Severe Thunderstorm Warning 38, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
401 PM MDT Thu Jul 25 2024

.SYNOPSIS...Moisture will be in place across the entire state
through Friday, resulting in the threat of showers and storms
areawide through Friday evening. Drier air will move in for the
weekend with increasing winds. Dry conditions with lighter winds
and gradually warming temperatures will greet us for early next
week.

&&

.KEY POINTS...

*Confidence is increasing that a quasi-squall line will move
 through northern Utah late this afternoon through this evening,
 bringing a threat of damaging wind gusts, especially across the
 Great Salt Lake Desert, Dugway and perhaps extending into the
 Wastach Front and Cache Valley late this evening.

*Plentiful moisture will support an increased flash flood risk
 across southern Utah through this evening.

*Similar risk for damaging wind gusts exists across the same areas
 Friday afternoon and evening.

.SHORT TERM (Through 12Z/6AM Saturday)...Early Thursday afternoon,
a remnant MCV could be seen departing Utah and exiting into
Wyoming and Colorado. The southern extension of this feature has
stalled across southern Utah, however. This has served as a focus
for convective initiation so far today, and will continue to do
so through the evening hours. Mid-level flow parallels this
boundary, leading to a setup that favors training cells (cells
that track over the same/similar areas multiple times). Although
PWATs have fallen slightly thanks to mixing this afternoon, they
remain in the 0.8"-1.0" range, highest over northwest and
southwest Utah, where these values are in the 175% to 200% of
normal range.

Elsewhere, an upstream shortwave can be seen moving across Nevada,
and this feature is forecast to track across northern and central
Utah from west to east from late this afternoon through this
evening. This feature will lift the capping inversion left behind
in the wake of the MCV. We can already see this process unfolding
across eastern Nevada, as a cumulus field is increasing in areal
coverage, and cumulus is growing in the vertical as well,
signaling widespread convective initiation here. CAMs strongly
suggest this convection will evolve into a loosely-defined squall
line, which is expected to reach the NV/UT border between 6PM and
7PM, then gradually move eastward through the evening, extending
from just west of the I-15 corridor southward across the Great
Salt Lake and Tooele, and finally extending southward into central
Utah between 7PM and 10PM. There is some uncertainty as to
whether this feature will remain fully intact as it reaches the
Wastach Front, as CAMs are divided nearly 50/50 on this. This
uncertainty seems reasonable as deep-layer shear will begin to
wane by this time. Even if the feature does fall apart over the
Great Salt Lake, enough remnant wind should be available to blast
into the Wasatch Front thanks to a well-developed cold pool, even
if precip and lightning weakens. Worst case scenario wind gusts
(HREF 90th percentile) are in the 60-75 mph range across a wide
swath that includes the Great Salt Lake Desert and Dugway,
including the I-80 corridor and Wendover. This weakens into the
50-60 mph range for the Tooele Valley down into central Utah as
well as the Wasatch Front and Cache. The Storm Prediction Center's
SWODY1 outlook has been upgraded to a sight risk for much of
these areas to account for this damaging wind threat, and SPC has
issued an MCD with a 40% watch issuance probability.

On Friday, another shortwave embedded within deep-layer
southwesterly flow is forecast to move across northern and central
Utah during the afternoon and evening. When combined with very
similar values of SBCAPE (250-750J/Kg) and deep-layer shear
(15-25kts), this will set the stage for a similar setup for Friday
afternoon and evening, with damaging microburst and outflow winds
once again, for nearly the same areas. Highest wind probabilities
do extend a bit further south across central Utah, however. One
difference is that CAMs suggest the window for the worst
conditions will be earlier in the day (and this makes sense in the
absence of a capping inversion left behind by this moring's MCV),
with window for strongest gusts developing across the west desert
and Great Salt Lake Desert between 2PM-3PM, spreading across
central Utah and the Wasatch Front between 4PM and 8PM. Worst case
scenario wind gusts are in the 55-65 mph range. Showers and
thunderstorms will once again be capable of heavy rainfall, but
faster storm motions will limit flash flood threat, except where
convection trains.

.LONG TERM (After 00Z Sunday/6PM Saturday), Issued 423 AM MDT...
Showers and thunderstorms will be near the Idaho border by
Saturday evening, with drier conditions building northward with
southwest flow. Gusty southwest winds, strongest throughout
southern Utah, will diminish through the evening.

Dry conditions are forecast throughout southwest Wyoming and Utah
Sunday. As ridging to the southeast starts to retrograde, the
pressure gradient will weaken, and winds will be lighter. Wind gusts
around 25 mph are likely for much of southern Utah and southwest
Wyoming.

There is good agreement with models that the ridge will retrograde
while building north through much of the week. Dry conditions will
last through Monday, with similar wind speeds to the previous day.

As the ridge builds into the Four Corners area Tuesday, there will
be enhanced moisture advection, with precipitable water values
around 0.8" into southern Utah. Isolated showers and thunderstorms
are likely during the afternoon. They will primarily be around
terrain for much of southern Utah and be capable of gusty, erratic
outflow winds.

As the ridge builds north and west Wednesday, precipitable water
will increase, but with a lack of synoptic forcing and a dry
environment in the lower levels, showers or thunderstorms that build
will likely not last, but be capable of gusty, erratic outflow winds.

As the ridge tracks into Utah, temperatures will warm from roughly
near normal for the end of the weekend to around 5F warmer than
normal Wednesday.

&&

.AVIATION...KSLC...Northwesterly winds around 10kts will continue
through the afternoon, with gusty outflow winds out of the west-
northwest expected to arrive around 03-04z. These winds could be
very strong, likely reaching 25-30kts. There is a 10% chance of
winds exceeding 40kts. A brief period of moderate rainfall may
follow around 04-06z, though showers may dissipate before reaching
KSLC. There is a 30% chance for thunder if these showers make it to
the terminal.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered showers and
thunderstorms will continue to develop across the area this
afternoon and evening, with periods of heavy rainfall and some
outflow winds expected. Of note, a conglomerate of showers and
thunderstorms is expected to move across the northwestern quadrant
of Utah this evening, reaching KENV after 23z and the Wasatch Front
around 03-04z. This area has the highest chance of very strong wind
gusts, potentially in excess of 50kts. This feature will gradually
decrease in strength as it moves across the northern portion of the
state through the evening.

&&

.FIRE WEATHER...Increasing moisture along with a series of weather
disturbances crossing the area will maintain a threat of scattered
to numerous showers and thunderstorms across the area through
Friday, with some of these storms capable of producing wetting
rain, and perhaps locally moderate to heavy rain. By Saturday,
showers and thunderstorms will become more confined to northern
Utah while significant drying will take place across southern and
central Utah. Winds will increase from the southwest on Saturday,
leading to the potential for critical fire weather conditions due
to gusty winds and low humidities across southern and portions of
central Utah. Humidity recoveries Saturday night are also
expected to be relatively poor across southern and central Utah.
The dry airmass will overspread the entire forecast area by Sunday
and continue through the first half of next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Fire Weather Watch from Saturday afternoon through Sunday
     evening for UTZ482>484-488-489-492>498.

WY...None.
&&

ADeSmet/Cunningham/Wilson

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

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