Flash Flooding in Utah, July 25, 2022

Area Forecast Discussion
National Weather Service Salt Lake City UT
406 AM MDT Mon Jul 25 2022

.SYNOPSIS...A tale of two regimes will continue with northern Utah
remaining dry while the southern half of Utah continues to see a
monsoon surge. Dry air will continue to shift southward Tuesday,
with only the southern third of the state seeing a threat for
convection.

&&

.SHORT TERM (Through 12Z Wednesday)...Morning upper air and
satellite analysis indicates a trough is shifting southeastward
across eastern Montana. A pronounced monsoonal pattern is in place
across the Desert Southwest, with broad ridging across the
southern United States.

Several weak MCVs are embedded within the flow across Arizona, the
first of which is shifting into southwestern Utah early this
morning. Persistent convection along a subtle boundary has
continued through the overnight hours and is now intensifying and
lifting to the northwest as the MCV shifts into the state. Main
threat with this convection will be heavy rain.

Attention will then turn to convective development this afternoon
and evening...mainly south of I-70. HREF mean SBCAPE values range
from around 500 J/kg to 1000 J/kg. Deep layer shear will be light
with PWATs around 1-1.50"...with the highest values near the
UT/AZ border. Storm motions look to initially favor a motion
toward the southeast, which is most favorable for Capitol Reef,
the Swell and the Grand Staircase.

Looking toward mid to late afternoon into early evening, given
weak storm motions and relatively high instability, several CAMS
(and conceptual models) suggest an area of convection will develop
across the higher terrain areas of southwest Utah and then
gradually shift south and west along the instability gradient into
Zion National Park and lower Washington County including St.
George. This type of pattern can also bring a flash flood threat
to the St. George metro area. The probability of exceeding the
average return interval of a 5 year 6 hour precipitation total is
around 40-50% (which is elevated) for portions of Washington
County this evening.

Convection will gradually decrease in coverage overnight. Moisture
and instability is expected to be more pinned toward the
Utah/Arizona border region Tuesday, though parameters from Iron
County to Garfield County and south are expected to be similar to
Monday. Another round of convection is expected in these areas
with slow storm motions and high ambient moisture levels
supporting the threat for flash flooding. Day shift may need to
evaluate an extension of the Flood Watch for these areas into
Tuesday evening.

Smoke from the Yosemite fires may bring hazy conditions into
portions of northern Utah again today.

.LONG TERM (After 12Z Wednesday)...A broad ridge of high pressure
is progged to be in place over the Great Basin to start the
period, with deeper moisture, precipitable water values near an
inch, over southern Utah.

Given weak low to mid level flow, deeper moisture and convection
should remain primarily over southern Utah on Wednesday. A return
of a weak southerly flow will push moisture northward into
portions of central and eastern Utah on Thursday. Given warm
temperatures aloft, expect high temperatures around 5 degrees
above normal for northern Utah, with readings remaining near
normal across central and southern Utah given greater sky cover.

With the ridge of high pressure amplifying over the western
portion of Great Basin on Friday and Saturday, should see a west
to northwest flow aloft suppress moisture to the south. This
stated, will likely be enough residual moisture to support low
chances for afternoon and evening showers and thunderstorms over
primarily southern Utah. Readings will likely be above normal
across majority of the forecast area.

Differences emerge in ensemble solutions for Sunday and Monday,
with GEFS indicating ridge axis becoming centered to the east of
forecast area, and thus, a southerly return of moisture. ECMWF is
a bit father westward with ridge axis, with CMC maintaining ridge
axis to the west of forecast area, and thus, offering a drier
solution.

&&

.AVIATION...KSLC...VFR conditions will prevail throughout the
entire period for the KSLC terminal. A southeast drainage wind
will shift to a northwest lake breeze around 18Z, enhanced a bit
by a weak westerly wind aloft.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will prevail
across northern and central Utah and extreme southwest Wyoming
throughout the period. With monsoonal moisture in place across
southern Utah, expect convection to develop this afternoon, as
early as 18Z in favored terrain areas including KBCE. Could see
MVFR/IFR conditions with thunderstorms. Showers and thunderstorms
will likely persist into the evening.

&&

.FIRE WEATHER...Another day of monsoonal moisture is expected to
bring a significant threat of wetting rain, particularly south of
I-70. Dry air will continue to push this deeper moisture south on
Tuesday, with only the southern third of Utah expected to see
showers and thunderstorms. This will continue to be the case
Wednesday with the potential for northward moisture advection
Thursday into Friday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flood Watch from 11 AM MDT this morning through this evening for
     UTZ123-124-128>131.

WY...None.
&&

Kruse/Barjenbruch

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

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php