Flash Flood and Debris Flow in Arizona, August 24 to 26, 2022

Area Forecast Discussion
National Weather Service Las Vegas NV
1209 PM PDT Wed Aug 24 2022

.SYNOPSIS...Monsoon moisture returns for much of the area on
southeast flow today, bringing increased thunderstorm chances for
today, Thursday, and possibly Friday. Gradual drying trend is
expected for the weekend which will continue into next week.

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.SHORT TERM...through Friday.

Starting to see convection initiate across the high terrain this
afternoon near the Spring Mountains, with additional activity
firing across the Colorado Plateau. Given the well mixed and
unstable 12z sounding sampled this morning at VEF, expecting
activity to develop quickly across the higher terrain before
propagating southward on outflow boundary interactions. Current
SPC Mesoanalysis suggests 500-1000 mixed layer CAPE extending
southern Lincoln county southward, before increasing to over 1500
j/kg near the Colorado River Valley.

Still favoring the 12z HREF's overall convective evolution, with
early day through late afternoon scattered activity favoring
Clark and Mohave counties before increasing in coverage and
propagating southward into the Colorado River Valley through the
evening. The Flash Flood risk will be greatest in this region
where storms will be developing into an increasingly moist and
unstable airmass within an environment of relatively weak
steering flow, leading to slow moving strong thunderstorms. The
Flash Flood Watch for these areas continues to look good through
the evening hours. In addition to the flood threat, at least an
isolated risk of damaging wind gusts will exist as a strengthening
cold pool moves into the hot and well-mixed Lower river valley.
Greatest risk for strong winds will exist from Laughlin to Lake
Havasu, though storms most anywhere today will be capable of
strong microburst winds.

Similar conditions will continue tomorrow though uncertainty
exists in morning cloud cover and overall storm coverage tomorrow.
With the concerns about how things will organize today and
ultimately influence tomorrow, held off on any watch Flash Flood
Watch extensions for now, though additional watches will be
possible tomorrow for some areas. Lingering storm chances will
continue into Friday across mainly eastern Clark, Mohave, and
Lincoln counties.

.LONG TERM...Saturday through early next week.

An increasing westerly flow aloft will develop over the weekend
as weak troughing develops over the PacNW before sagging into the
Great Basin. This will suppress our monsoon high to the south and
allow some drier air to work into the region. This will lead to a
decreasing trend in thunderstorm chances, with only very isolated
storm coverage over the weekend across Northwest Arizona, and
potentially dry conditions area-wide by early next week.
Meanwhile, temperatures will hover near seasonal normals for late
August.

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.AVIATION...For Harry Reid...In general, winds will favor diurnal
trends today. However, there will be substantial deviations from
this pattern today due to thunderstorm activity in and around the
Las Vegas Valley creating gusty, erratic winds from their direction.
Vicinity showers could start as early as 19Z, with highest chances
of storms directly impacting the terminal between 22 and 03Z. After
sunset, winds will fall back into their diurnal patterns, generally
below 8 kts. There is an indication that light showers persist
around the Las Vegas Valley through the entire overnight period, but
confidence at this time is not high enough to warrant including it
in the prevailing TAF. SCT-BKN down to 5 kft possible this afternoon
with stronger, overhead thunderstorms.

For the rest of southern Nevada, northwestern Arizona, and
southeastern California...Winds will tend to favor diurnal trends
today, though convection at and around TAF sites will result in
gusty, erratic winds from the direction of the storms. KBIH, KDAG,
and the Las Vegas Valley will see greatest chances this afternoon
from 19 to 00Z, with KIFP and KEED ramping up around sunset and
lasting through 05Z. There is an indication that light showers
persist in and around the Las Vegas Valley through the overnight
period, but confidence at this time is not high enough to warrant
including it in the prevailing TAF. SCT-BKN down to 5 kft possible
this afternoon with overhead thunderstorms.
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.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

DISCUSSION...Outler
AVIATION...Varian

For more forecast information...see us on our webpage:
https://weather.gov/lasvegas or follow us on Facebook and Twitter

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LAS VEGAS NV
411 PM PDT WED AUG 24 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0249 PM     FLASH FLOOD      4 E MOUNTAIN PASS       35.46N 115.47W
08/24/2022                   SAN BERNARDINO     CA   LAW ENFORCEMENT

            CHP REPORTED THEY WERE REQUESTING CLOSURE OF
            INTERSTATE 15 DUE TO FLOODING AT MOUNTAIN
            PASS BETWEEN NIPTON ROAD AND BAILEY ROAD.

0328 PM     FLASH FLOOD      8 NE MOUNTAIN PASS      35.54N 115.42W
08/24/2022                   SAN BERNARDINO     CA   LAW ENFORCEMENT

            CHP REPORTED FLOODING HAD STOPPED TRAFFIC ON
            SOUTHBOUND INTERSTATE 15 AT YATES WELL ROAD.
            OFFRAMP AND ONRAMP WERE ALSO FLOODED.

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