Flash Flood in Utah, August 25, 2023

Area Forecast Discussion
National Weather Service Salt Lake City UT
342 PM MDT Fri Aug 25 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 Sunday)...Lingering moisture
characterized by GOES-derived PW values in the 0.75 to 1 inch
range, and dew points in the 50s, remains across the forecast
area this afternoon. Mid level height rises coupled with somewhat
marginal mid level lapse rates near 7 C/km have limited
convection through mid afternoon, however over the last hour have
seen scattered convection spreading off the higher terrain toward
Capitol Reef as well as the Grand Staircase areas. Given the ample
moisture and slow storm motions, very heavy rainfall will be
possible with these storms through the remainder of the afternoon
and evening.

Little change in the overall pattern is expected Saturday, as
lingering moisture again supports scattered afternoon convection
initially over the higher terrain, but eventually spreading into
areas near Capitol Reef, the San Rafael Swell and the Grand
Staircase region. As such have maintained higher PoPs over these
areas, along with a Probable flash flood potential for Capitol
Reef.

.LONG TERM (After 12Z Sunday)...The synoptic pattern will remain
fairly stagnant through midweek as a ridge over the desert southwest
brings mostly dry and hot weather to the entire CWA. Low pressure
off the PNW coast will continue to meander before eventually
progressing inland. This will help to dislodge the aforementioned
ridge and bring back more of a troughing pattern to the west that
will lead to a moist southwesterly flow and cooler temperatures.

There is good agreement amongst ensemble members in regards to the
upcoming pattern. As a low sits off the PNW coast and a ridge over
the desert southwest a dry westerly flow will prevail over the CWA.
This will lead to temperatures 5-10 degrees above average, with
temperatures peaking on Tuesday in the mid 90s for most valley
locations with low 100s in lower Washington county.

Wednesday is a transition day as the low off the PNW coast moves
inland bringing temperatures back to near normal across the
northern CWA, but temperatures will remain elevated across
southern Utah. As this low moves onshore it will phase back in
with the mean flow and setup a trough across the western CONUS
while dislocating the ridge. Our CWA will be on the eastern side
of this trough. As such, increased southwesterly flow will advect
moisture back into the region leading to afternoon convection
Thursday and Friday. Additionally, temperatures will cool back
down to near normal.

&&

.AVIATION...KSLC...Northwest winds will switch to the southeast
between 03-04Z, with VFR conditions prevailing through the TAF
period.

.REST OF UTAH AND SOUTHWEST WYOMING...VFR and mostly diurnal
winds are expected through the TAF period. Scattered showers and
thunderstorms will persist through early evening, mainly east of
I-15. Temporary MVFR visibility in heavy rain, and gusty erratic
winds will be possible with these storms.

&&

.FIRE WEATHER...Lingering moisture across the region continues to
result in elevated humidity values this afternoon, and will again
allow for good to excellent humidity recovery tonight. Despite
this moisture, the airmass remains fairly stable which is limiting
the coverage of showers and thunderstorms this afternoon.
Isolated to widely scattered showers and thunderstorms will linger
into the evening hours, and locally heavy rain is possible with
any storms which do persist.

High pressure will slowly build across the region bringing a
gradual warming and drying trend beginning Saturday, and continuing
through early next week. Lingering moisture will again allow for
afternoon and evening showers and thunderstorms capable of
producing wetting rainfall Saturday, with the threat decreasing
on Sunday. Southwesterly flow will increase Tuesday and continue
into Wednesday, as a series of weak weather systems brush by
the area to the north. This will result in locally breezy
conditions across western valleys, while daytime humidity values
fall into the teens.  By Thursday another surge of monsoon
moisture will spread across the region, and remain in place
through much of Labor Day weekend.

&&

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

Seaman/Mahan

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