AWUS01 KWNH 221322
FFGMPD
AZZ000-UTZ000-221920-

Mesoscale Precipitation Discussion 0919
NWS Weather Prediction Center College Park MD
921 AM EDT Thu Aug 22 2024

Areas affected...southern UT...northern AZ

Concerning...Heavy rainfall...Flash flooding likely

Valid 221320Z - 221920Z

Summary...Localized short-term totals in excess of 2" are likely
to lead to isolated to widely scattered instances of flash
flooding. An instance or two of flash flooding may be significant,
with particular concern for slot canyons, normally dry washes, and
other sensitive low-lying areas).

Discussion...The Southwestern U.S. remains wedged between an upper
low/trough to the northwest (off the Northwest U.S. coast) and a
upper high/ridge to the southeast (over TX), enhancing the
seasonal monsoonal pattern with moisture streaming northward in
the lower to mid levels (925-700 mb). Widely scattered convection
is evident via GOES and MRMS imagery over portions of southern UT
and northern AZ at this early morning hour, unusual for even this
more rainy time of year. The mesoscale environment is
characterized by pockets of 500-1000 J/kg of SB CAPE, a broad
plume of precipitable water values ranging from 0.9 to 1.3 inches
(between the 90th percentile and max moving average, per VEF and
FGZ sounding climatology), and effective bulk shear of 20-30 kts.
This parameter space should continue to support at least isolated
convection through midday, despite somewhat limited CAPE and
unfavorable diurnal timing, particularly because convection is
already ongoing and shear is anomalously high (near the 90th
percentile, owing to the unseasonable strength of the Northwest
U.S. trough).

While hi-res CAMs notoriously struggle with monsoonal convection,
there is some HREF signal (from the 06z run) to support a flash
flood risk in the near term. While the HRRR is consistently one of
the weakest members of the ensemble, there is still a 10-20%
chance for localized 2" exceedance, per 40-km neighborhood method
via the 06z HREF. These values are quite significant across this
hydrologically sensitive portion of the country, corresponding to
20-30% chances of exceeding the 10-year average recurrence
interval (ARI), and even a 10% chance of exeecind the 100-year ARI
(maximized over portions of southwestern UT, where much of the
rainfall so far this morning has already occured). Given this
favorable enviornment and the local sensitivity, isolated to
scattered instances of flash flooding are likely (and may be
locally significant, particularly in the vicinity of slot canyons,
normally dry washes, and other sensitive low-lying areas).

Churchill

ATTN...WFO...FGZ...GJT...SLC...VEF...

ATTN...RFC...CBRFC...NWC...

LAT...LON 38531152 37410974 35441022 34611246 35131352
35791393 37291388 38061317
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