154
FXUS65 KSLC 171947
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
147 PM MDT Thu Jul 17 2025

.SYNOPSIS...A potent moisture surge will bring the threat of
thunderstorms with heavy rain to southern Utah today, spreading
into central Utah Friday. Drier air will gradually work into the
region Saturday into Sunday.

&&

.SHORT TERM (Through 12Z Saturday)...An active day of convection
is expected across much of southern Utah. Early afternoon upper
air and satellite analysis indicate multiple MCVs embedded in
southerly flow across Arizona. Current mesoanalysis indicates
SBCAPE values in excess of 1000-1500 J/kg across southern Utah
coupled with deep layer shear around 15-25 kts. Precipitable water
values have surged over 1" across southern Utah, an increase of
around 0.4-0.5" from 24 hours ago.

With deep moisture in place, HREF max hourly rainfall rates
continue to show values in excess of 1.5"/hour. While this is an
extreme, given the CAPE/shear profile, rainfall rates in excess of
1"/hour are likely across southern Utah, particularly southwestern
and south central portions. The Weather Prediction Center upgraded
southwestern Utah to a slight risk for excessive rainfall,
implying a greater than 15% chance of rainfall rates exceeding
flash flood guidance within 25 miles of a point.

Given the combination of forcing, instability, shear and
available moisture, issued a flood watch for flash flooding for
portions of southern Utah including Zion National Park, the
greater St. George area, southwestern Utah including Cedar City
and mainly the western portion of the Grand Staircase through
midnight.

Storm genesis is already ongoing as of press time, and given
trends in CAMS, expect the highest threat of strong thunderstorms
capable of heavy rain to coincide with the flood watch area. In
particular several CAMS suggest storm morphology and trends will
focus heavy rainfall on Zion National Park and the St. George area
later this afternoon and evening.

Isolated high-based convection will be possible further north this
afternoon and evening, with the main threat occasional gusty,
microburst winds.

Guidance suggests a mid-level dry slot will shift the convective
threat north of Bryce Canyon Friday afternoon and evening...with
recent burn scars in central Utah as well as normally flood prone
areas most at risk for flash flooding from heavy rainfall. Across
far northern Utah, sufficient deep layer shear and instability
may allow for strong convection with an attendant wind
threat...as the last few runs of the HRRR have indicated.
Something to watch.

.LONG TERM (After 12Z Saturday)...

Issued 419 AM MDT

By Saturday, a longwave trough will dig into
western Canada, while an expansive ridge of high pressure from the
southern plains into the southeast U.S. amplifies. Flow for the
region will become more southwest, which will advect drier air
into southwest Wyoming and Utah. Expect slightly less coverage of
showers and thunderstorms than prior in the week, but enough
moisture and forcing will be in place for gusty, erratic outflow
winds and localized flash flooding if a thunderstorm producing
heavy rain tracks into a vulnerable location.

The longwave trough will dig into the PacNW Sunday, while ridging to
the southeast amplifies. This set up will bring a downtrend in
precipitation, with most showers and thunderstorms isolated and
around mountainous terrain. The approaching longwave trough will
enhance southwest flow. Southwest winds will likely be below Red
Flag Warning criteria, but gusts will likely top out around 25 mph
for southwest Utah. Relative humidity will range from around 10-20%.

Critical fire weather conditions become more likely Monday as the
longwave trough will dig slightly further south and the ridge to the
southeast will be largely unchanged. Flow will be west to southwest,
gusting in excess of 25 mph throughout southwest Wyoming and most of
western Utah. Relative humidity values will likely range from the
single digits to teens.

Ensembles vary on the track of the longwave trough into the week,
with some members breaking off a closed low that would track
southwest to off the west coast. Other members slide the trough
eastward, with it remaining north of Utah. The ridge to the
southeast will likely retrograde into the week, which will allow for
lighter winds. It would also bring better moisture advection, with
more of a southerly flow. Models and ensembles are in good agreement
on that, with a general uptrend in precipitable water values Tuesday
through much of the week.

Valley high temperatures from the weekend through much of the week
will be near normal in the low or mid 90s.

&&

.HYDROLOGY...A flood watch for flash flooding has been issued for
portions of southern Utah through midnight. Convective allowing
models have suggested rainfall rates in excess of 1"/hr are
possible with the stronger storms, with a portion of the guidance
focusing on especially flood prone areas of Zion National Park and
the greater St. George area. Those in the flood watch should
continue to monitor forecasts and prepare for potential warnings.
For those with outdoor plans, heavily suggest refraining from
entering or camping in/near slot canyons and normally dry washes
through tonight.

&&

.AVIATION...KSLC...VFR conditions will prevail for the KSLC
terminal through the period. Dry and clear conditions will
transition to more scattered and broken CIGs this afternoon as
convection tries to develop. There is a 30% chance of storms
impacting the terminal through sunset with storms capable of
frequent lightning, heavy rain reducing VIS, and gusty erratic
outflow winds. Winds will become light and southerly overnight.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will prevail
for most of the airspace through the period. Afternoon
convection, primarily across the southern and central airspace,
lasting until ~06Z will be capable of producing frequent
lightning, heavy rain reducing VIS, and gusty erratic outflow
winds. Light precipitation will slowly shift north through the
central portions of the airspace, with little impact to terminals.

&&

.FIRE WEATHER...A potent monsoon surge combined with a series of
convectively induced features will bring a heightened threat of
wetting rains and flash flooding to southern Utah today. The best
threat of heavy rain and flash flooding will shift somewhat
northward to include much of Utah Friday. By Saturday, drying will
begin to clear the deep moisture out of the region.

As a disturbance approaches the Pacific Coast Monday, gusty
southwest winds will combine with low humidities to bring the
threat of critical fire weather conditions to western Utah. This
threat will continue into Tuesday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch until midnight MDT tonight for UTZ122>124-128.

WY...Red Flag Warning until 8 PM MDT this evening for WYZ277.

&&

$$

Kruse/Mahan/Wilson

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