Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
542 AM MDT Tue Aug 17 2021

.SYNOPSIS...A slow moving upper level system will impact the area
with a potential for strong thunderstorms and heavy rainfall through
at least Wednesday. Expect much cooler temperatures are
expected Wednesday through the end of the work week.

&&

.SHORT TERM (THROUGH 12Z FRIDAY)...An upper low is noted on
satellite imagery this morning moving onto the PacNW. Ahead of this
low, the flow aloft is turning more southwesterly across Utah.
Meanwhile, a couple of subtle shortwave disturbances, one which
crossed southeast Idaho overnight and another currently moving into
northeast Nevada, has brought some clouds and high-based
showers/virga to northern Utah, and these remain in place this
morning. Moisture will increase from the south today as the
southwest flow becomes more established and stronger later this
morning. Latest estimates of PWATs indicate between 0.6-0.8 inches
across southern Utah this currently. This should rise to between 0.8-
1.0 inch across central and southern Utah by midafternoon.
Furthermore, the southwest flow will cause the smoke to thin out,
especially at the lower levels owing to increased mixing.

Expect convection to develop especially across portions of central
and southern Utah this afternoon as the airmass destabilizes under
strong surface heating, with the HREF showing mean MUCAPE values
upwards of 1000J/kg. Expect storms in this area to mostly initiate
over the higher terrain and drift into the eastern valleys.
Dewpoints are still largely in the 30s, but the airmass should
moisten quickly and a potential for locally heavy rain exists with
the strongest storms. An area of more focused convection is expected
to extend from the west-central Utah valleys through Utah County and
the Uintas, as the aforementioned subtle shortwave trough shears out
across northern Utah ahead of the larger upstream storm system.
Stronger shear will be in place across northwest Utah, and the best
combination of instability and shear looks to be in this general
area. Therefore, in addition to locally heavy rain, there will also
be a better potential for hail and gusty winds across central Utah
today.

A cold front associated with the upper low will move into northwest
Utah by early this evening, then slowly make its way across the
northwestern third of Utah tonight. Given large-scale dynamics at
play, showers and thunderstorms will continue through the night,
ahead of and near the frontal boundary. Particular focus will remain
in the general Utah County through Uinta Mountains corridor.

Greater shear will be present across a larger portion of the area
tomorrow as the upper low moves closer to Utah. This will maintain a
severe threat with hail and gusty winds. Even though instability may
decrease a bit owing to cloud cover and cooler temperatures
associated with existing precipitation and outflows, hi-res models
continue to indicate sufficient instability with over 500J/kg of
CAPE across much of central and southern Utah. As the cold front
moves into central Utah, it will encounter greater and deeper
moisture, and widespread heavy rain will be possible near and ahead
of the boundary. Have extended the existing Flash Flood Watch for an
additional 24 hours as a result.

Temperatures will be much cooler and well below climatological
normals behind the cold front. Northern Utah valleys are forecast to
see afternoon max temperatures in the 70s tomorrow, although the low
end of the spread in the NBM indicates upper 60s possible closer to
the Idaho border.

Greater uncertainty exists with the evolution of the upper low on
Thursday as far as how quickly it starts to shift east of the area.
In a solution such as that shown in the NAM, where the low center
will be over northeast Nevada and northwest Utah and the upper jet
extending across central Utah, Thursday could remain fairly active
and wet. However, the GFS, which has been faster with the
progression of the storm, suggests mainly just some lingering
showers across eastern Utah and southwest Wyoming and decreasing
throughout the day. Nevertheless, Thursday should see at least some
unsettled weather continuing, with temperatures 15-20 degrees below
normal across northern Utah and around 10 degrees below normal
across much of the south.

.LONG TERM (AFTER 12Z Friday)...The midweek trough exits rapidly
east and ushers in a quick ramp up of temperatures throughout the
forecast area, particularly in the north. A solid westerly flow
yields to a potent southwest flow by Saturday morning, heralding the
imminent arrival of another trough digging into to the PacNW. Though
not as impressive as the last trough, and more relegated to the
northern region, Saturday rain totals could be significant without
the cooldown brought earlier in the week.

A much drier airmass and southwest flow regime engulfs the region by
Monday and could persist into midweek if current model solutions
hold. Temperatures are expected to settle in near normals by the end
of the forecast period.

&&

.AVIATION...KSLC...Low VFR surface visibility from regional
wildfire smoke is expected to continue into early afternoon with
periods of MVFR visibility possible. Though surface vis shows
improvement after 20z, vertically integrated smoke will still likely
persist. Southerly winds will continue through the afternoon, with a
20% chance of becoming westerly between 20-23z. Winds may reach AWW
criteria during the mid to late afternoon. Gusty and erratic winds
from vicinity showers and possible thunderstorms could impact the
terminal after 21z.

REST OF UTAH AND SOUTHWEST WYOMING...Low VFR surface visibility is
expected to improve into the late afternoon as the southwest flow
regime settles in. Vertically integrated smoke will still likely
persist throughout the day. Thunderstorms are possible along the
higher terrain and may impact KBCE and KHCR after 21z. Expect gusty
and erratic winds with passing showers for most terminals today.

&&

.FIRE WEATHER...Southwest flow will increase today ahead of a storm
system moving into the Pacific Northwest. This southwest flow will
draw moisture into the area, leading to the development of showers
and thunderstorms by midafternoon today. Some storms will be capable
of producing heavy rain, especially over the mountains of central
and southern Utah and into the eastern valleys. In addition, gusty
winds and hail will be a possibility. A fairly significant cold
front will enter northern Utah tonight and slowly push south through
the day tomorrow and into tomorrow night. Widespread heavy rain can
be expected to continue near and ahead of this cold front, along
with a continued possibility of hail and gusty winds. Much cooler
temperatures will settle into the area behind the front. Lingering
showers will continue Thursday and possibly into Friday, with
temperatures remaining below normal. Another storm system looks to
cross northern Utah late Saturday, then the airmass will trend drier
for the beginning of next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Red Flag Warning from noon today to 9 PM MDT this evening for
     UTZ478-479-492.

     Flash Flood Watch from noon MDT today through Wednesday evening
     for UTZ111-113-117-120-121-125>131.

WY...Red Flag Warning from noon today to 9 PM MDT this evening for
     WYZ277.

&&

Cheng/Verzella

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

Hazardous Weather Outlook
National Weather Service Salt Lake City UT
631 AM MDT Tue Aug 17 2021

UTZ101>131-WYZ021-181300-
Great Salt Lake Desert and Mountains-Tooele and Rush Valleys-
Eastern Box Elder County-Northern Wasatch Front-Salt Lake Valley-
Utah Valley-Cache Valley/Utah Portion-Wasatch Back-
Bear Lake and Bear River Valley-Wasatch Mountains I-80 North-
Wasatch Mountains South of I-80-Western Uinta Mountains-
Wasatch Plateau/Book Cliffs-Western Uinta Basin-
Western Millard and Juab Counties-Eastern Juab/Millard Counties-
Central Mountains-Sanpete Valley-Sevier Valley-Castle Country-
San Rafael Swell-Southwest Utah-Lower Washington County-
Zion National Park-Southern Mountains-Upper Sevier River Valleys-
Bryce Canyon Country-South Central Utah-
Capitol Reef National Park and Vicinity-Western Canyonlands-
Glen Canyon Recreation Area/Lake Powell-Southwest Wyoming-
631 AM MDT Tue Aug 17 2021

This Hazardous Weather Outlook is for the western two thirds of
Utah and southwest Wyoming.

.DAY ONE...Today and Tonight

Showers and thunderstorms will develop across the area this
afternoon ahead of an approaching low pressure system. The
strongest thunderstorms can be expected south of I-80, and will
be capable of producing locally heavy rainfall, hail, and strong
gusty winds.

Hazardous fire weather conditions are expected across portions of
west-central and northern Utah due to gusty winds, low humidities,
and thunderstorms this afternoon.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Widespread moderate to heavy rainfall is expected on Wednesday,
along with some strong to severe thunderstorms capable of
producing hail and strong gusty winds. Lingering showers
and thunderstorms will continue through at least Thursday,
maintaining some potential for locally heavy rain. Elevations
above around 10,000 feet may even see a changeover to snow for a
time Wednesday night into Thursday morning behind a cold front.

Those near recent burn scars and near areas prone to flash
flooding should monitor the forecast over the next several days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information
about observed severe weather to the NWS while following all
local, state, and CDC guidelines.

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

For information on potential road travel impacts visit...
http://www.udottraffic.utah.gov/roadweatherforecast.aspx

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