Severe Thunderstorm Warning 45, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
334 PM MDT Thu Aug 8 2024

.SYNOPSIS...A monsoon surge will bring the threat of heavy rain to
central and southern Utah into the weekend. More isolated
thunderstorms are expected across northern Utah.

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.SHORT TERM (Through 12Z Saturday)...An elongated mid level ridge
axis stretches roughly along the Utah/Arizona border this
afternoon, as boundary layer moisture has spread into southern and
portions of central Utah. Although not particularly rich, ample
boundary layer moisture characterized by dew points in the low to
mid 50s, along with slow storm motions in proximity to the upper
ridge, are supporting convection with occasional deep cores
capable of producing heavy rainfall. Given the somewhat limited
moisture, this convection is becoming somewhat outflow dominant
as it slowly grows upscale and builds off the terrain. Model
guidance is in fairly good agreement propagating this activity
south- southwest toward Iron/Washington/western Kane counties over
the next few hours. Heavy rain/isolated flash flooding as well as
the potential for outflow winds in the 40-50 mph range will be
the main threats with this activity.

This moisture remains in place through Friday, creeping a bit
further northward with time. This will bring a chance for
afternoon convection up to roughly I-80 during the
afternoon/evening. Although moisture content will increase a bit
across southern Utah with lower LCLs and slightly higher dew
points, the overall flash flood threat will be dependent on
lingering overnight/early morning convection and the ability of
the airmass to recover. Have maintained "Probable" Flash Flood
Potential across most of southern Utah for now, with the potential
for greater coverage should the airmass sufficiently destabilize.

.LONG TERM (After 12Z Saturday), Issued 440 AM MDT...
The period will mainly consist of zonal flow as the jet stream
stays positioned across the northern CONUS. High pressure across
the southwest and southern CONUS will keep moisture in place for
the entire CWA through the weekend and early next week before a
drier air mass begins to filter in by the middle of next week.

Saturday will be the hottest day of the period, particularly
across northern Utah (~5 degrees above normal), before higher
moisture advects further north early next week. Saturday and
Sunday will pose a flash flood risk across central and southern
Utah as PWATs will be near the 90th-95th percentile with ensemble
means near 1.3". PWATs will remain elevated across southern Utah,
but gradually taper through midweek closer to normal values for
this time of year. As such, the flash flood threat will also start
to taper. Across northern Utah/SW Wyoming, PWATs peak on Monday
with ensemble means ~0.9", which would coincide with the 90th
percentile for this time of year. Additionally, an approaching
shortwave will lead to more widespread convection. Modest
instability on Monday could result in strong to severe convection
across northern Utah/SW Wyoming. PWATs lower by midweek closer to
climatological normals resulting in less of a flash flood risk.

Guidance has started to trend away from a deeper PNW trough for
early next week. Instead, ~49% of guidance has a modest trough
moving into the Great Basin that could lead to some shortwave
ridging ahead of the main passage of the trough by midweek.
However, 51% of the members keep more of a zonal pattern with the
potential for embedded shortwaves to bring more convection.

&&

.AVIATION...KSLC...Light northwest winds will transition to
southeast around 03Z with scattered to broken clouds. Northwest
winds are likely by around 19Z. Showers and thunderstorms will
likely be to the south, with around a 15% chance for a shower or
thunderstorm capable of gusty, erratic outflow winds after 19Z.

.REST OF UTAH AND SOUTHWEST WYOMING...Showers and thunderstorms
capable of heavy rain, lightning, and gusty, erratic winds will last
through around 06Z in southern Utah. Elsewhere, there will be dry
conditions with light winds. Scattered showers and thunderstorms
capable of heavy rain, lightning, and gusty, erratic winds are
likely south of KSLC after 19Z.

&&

.FIRE WEATHER...Monsoon moisture will remain across central and
southern Utah through the remainder of the week. This will fuel
mainly afternoon and evening showers and thunderstorms capable of
producing heavy rainfall. With this moisture in place, daytime RH
will remain elevated with good to excellent overnight recovery.

Meanwhile, a warmer and drier airmass remains across northern and
west central Utah where RH values are in the 10-15% range this
afternoon. This warmer and drier airmass will largely remain in
place through Friday, with a slight upward trend in RH as moisture
to the south begins to creep northward into the area. This will
introduce a slight chance for afternoon and evening showers and
thunderstorms mainly over the terrain. By Saturday this moisture
will have spread across all but far northwest Utah, resulting in
a continued upward trend of 5-10% in afternoon RH as well as a
better chance for showers and thunderstorms.

Southwesterly flow will spread across the region beginning early
next week. This will result in a gradual drying trend from west
to east as the monsoonal moisture is slowly pushed out of the
region.

&&

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

Seaman/Wilson/Mahan

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