Flash Flooding and Damaging Winds in Arizona, August 12 to 15, 2022

Area Forecast Discussion
National Weather Service Las Vegas NV
1008 AM PDT Fri Aug 12 2022

.SYNOPSIS...Moist monsoonal flow will remain in place through
the week keeping temperatures at or below average with daily
thunderstorm chances, mainly for the eastern Mojave Desert and
Southern Great Basin. Further west across the Sierra and Western
Mojave Desert, generally dry weather expected over the weekend
with increasing storm chances next week.

&&

.UPDATE...Showers and isolated thunderstorms were develop early this
morning across eastern Lincoln and northern Clark counties aided by
an upper level circulation which was depicted in the SPC 700-400mb
differential vorticity advection analysis. However, this was not
indicated by the latest HRRR which still forecasts an active day of
thunderstorms driven mainly by surface heating and outflow
interaction. Another feature to watch is the crest of an easterly
wave between Lake Havasu and Yuma which was initiating thunderstorms
this morning. This may enhance thunderstorm development up into
southeast San Bernardino County. Meanwhile, these disturbances and
surface heating have a very moist and unstable air mass to work with
over Clark, Lincoln, Mohave and eastern San Bernardino counties
where a Flood Watch is in effect. Slow moving storms and heavy
rainfall can be expected anywhere in this area today. No significant
changes needed to the going forecast. -Adair
&&

.PREV DISCUSSION...238 AM PDT

.SHORT TERM...through this evening.

After an eventful evening, convection has ceased early this
morning with the radar devoid of any echoes as of 2am. Moisture
remains plentiful today with a continued weak steering flow of
~5 knots or less with in an environment characterized by 150-180%
of normal precipitable water values. This translates to another
day of slow moving thunderstorms capable of very heavy downpours
and flash flooding. This evenings 00z HREF guidance depicts the
greatest risk for heavy precipitation across the mountains of
Clark and Mohave counties, as well as eastern Lincoln County.
Given the anomalous moisture and slow storm motion expected,
issued a broad Flash Flood Watch for today through midnight for
most of Lincoln, Clark, eastern San Bern, and Mohave Counties.

While I don't necessarily anticipate a repeat performance in the
Las Vegas Valley with widespread activity, storms forming near the
Spring Mountains or McCullough Range may tend to drift into the
valley in the afternoon with an attendant flood risk.

.LONG TERM...This weekend and much of next week.

Little change over the weekend with rich monsoonal moisture
remaining entrenched across the Mojave Desert with a weak
southerly steering flow persisting. While daily storm coverage
remains uncertain, flash flood risk will persist given continued
slow storm motion and efficient rainfall rates. With cloud cover
and scattered precipitation, temperatures will be kept in check
and remain generally at or below average across the region.

As we head into next week, the monsoon shows little sign of
slowing down with rich moisture locked in place across the Mojave
Desert and southern Great Basin. Meanwhile, high pressure builds
into the northern Great Basin which could shift the steering flow
to a more north or northeasterly direction for a few days, a
favorable pattern for thunderstorm complexes moving into the lower
deserts. Aside from the daily storm chances, temperatures will
remain at or below normal through the period, with the greatest
departures from normal across the eastern Mojave Desert where
deeper monsoonal moisture will be hanging around.

&&

.AVIATION...For Harry Reid....Expect the remainder of the night to
be dry with light winds and a variable amount of clouds at all TAF
sites. Expectations are for storms developing between 11 am and noon
today before potentially moving or developing in the valley. The
primary concerns for terminal impacts include brief heavy rain,
gusty outflow winds, and lightning. Outside of any thunderstorm
influences, expect southerly winds around 6-8 kts.

For the rest of southern Nevada, northwest Arizona and southeast
California...Expect the remainder of the night to be dry with light
winds and a variable amount of clouds at all TAF sites. Expectations
are for storms developing between 11 am and noon today before
potentially affecting TAF sites in the Las Vegas Valley and the
Colorado River Valley after 21z and lasting through mid-late
evening. Possible storm impacts at TAF sites include Brief heavy
rain, gusty outflow winds, and lightning. KDAG could see isolated
storms after 22z while KBIH should remain dry.
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

DISCUSSION...Outler
AVIATION...Salmen

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LAS VEGAS NV
1220 PM PDT FRI AUG 12 2022

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

1000 PM     FLASH FLOOD      4 NNE SPRING VALLEY     36.16N 115.22W
08/11/2022                   CLARK              NV   PUBLIC

            SOCIAL MEDIA VIDEO SHOWS STREET FLOODING ON
            WEST CHARLESTON BOULEVARD WITH VEHICLES
            DRIVING THROUGH IT. ESTIMATED TO BE 1-1.5
            DEEP.

&&

MW