Flash Flood Warning 58, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
218 AM MDT Sat Aug 26 2023

.SYNOPSIS...A warming and drying trend is expected Saturday into
early next week, with the potential for another monsoon surge
later in the week.

&&

.SHORT TERM (Through 12Z Monday)...Another quiet overnight period,
after a day with several flash flood warnings. Early morning upper
air and satellite analysis indicates gradually rising heights as
high pressure builds into the area. Mesoanalysis indicates
precipitable water values have fallen to 0.80-1.00" across the
state. These values are closer to the 90th percentile or lower,
rather than the near climatological maxes several days ago.

HREF ensemble mean SBCAPE values are expected to range from around
250 to perhaps 750 J/kg across eastern and southern Utah this
afternoon. Deep layer shear in this area is around 5-10 kts, with
storm motions favoring an east to northeast trajectory. While not
the most conducive parameters for long-lived convection, expect
another day of a conditional threat of flash flooding. Given
the storm motion is more east to northeast today as the upper
level ridge shifts eastward, the main threat of flash flooding is
expected across Capitol Reef National Park and the San Rafael
Swell. Typically, storms moving in this direction aren't as high a
threat of flooding for the Grand Staircase, however, given light
flow, it is possible cold pools may push storms down drainage in
the Staircase.

Across far northwestern and western Utah, a slightly different
regime is in place. Expect SBCAPE values to average around 500
J/kg, but deep layer shear is a bit higher in this area, on the
order of 20-25kts. With a relatively well mixed, deep subcloud
environment in place, expect any thunderstorms may bring the
threat of strong and gusty microburst winds in this area.

Sunday looks quite a bit quieter, with convection restricted to
near the Utah/Idaho/Wyoming border area and perhaps associated
with higher terrain. With a drier airmass in place, the flash
flood threat will be considerably lower than the last few days for
most locations, with a possible rating in the flash flood
potential product for Capitol Reef and the Swell.

.LONG TERM (After 12Z Monday)...The beginning of the week will
see dry conditions with warming temperatures as a ridge sits over
the CWA and west to northwest flow will be situated aloft.
Temperatures for Monday and Tuesday will run 5F to 10F above
normal. This means most valley locations will see highs in the mid
to upper 90s and areas near lower Washington County and Zion NP
will see highs in the low 100s.

During this time, a low pressure system will be stalled off the
PacNW coast. This will move inland starting on Tuesday. As this
low moves inland, Wednesday becomes a transition day as this low
moves by to the north and moves the ridge. This low will bring
cooler temperatures with it as temperatures will go back to near
normal for the middle of the week.

By Thursday, there is good agreement amongst ensemble members
that after the low moves by, a trough develops off the California
coast and the ridge sets up to the east. The CWA will then see
enhanced southwesterly flow through the end of the week. This flow
direction will bring back monsoonal moisture as PWAT values
increase to be between 0.7" and 1.0" across the CWA. With this
moisture and increased southwesterly flow, afternoon showers and
thunderstorms will be possible through the latter part of the
extended period. Temperatures towards end of the week could be as
low as 5F to 7F below normal.

&&

.AVIATION...KSLC...Southeast winds expected through the overnight
period. Northwest wind shift is expected between 18Z and 20Z.
There is a low chance (15%) that showers or thunderstorms could
impact the area during the afternoon. With these storms, winds
could become gusty and erratic and there could be brief reductions
in visibility. VFR conditions expected to prevail.

.REST OF UTAH AND SOUTHWEST WYOMING...Isolated showers and
thunderstorms are possible for much of Utah and southwest Wyoming,
with the greatest chance being in eastern Utah. With these storms
winds could become gusty and erratic and there could be brief
reductions in visibility. VFR conditions and predominantly diurnal
winds expected for the TAF period.

&&

.FIRE WEATHER...While deep moisture continues to decrease across the
region, sufficient moisture remains in place for another round of
slow moving thunderstorms across mainly portions of eastern and
southern Utah. A few thunderstorms with strong gusty winds are
possible near the Utah/Idaho/Nevada borders in northwestern Utah.

High pressure will build into the area through the weekend and
remain in place through early to mid-week next week. This will
bring increasingly dry conditions, with valley minimum humidities
falling to near 15-25% by Sunday, gradually lowering to 10-15% by
Tuesday. As a result of this dry airmass, overnight recoveries
will worsen through at least midweek.

Guidance continues to indicate the next monsoon surge will begin
Thursday, with overnight humidities increasing as early as
Wednesday night.

&&

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

Kruse/Cecava

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