Severe Thunderstorm Warning 76, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
422 AM MDT Thu Aug 17 2023

.SYNOPSIS...Monsoonal moisture will remain in place across the
region, bringing daily chances for showers and thunderstorms
across the area. A moisture surge from Tropical Storm Hilary is
slated to move across the Great Basin region and may bring
significant impacts to the forecast area late in the weekend/
early next week, but there are still some forecast challenges to
work through in that regard.

&&

.SHORT TERM (Through 12Z/6AM Saturday)...The eastern Great Basin
region remains sandwiched between two large scale synoptic
drivers; an offshore low near California and a developing high
over the central Rocky Mountains. Deep atmospheric moisture is
noted between these two features and will continue to drive
diurnally driven showers and thunderstorms across the forecast
area. High terrain and adjacent valley areas from roughly Zion NP
north-northeastward through Uinta Co., Wyoming will see the
greatest chances for seeing any sort of significant precipitation
with scattered storms, whereas areas in eastern and northwestern
Utah will see more isolated storms. Storm motions are expected to
be relatively slow (10-20kts) and on a SW-NE trajectory. For areas
sensitive to heavy rainfall (i.e. slickrock areas, dry washes,
slot canyons, and recent burn scars), the threat of flash
flooding will be elevated... especially for areas like Capitol
Reef NP, Grand Staircase-Escalante NRA, and the various large burn
scars of central and northern Utah. Temperatures will remain near
seasonal today, however, any passing showers will bring potential
to drop temperatures 10-15 degrees cooler than forecast.

Moisture will remain spread across the region through the end of
the week, and into the weekend, keeping chances for showers and
thunderstorms in the forecast for Friday. The caveat here is that
Friday will feature a bit more support from the offshore low,
which will increase the overall coverage of storms, especially
across the southern half of the forecast area. The risk of flash
flooding will remain elevated on Friday and spread a little
further north into northern Utah, where areas featuring steep,
rocky terrain or recently burned areas will see an increased
threat.

.LONG TERM (After 12Z/6AM Saturday)...A complex pattern will be in place
to start the long term period. A ridge over the central CONUS and a
cutoff low off the coast of California will keep a moist southerly
flow advecting into the entire CWA. This will help to develop
afternoon convection on Saturday with the highest probabilities
across southern Utah.

In addition to the typical monsoonal moisture plume, tropical storm
Hilary will be moving up the Baja of California coast with impacts
to portions of the desert SW Sunday through Tuesday. The highest
confidence track currently keeps a bulk of this tropical moisture
across California and Nevada. However, any deviations further east
would result in a greater flash flood potential to Utah and SW
Wyoming. A further track west/east would also result in temperatures
near normal/below normal. Currently there is a marginal risk of
flash flooding across Utah Saturday and Sunday, according to WPC's
excessive rainfall outlook.

Once the remnants of this tropical cyclone passes by to the north
Tuesday the focus returns to a more typical monsoonal moisture plume
that will be moving in from the south/southeast as the
aforementioned ridge remains in place across the center of the
country. This southeasterly flow will be drawing moisture up from
the Gulf of Mexico as tropical moisture will be moving into northern
Mexico/southern Texas. This moisture will remain in place through
the end of the period as ensemble mean PWATs across northern Utah
remain ~1" with near 1.15" PWATs across southern Utah. Expect
temperatures to remain near normal, if not slightly below normal.

&&

.AVIATION...KSLC...Mostly VFR conditions will prevail for the KSLC
terminal through the remainder of the period. Monsoonal moisture
will result in isolated afternoon convection that could briefly
reduce flight categories. Gusty and variable outflow winds could
result from any nearby convection. Otherwise, winds will likely
remain southerly for most of the day with only a brief window of
northerly lake breeze winds in the afternoon.

.REST OF UTAH AND SOUTHWEST WYOMING...Mostly VFR conditions will
prevail for most terminals as a monsoonal plume moving through the
entire region will result in afternoon convection that could create
brief flight category reductions. Additionally, winds will primarily
be out of the south with gusty variable winds associated with any
nearby convection. Convection over the terrain could lead to brief
mountain obscuration.

&&

.FIRE WEATHER...Deep atmospheric moisture will spread across a large majority
of the region today, bringing increased risk of showers and
thunderstorms across Utah. Storms will be diurnally driven and
will favor higher terrain areas, as well as adjacent valley areas.
Due to increased atmospheric moisture content, wetting rain
chances will be anywhere from 30 to 60 percent across the high
terrain (excluding northwest Utah). Heavy rainfall with isolated
flooding, lightning, and gusty outflow winds will be the primary
threats in passing storms. Outside of storm chances, humidity will
continue to be moderated across a majority of Utah with the
exception of portions of northwest and eastern Utah. In these
areas, humidity may drop as low as 20%.

Deep layer moisture will remain in place tomorrow with coverage
of showers and thunderstorms expected to increase across the
southern half of Utah. CWRs will see a significant increase in the
southern and central mountains, as well as the Uintas and
southern Wasatch, and adjacent valley areas with minimal change
expected across far northern Utah.

A much more significant surge of moisture is expected to push
into Utah this weekend, bringing much higher daytime humidity
values, coverage of showers and thunderstorms, and increased
chances for widespread wetting rains. Moisture will remain in
place through at least mid-week next week, keeping the chance for
daily showers and thunderstorms in the forecast.

&&

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

Webber / Mahan

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
245 PM MDT Thu Aug 17 2023

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

0225 PM     Flash Flood      Fruita                  38.28N 111.25W
08/17/2023                   Wayne              UT   Park/Forest Srvc

            Sulphur Creek flash flood reached the
            Visitors Center at 225 PM. Fremont is out of
            banks near the Fruita Campground.
            Evacuations are in progress.

&&

CRK

Preliminary Local Storm Report
National Weather Service Salt Lake City UT
301 PM MDT Thu Aug 17 2023

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

0300 PM     Flash Flood      1 N Zion National Park  37.26N 112.95W
08/17/2023                   Washington         UT   Park/Forest Srvc

            Park Service reports ongoing flash flooding
            at Refrigerator and Echo Canyons.

&&

CRK

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