Severe Thunderstorm Warning 64, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
311 PM MDT Tue Aug 13 2024

.SYNOPSIS...With monsoonal moisture and and unstable airmass
remaining in place through this evening, scattered to numerous
showers and thunderstorms will continue into much of this evening.
Drier air will move in from south to north tonight through the end
of the week, resulting in decreasing coverage of showers and
thunderstorms and increasing fire danger. Monsoonal moisture is
expected to return to southern Utah by Saturday and for the
remainder of the area by Sunday.

&&

.SHORT TERM (Through 12Z/6AM Thursday)...Early Tuesday afternoon
analysis revealed a shortwave rotating through southern Idaho with
an impressive mid-level dry slot extending south from the
aforementioned wave , cutting across eastern Utah, with
increasingly dry conditions to the west, with conditions
remaining moist to the east of this line. Associated with the
trough was deep-layer shear of 25-35kts that was in place across
the northern 3/4 of the area. In addition, at the surface, a moist
airmass was in place with widespread dewpoints in the 50s, and
even a few low 60s dewpoints near Kanab. Across southwest Utah,
dewpoints had dropped into the 40s thanks to efficient mixing and
dry advection. Except for southwest Utah, the moist environment
was resulting in a wide swath of SBCAPE in excess of 2000J/kg --
exceptions again being southwest Utah due to dry advection and the
Uinta Basin, Castle Country and the Swell where earlier
shower/thunderstorm activity was inhibiting destabilization here.
Across southern Utah, convection has largely been more robust than
forecast, with numerous Flash Flood Warnings in effect as of 3PM
MDT. Heading through the evening, CAMs are in good agreement that
existing convection will continue to migrate northeastward across
the remainder of central, eastern and southeast Utah through the
early evening hours. Given remaining CIN, uncertainties surround
convective maintenance across the Swell, Castle Country and the
Uinta Basin, where extensive cloud cover is still in place.
However, large- scale forcing could aid in overcoming this a bit.
Farther north, CAMs continue to advertise convective development
across the Great Salt Lake Desert, Great Salt Lake, Wasatch Front
and northeast Utah. However, differences remain regarding timing
and placement, leading to some degraded confidence in timing and
coverage. Recent trends suggest increasing vertical development of
cumulus across these areas, suggesting CAMs do have some merit.
Radar trends to continue to indicate increasing convection east
of Wendover as well. However, we do not think convection across
the Wasatch Front will be quite as extensive as last night, but
could still be robust with gusty winds, hail and heavy rainfall.
Sufficient shear, low LCLs and modest low-level helicity suggests
the potential for a few rotating updrafts as well into the early
evening hours across northern Utah.

Once we get through much of the evening hours, models are in good
agreement that convective activity will not be as widespread
tonight when compared to the last few nights, and this makes sense
given subsidence in the wake of the shortwave as well as deep-
layer dry advection. That said, can't rule out an occasional
shower near the Idaho border or over the eastern valleys
overnight, but not anticipating anything organized or widespread.
On Wednesday, dry air overspreads the southern half of the state
of Utah, significantly limiting convective potential. CAMs do
continue to carry a few showers and thunderstorms near the Idaho
border, SW Wyoming and across northeast Utah, with a cell or two
perhaps expanding south across the Wasatch Front and Swell. Given
dryer air in place tomorrow, threat transitions to gusty winds,
and this threat is maximized across SW Wyoming and NE Utah
Wednesday afternoon/early evening. Any convective activity
Wednesday is expected to quickly wane by sunset.

.LONG TERM (After 12Z/6AM Thursday)...Long term forecast period
begins with moderately strong (~60-80 kts) upper level westerlies
atop the area while a weak midlevel shortwave ridge begins to
shift east. As a whole, this setup will yield minimal moisture
transport into the region (PWAT values around 60-90% of seasonal
normal), and limited broader scale support for ascent across much
of the region. The exception to this appears to be the northern
reaches of the forecast region (high Uintas, SW WY, UT/ID border
area) where modest grazing shortwave energy looks to trigger some
isolated to scattered afternoon/evening convection. While coverage
is expected to be far less than what was seen earlier on in the
week, the stronger jet flow aloft will add a bit of deep layer
shear (~15-30 kts) which may help lead to a little more
organization in what does ultimately form. With loss of daytime
heating, activity will wane fairly quickly moving into the
overnight hours. The drying trend looks to continue into Friday
with even less identifiable sources for sufficient lift, thus only
expecting an isolated shower or two over some of the high terrain
(primarily high Uintas).

For the weekend, guidance continues to support broad ridging
redeveloping from the southern Plains into the Four Corners region
while troughing amplifies through the PacNW. With the placement of
these features, the local forecast region will find itself placed
within stronger south to southwesterly flow, which will begin a
moistening trend. This will allow for a return of the daily isolated
to scattered diurnal convective threat. On Saturday coverage looks
to be highest east of the I-15 corridor and south of the I-70
corridor, while Sunday will see more scattered activity creep north
up to around the I-80 corridor. While neither day looks particularly
prolific at the moment, those planning any recreation in rain
sensitive areas (slot canyons, dry washes, slickrock areas) should
keep an eye on the forecast and remain weather aware.

Moving into the upcoming week, general ensemble consensus favors the
larger scale synoptic features remaining more or less in place and
resulting in some sort of continuation of the ongoing pattern. That
said, a subset of ensemble members (~35%) show scenarios where the
ridge ends up a bit flatter or centered nearer to the forecast
region, both of which would result in a bit less moisture transport
into the region and more limited daily precipitation chances. For
now, opted to maintain NBM guidance which carries continued isolated
to scattered daytime convection across much of the area.

&&

.AVIATION....AVIATION...KSLC...Thunderstorms are forecast to impact
the terminal early this evening with risk for gusty erratic wind,
lightning and briefly reduced visibility. Thunderstorms are most
likely between 5 pm and 8 pm. Winds should shift out of a
southerly direction thereafter with a gradual decreasing speed
trend late this evening.

.REST OF UTAH AND SOUTHWEST WYOMING...Scattered to numerous showers
and thunderstorms will continue to impact much of the state into the
evening hours. Periods of very strong wind, heavy rain and lightning
are forecast as these thunderstorms track across any terminal. Brief
periods of MVFR or IFR VIS are possible as a result. Thunderstorm
development will significantly decrease after sunset, allowing for
widespread VFR conditions.

&&

.FIRE WEATHER...Early Tuesday afternoon, moist air and numerous
showers and thunderstorms were ongoing east of I-15 across
central and southern Utah, while dry air was quickly moving into
southwest Utah. Across northern Utah, isolated to scattered
showers and thunderstorms are expected through the first half of
the evening. Tonight through Wednesday, dry air will gradually
spread northward and move into central Utah as well. Isolated to
scattered showers and thunderstorms are once again expected on
Wednesday, especially near the Idaho border and over northeast
Utah. Otherwise, pockets of critical fire weather conditions will
develop over southwest Utah Wednesday afternoon due to the
combination of southwest gusts to 25 mph and minimum humidity
values near 15%. These same conditions will overspread most of the
southern half of Utah on Thursday. By Friday, breezy southwest
winds of 15-25 mph with isolated gusts to 30 mph will combine with
afternoon humidity down into the teens to bring critical fire
weather conditions to most low to mid elevation areas throughout
the state. On Saturday, another monsoonal surge will move into
southern Utah, bringing an increase in humidity along with an
increase in shower and thunderstorm activity. This monsoon surge is
expected to overspread the state Sunday and remain in place
through Monday. Conditions are forecast to begin to dry out once
again by next Tuesday.

&&

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

ADeSmet/NDeSmet/Warthen

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

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