Severe Thunderstorm Warning 76, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
201 PM MDT Sat Aug 17 2024

.SYNOPSIS...A robust monsoon surge will begin across the region
today, continuing through early next week. Tuesday will trend drier,
with another potential monsoon surge Wednesday into the end of the
week.

&&

.SHORT TERM (THROUGH 12Z MONDAY)...Strong southwesterly flow
aloft is in place over Utah this morning in between high pressure
centered over New Mexico and a closed low spinning off the PacNW
coast. Meanwhile, a weak shortwave disturbance is noted near the
southern tip of Nevada. This pattern has very quickly advected
deep monsoonal moisture to the entire area, 1.3" per the latest
SPC mesoanalysis. The same mesoanalysis also indicates 500-1500
J/Kg of most unstable CAPE and up to 40kt effective shear
associated with the moisture surge. Combined with a shortwave
disturbance quickly lifting northeastward, we have a recipe for
severe weather and flash flooding, including gusty winds, large
hail, and heavy rain in a brief period. Also of note, the storms
are moving quickly with the strong steering flow aloft, quickly
moving northeastward around 30-40mph. Those with outdoor plans
should stay tuned to weather conditions.

Behind this shortwave later this afternoon, northern Utah may see
a bit of a lull in convection as the airmass becomes suppressed, while
over the mountains and southern and eastern valleys, daytime
heating and instability within the moist airmass will result in
showers and thunderstorms filling in over those areas. We have
around 60% confidence in this scenario. The other scenario is
enough clearing is present for destabilization and additional
storms to fire across nearly the entire area. Either way, high res
models suggest another round is likely this evening.

Multiple threats will be associated with thunderstorms today, owing
to the abundant moisture, larger scale ascent with the shortwave,
and increased shear ahead of the aforementioned low off the PacNW.
The initial round of thunderstorms with the shortwave may be
accompanied by strong and gusty outflow winds. More than half of the
members of the 00z HREF show at least an area of brief 50+ mph winds
from mid-morning through the early afternoon hours. Some of the
members continue to depict areas of 50+ mph gusts even into the late
afternoon, associated with the afternoon thunderstorms across the
rest of the area. The overnight SPC update to their Convective
Outlook now has a Slight Risk over a good chunk of Utah. With the
moisture spreading into the area, heavy rain is also anticipated.
The HREF probability-matched mean 6hr QPF has a swath of between 1
to greater than 1.5 inches along the spine of Utah between 18-00z.
The latest WPC Excessive Rainfall Outlook has a Slight Risk for
exceeding flash flood guidance from southwest into central Utah.
With some sensitive burn scars and flash-flood prone areas within
this region, a Flash Flood Watch is in effect for portions of
southern and central Utah.

Showers and thunderstorms are expected to generally wind down
overnight. However, some CAMs do want to maintain isolated to widely
scattered convection across portions of northern Utah through the
night. Tomorrow, widespread convection can again be expected, with
the moisture remaining in place. While the wind threat will be
reduced, aided in part by moistening of the low levels, the
potential for heavy rain and associated flash flooding will remain
elevated.

.LONG TERM (After 12Z/6AM Monday), Issued 433 AM MDT... The
forecast remains largely unchanged, with monsoonal moisture
bringing daily showers and thunderstorms to much of the forecast
area, especially further south and east. An elevated flash flood
risk will continue.

The longwave pattern will be fairly stagnant, at least through
Thursday, with a ridge to the east over CO/NM and a trough off the
coast of the PacNW. Subtle changes in flow direction over Utah and
southwest Wyoming will limit precipitation chances mid-week,
especially further north and west. Tuesday will likely be the driest
day, with precipitation chances across mainly higher terrain, with
the best moisture and instability over eastern Utah. While Wednesday
remains somewhat drier, too, deterministic guidance suggests a
shortwave trough sliding over northwestern Utah on Wednesday,
perhaps providing enough lift for a wider coverage of showers and
thunderstorms. Additional moisture will likely filter across the
area by Thursday.

The main area of uncertainty in the long-term period is the inland
movement (or lack thereof) of the aforementioned PacNW trough. This
trough would likely bring higher amounts of shear to the area as it
approaches, lending to more organized convection. Additionally, a
much drier air mass would likely move in behind the trough.
Currently, the best timing of this occurrence looks to be Friday
(~30% chance) or Saturday (~70% chance), but there is still plenty
of uncertainty in the exact progression of the trough.

&&

.AVIATION...KSLC...Intermittent showers and thunderstorms will
impact KSLC this afternoon/evening. Winds will be gusty and
erratic at times, with a 10% chance of exceeding 50kts. Later in
the afternoon/evening, there is a higher chance of periods of
moderate- heavy rain. Periods of mountain obscuration are possible
given SCT- BKN cloud decks near 6000ft AGL. There is uncertainty
with how long these showers/thunderstorms stick around, ending
anywhere between 01z and 08z.

.REST OF UTAH AND SOUTHWEST WYOMING...Widely scattered showers
and thunderstorms will continue this afternoon and evening,
lasting well into the evening across northern UT, mainly north of
KDTA. Main threats will be gusty and erratic outflow winds,
frequent lightning, and even moderate to heavy rainfall later in
the day. Most areas will see gusts around 25- 40kts, though there
is a 5-15% chance of exceeding 50kts across the forecast area.

&&

.FIRE WEATHER...Moisture surging in from the south today will
accompany a weak weather disturbance rotating around high pressure
centered over New Mexico. This moisture and associated
disturbance has brought multiple rounds of showers and
thunderstorms across the area today, which will continue through
Sunday. The thunderstorms will generally be wet, and have the
potential of bringing heavy rain, small hail, and gusty winds in
excess of 40 mph, and locally up to 60 mph, to the area.

Moisture decreases slightly Monday, maintaining a more limited threat
of heavy rain to the higher terrain east of I-15 . The airmass
will be drier still on Tuesday, with more limited shower and
thunderstorm activity. Monsoonal moisture will then begin to make
another return on Wednesday, increasing across the area through
late week for more daily rounds of thunderstorms with elevated
wetting rain potential.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch until 10 PM MDT this evening for UTZ113-117-123>126-
     128-131.

WY...None.
&&

Van Cleave/Traphagan/Cunningham

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

Preliminary Local Storm Report
National Weather Service Salt Lake City UT
307 PM MDT Sat Aug 17 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0155 PM     Tstm Wnd Gst     5 ENE Almy              41.35N 110.91W
08/17/2024  M72 MPH          Uinta              WY   Mesonet

            Measured at mesonet site Medicine Butte.

&&

DVC

Special Weather Statement
National Weather Service Salt Lake City UT
353 PM MDT Sat Aug 17 2024

UTZ122-125-172230-
Southwest Utah UT-Southern Mountains UT-
353 PM MDT Sat Aug 17 2024

...A strong thunderstorm will impact portions of central Iron County
through 430 PM MDT
...

At 352 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles west of Enoch, or 9 miles northwest of Cedar City, moving
northeast at 25 mph.

HAZARD...Wind gusts up to 50 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of central Iron
County.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

A Severe Thunderstorm Watch remains in effect until 700 PM MDT for
southern Utah.

&&

LAT...LON 3772 11320 3777 11331 3810 11315 3796 11288
TIME...MOT...LOC 2152Z 212DEG 23KT 3779 11320

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...50 MPH

STW

Special Weather Statement
National Weather Service Salt Lake City UT
402 PM MDT Sat Aug 17 2024

UTZ120-121-172230-
San Rafael Swell UT-Castle Country UT-
402 PM MDT Sat Aug 17 2024

...A strong thunderstorm will impact portions of southwestern Emery
County through 430 PM MDT
...

At 401 PM MDT, Doppler radar was tracking a strong thunderstorm 27
miles northwest of Hanksville, moving east at 35 mph.

HAZARD...Wind gusts up to 50 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of southwestern Emery
County.

This includes the following highways...
 Interstate 70 between mile markers 100 and 137.
 Utah Route 24 between mile markers 139 and 149.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

A Severe Thunderstorm Watch remains in effect until 700 PM MDT for
central Utah.

&&

LAT...LON 3859 11102 3875 11130 3913 11077 3869 11034
TIME...MOT...LOC 2201Z 250DEG 30KT 3869 11102

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...50 MPH

STW

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