isolated thunderstorms
remains. However, much lower PoPs represent the greater
uncertainty in the forecast at this point, and storms are likely
to remain terrain-tied, initiating off the northern mountains. In
the western valleys, afternoon wind gusts could climb into the
upper 20 mph range, but are not expected to become impactful at
this time. With a mix of cloud cover in the forecast area and
little change in H7 temps, daytime high temps are expected to
remain unchanged day over day through Saturday.

.LONG TERM (After 12Z Sunday)...The period will begin with a
building ridge ahead of a deepening west coast trough. This will
provide a dry period for the CWA along with above average
temperatures. There is some uncertainty as to how quickly this
trough moves onshore, but most guidance is indicating a track that
will bring impacts by midweek resulting in increased precipitation
chances and cooler temperatures for most of the area.

A building ridge on Sunday will result in southerly flow bringing
drier air into the region. Temperatures across northern Utah will be
running ~5 degrees above climo through Tuesday, however southern
Utah will be right around climo.

Cluster members are in high agreement on the phase and amplitude of
a rare west coast September trough. Ahead of this trough
southwesterly flow aloft will increase. These winds will likely mix
to the surface during the day Sunday-Tuesday resulting in winds
gusts on the order of 20-30 mph, especially across southwestern Utah
and southwestern Wyoming. Uncertainty exists in terms of the timing
that this trough tracks across the Intermountain West. Roughly 40%
of solutions bring the trough onshore by 00Z Wednesday whereas the
other 60% bring the trough onshore by 00Z Thursday. Regardless, all
members are in agreement that this trough will track across the
northern intermountain west.

Moisture advection will begin ahead of this trough that, combined
with synoptic scale ascent, will bring a return of precipitation
chances late Tuesday-Thursday as well as cooler temperatures a few
degrees below climo.

&&

.AVIATION...KSLC...There is a small chance of an isolated
thunderstorm impacting the terminal or its vicinity between 22-23z.
However, the main threat of thunderstorms is most likely between 05-
09z, and may occur as early as 04z. Impacts include periods of gusty
and erratic outflow winds and a 30% likelihood of MVFR CIGs between
05-09z. There is low confidence in prevailing wind direction after
23z. If storms develop to our south around 22-23z, southerly winds
would prevail until roughly 03z. After 03z, winds out of the west to
northwest are likely to prevail, outside of erratic outflow winds.
Southerly flow will return after 09-10z.

.REST OF UTAH AND SOUTHWEST WYOMING...Isolated showers and
thunderstorms will continue to develop through the afternoon. After
00z, a strong cluster of showers and thunderstorms are expected to
move from WSW to ENE across northern UT, north of KMLF. These storms
are expected to diminish after 12z. Within this cluster of storms,
MVFR to IFR conditions are possible due to both VIS and CIGs. Gusty
and erratic outflow winds are likely.

&&

.FIRE WEATHER...A storm system impacting northern and central
Utah will bring showers and thunderstorms overnight Thursday into
Friday morning. Wetting rains are likely for much of the northern
and central mountains as well as the eastern valleys.
Thunderstorms will become increasingly isolated and limited in
coverage on Friday, though these will be associated with higher
chances for gusty and erratic winds and decreasing heavy rainfall
potential.

As drier air continues to work into the state from the southwest
into the weekend, the potential for showers and thunderstorms will
become constrained to far northern Utah and areas where high
terrain will encourage convective development in the afternoon on
Saturday. Much drier air and breezy conditions will set in
statewide by Sunday, continuing into early next week.

&&

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

Wessler/Mahan/Cunningham

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

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

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