Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
331 PM MDT Sat Aug 19 2023

.SYNOPSIS...Moisture from Hurricane Hilary will impact the area
tonight through early next week, with western Utah seeing
high winds Monday. Monsoonal moisture fills back in and lingers
through the rest of the week.

&&

.SHORT TERM (Through 12Z Tuesday)...Currently sitting in a potent
environment on the verge of a productive evening, seeing the
mesoanalysis and recent CAMs continuing to increase the
instability and shear while moisture continues to stream in.
Satellite shows nearly clear skis across the forecast area with a
broad region of high cirrus to the west with a few bands of
convective development approaching through southern Nevada. Models
handling the initial convection fairly well, suggesting limited
development through the early evening outside of perhaps two
quasi-organized features impacting northeastern Utah and southwest
Wyoming posing a threat to recent burn scars, and a consistently
represented feature moving north through Washington and Kane
counties then northwest through Zion National Park and off the
terrain through Iron and Beaver counties. Given little change in
these features and the maintenance of a healthy convective
environment into the early evening, still carrying and messaging
these features and potential impacts.

Mid-level water vapor satellite shows the tropical moisture
associated with Hilary surging through SE CA and western AZ at
this time, with models bringing that plume into western Utah
overnight, influencing conditions overhead by 12Z Sunday. This is
consistent with the increased northward speed and eastward shift
in track observed by the NHC this afternoon. While the greatest
coverage of stratiform rain will be constrained to eastern Nevada
and the far western edge of utah, CAMs depict a number of pulses
moving northward as periods of PVA and enhanced dynamic support
help destabilize the background environment, potentially as a
result of feeder bands moving northward from the hurricane's edge.
These could trigger storms bringing extremely heavy rainfall to
portions of southern Utah. Additionally, noting the highly-sheared
environment in an unusual ideal balance with tremendous amounts of
CAPE (relative to the usual for Utah), severe potential in these
bands is worth attention. Low-level helicity across southwest
Utah Sunday morning coupled with these factors could create an
environment with healthy low-level mesocyclones and well-formed
supercells. While probabilities for specific severe weather
impacts remain low (<5%), cannot rule out the potential for large
hail or an isolated tornado in far southwestern Utah.

While rain will continue through Sunday night and Monday morning,
models begin to diverge in their handling of instability and
moisture, with the most favorable environment shifting further
west into Nevada. However, latest runs of the CAMs suggest the
wind threat attempts to grab the attention of the forecaster, with
the remnants of Hilary driving rapid height falls in the H7 field
as it undergoes extratropical transition and is assimilated into
the trough to the west. Probabilities for impactful winds in
excess of 58 mph across western Utah are creeping up, as high as
70% chances for far northwestern Utah and exposed terrain of
western Utah, and lower confidence further south at 30%. Some
uncertainty as to the onset time of these winds, as early as 4 or
5 AM along the UT/AZ border if winds are able to overcome low-
level stability and mix to the surface. Otherwise, if stability is
able to persist at the lowest levels, will see winds tick up
along the UT/NV border mid-morning and persist throughout the day
as the storm completes it's transition into a potent mid-level low
through Monday evening.

.LONG TERM (After 12Z Tuesday)...Tuesday will see a shift from an
overtly tropical in nature system to more of a typical mid-
latitude trough with a healthy moisture tap, starting off with
elevated PWATs and ample warm advection into the region in the
tropical system's wake. As high pressure fortifies to the east, it
will stall the progression of this trough over the western Great
Basin and reinforce the stubborn monsoonal pattern. PWATs will
continue to run over an inch/over 150% of normal through much of
the extended forecast period, with continued diurnal thermally
driven convection across the forecast area through the week and
potentially into next weekend, leading to continued threat to slot
canyons, dry washes, burn scars, and steep terrain across the
forecast area. At this time, Tuesday through Wednesday evening
appears to still carry the greatest threat, with the aid of
dynamic support from the fading trough to the west.

Tropical influences from the Atlantic Basic and northern branch
of the jet could both work to erode the upper level ridge
solidifying the monsoonal setup in place by next weekend, but
ensemble spread remains broad and uncertain. Clusters show a 60/40
split in a wetter/drier scenario as a result of position of the
ridge axis, currently favoring maintenance of the current setup
versus a westward shift with dry air moving in overhead.

&&

.AVIATION...KSLC...Prevailing VFR conditions will be maintained over
the KSLC terminal through tonight. Shower chances increase this
evening, to 30% between the 00-04z timeframe, with a corresponding
15-25% chance of lightning within 5SM. Brief CIGS to around 8-10kft
AGL will be possible with showers. Otherwise, generally light north
winds will continue through late this afternoon, with increasing
likelihood of south winds after 00-01z.

.REST OF UTAH AND SOUTHWEST WYOMING...Areas of showers and
thunderstorms will continue across portions of the Wasatch Front and
Mountains and areas eastward through around 04z. Meanwhile, should
see increasing thunderstorm activity across southwest and portions
of south-central Utah late this afternoon through around 04-05z.
Some of the stronger storms will produce brief MVFR conditions or
lower along with local mountain obscurations, along with the
potential for outflow winds greater than 35kt. Later tonight (around
05z or later) into Sunday morning, steadier rain will fill in across
far western Utah, likely affecting KENV and KSGU with potential for
brief periods of MVFR visibilities.

&&

.FIRE WEATHER...The ongoing monsoonal moisture surge will begin to
incorporate additional tropical moisture from the soon-to-be
remnants of currently Hurricane Hilary over the next 24 hours.
With this, can expect the potential for strong storms and wetting
rains to increase area-wide, though at this time the current track
will bring the greatest impacts to far western Utah, especially
the Pine Valley mountains and potentially through the Grand
Staircase. Areas outside the main slug of moisture are still
facing a heightened potential for wetting rains through the
remainder of the weekend and into next week as the tropical storm
is assimilated into the pattern.

The secondary concern outside of heavy rainfall and lightning
will be the increased winds moving into western Utah on Monday
morning. With the slight eastward shift in Hilary's track in
recent observations and model runs, have seen increased support
for winds in excess of 45 mph across much of western Utah through
the day Monday, with the highest winds possible in mountainous,
exposed terrain and across Box Elder county in northwestern Utah
where winds could approach or exceeded 60 mph at times. Given the
remnant moisture in place, while there will be some rapid drying,
the decrease in min RH will be from the low 80s and high 70s to
the mid 30s, with a few areas perhaps approaching 25%. This will
help limit the critical nature of fire weather on these days but
not entirely eliminate the threat.

Monsoonal moisture will return in the wake of the tropical
moisture's departure, with daily shower and thunderstorm activity
continuing through the upcoming work week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...High Wind Watch from Monday afternoon through Monday evening for
     UTZ101-102-115-122.

     Flood Watch until midnight MDT tonight for UTZ112-113-117-119-
     128>131.

     Flood Watch through Sunday evening for UTZ115-122>127.

WY...None.
&&

Wessler/Cheng

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

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