Flash Flood in Nevada, August 23, 2023

Area Forecast Discussion
National Weather Service Las Vegas NV
817 PM PDT Wed Aug 23 2023

.SYNOPSIS...Continued thunderstorm activity is anticipated for
Thursday and Friday due to remnant moisture from tropical
depression Harold. The greatest rainfall chances will be on
Thursday afternoon and evening. Activity should subside and
temperatures return to near average values next week.

&&

.UPDATE...Outflow from thunderstorm activity over Mohave County
earlier this evening made it into the Las Vegas Valley and is
sparking a few thunderstorms. At 8 pm, storm development was
mainly east of the I-15 and storms were producing dangerous
lightning, gusty winds, and moderate to heavy rain. Once the
storm activity ends this evening, we are looking for a quiet
overnight period. An update for the overnight period had been sent.

&&

.PREV DISCUSSION...1217 PM PDT Wed Aug 23 2023...

.SHORT TERM...Tonight through Friday.

The primary forecast concern in the short term is thunderstorm
activity. The remnants of tropical depression Harold, currently over
southern New Mexico, will advect moisture into Arizona and southern
Nevada starting this afternoon. This initial round of thunderstorm
activity should peak in the late afternoon, then taper off into the
evening with the loss of surface heating. Today and this evening's
activity is limited to northwest Arizona and higher terrain in Clark
and Lincoln counties as that is where moisture and instability is
forecast to be the greatest.

Thunderstorm development will become more widespread on Thursday
afternoon and evening as the remnant low circulation gets pulled
northwest around an upper level high, sending it further west into
Arizona. The presence of the low will advect additional moisture
into the forecast area, particularly in northwest Arizona and
southern Nevada where PWATs reach 1.6 inches, 180 to 200 percent of
normal. However, while CAPE remains roughly the same (between 250
and 500 J/kg), forecast soundings show a decrease in surface to 6 km
bulk shear. This setup appears to favor heavy rainfall rather than
strong outflow winds. Flash flooding will be the main threat across
the forecast area, especially given the already saturated soil from
heavy rainfall over the past week. That being said, the highest POPs
are over Mohave, Lincoln, and Clark Counties, while the greatest
rainfall earlier this week and last week was over eastern
California.

On Friday, upper level flow turns to the southwest and PWATs fall to
around 1 inch over the forecast area. Lacking moisture and
instability should limit most storm activity to high terrain in
northwest Arizona and maybe Mt. Charleston. Temperatures remain
below normal but start to rise, a trend that will last through the
weekend and into next week.

.LONG TERM...Saturday through next Wednesday.

The monsoonal area of high pressure is forecast to shift westward
over the Desert Southwest this weekend into early next week
providing the area with a break in monsoon action. That said,
moisture trapped underneath the high will provide a slight chance of
showers and thunderstorms Mohave County each afternoon, favoring the
higher terrain. Temperatures are expected to climb back to around
normal by early next week, and winds are forecast to be light.

&&

.AVIATION...For Harry Reid...Expecting VFR conditions with winds
at or below 10kt through at least 22z. Thereafter, an area of
showers and storms over Arizona looks to generate east/southeast
outflow winds to reach the terminal between 23-01z. Wind gusts of
20-25kt may occur with this activity. This outflow may also
trigger additional shra/tsra with VCTS maintained in latest
forecast. Gusty winds of 25-35kt may accompany any stronger storms
along with heavy rainfall. By 04z, most activity looks to
dissipate, with light northeast winds resuming, becoming variable
by 10z.

For the rest of southern Nevada, northwest Arizona and southeast
California...Generally VFR conditions and winds 10-15kt or less are
expected through at least mid afternoon. KBIH will see slightly
stronger southeast winds through the Owens Valley during the day,
with gusts to 25kt. Elsewhere, an area of showers and storms over
western Arizona is set to send an outflow westward, which will bring
east/southeast winds to KEED/KIFP. The outflow may also trigger
showers and thunderstorms from 00z to 04z with gusty winds and
locally heavy downpours. Otherwise, terminals will largely follow
diurnally induced wind patterns with all terminals set to return to
VFR conditions by 05z-06z.

&&

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

&&

UPDATE...Salmen

SHORT TERM/LONG TERM...Meltzer
AVIATION...Austin

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