Flash Flood Warning 132, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
1041 AM PDT Wed Aug 10 2022

.SYNOPSIS...Another afternoon of thunderstorms expected today across
the eastern half of the area, with more isolated coverage in our
western locations. Heavy rain and flash flooding are the main
concerns. Storm chances and near-to-below normal temperatures stick
around into next week.
&&

.UPDATE...Mesoanalysis through the morning showed highly anomalous
moisture with an inch of higher PWATS and dewpoints over 60F in
place over much of southern Nevada through the Colorado River Valley
into western Arizona. In fact, the 12Z VEF sounding had recording
breaking PWATs of 1.7 inches (with the previously highest recorded
PWAT for today being 1.54 inches. Satelite imagery showed an area
of clouds associated with weak MCV lifting north into central
Nevada. By late morning, much of the region has at least some sun
to help heat the low levels. With a juicy atmosphere and surface
based CAPEs uncapped with values over 1000 J/Kg, we already have
isolated thunderstorms developing in the mountain area of Clark,
Lincoln, and Mohave County. High rain rates were already reported
with these storms.

Expectations for today have not changed much since the previous
forecast. Expecting convection late this morning through the
afternoon before diminishing this evening. Given the amount of
moisture, any storms will contain high rain rates and could lead to
flash flooding. The most widespread thunderstorm activity will be in
Lincoln, Clark, and Mohave counties this afternoon, thus the flash
flooding risk will be highest in these areas. Tall thunderstorms
with an uncapped atmosphere and progged ML CAPE to be uncapped with
values 1000-2000 J/Kg this afternoon could lead to wet microbursts
and strong winds. However, there is limited shear and DCAPE, so the
threat for widespread severe hail and winds is low. Made no changes
to the Flash Flood Watch that continues through this evening for
much of southern Nevada, western Arizona, and eastern San
Bernardino.
--Nickerson--
&&

.PREVIOUS DISCUSSION...203 AM PDT Wed Aug 10 2022
.SHORT TERM...Today and Thursday.

Overnight radar and satellite analysis shows scattered light showers
and a few thunderstorms across our central and northeastern zones.
These are likely being forced by remnant mesoscale perturbations
produced by yesterday's convection. This activity should continue to
drift northward with the mean upper-level flow, so a brief shower or
storm will be possible, mainly in our Nevada zones, through the
early morning.

The main thunderstorm activity will be this afternoon and evening,
across many of the same areas that were impacted yesterday. As a
result, the Flash Flood Watch was extended through midnight tonight.
The upper-level support for today is expected to arrive in the form
of an MCV produced by the ongoing convection in southern Arizona.
Not sure that guidance has a good grasp on this feature yet, it
looks like it'll arrive this afternoon or evening, providing lift
for thunderstorm development in additional to the usual orographic
component. Thus, scattered to widespread storms are forecast this
afternoon, primarily across Clark, Mohave, Lincoln, and northeastern
San Bernardino Counties. Primary concerns are heavy rain and flash
flooding, especially with PWATs of 1.5+ inches. However, gusty
outflow winds and frequent lightning will also be possible.
Thunderstorm activity should begin to wane in the evening hours.

Looking ahead to tomorrow, it's basically rinse and repeat. Deep
moisture stays in place and there remains no sign of larger-scale
features in the flow, so thunderstorm coverage will continue to rely
on mesoscale triggers from the previous day's convection. Heavy rain
and flash flooding potential will persist, primarily in Mohave,
Clark, and Lincoln Counties, but uncertainty remains in the
magnitude of these threats due to the aforementioned setup.

.LONG TERM...Friday through next week.

There is no sign of a pattern change in the extended forecast.
Ensemble guidance continues to show a dominant 500mb high over the
Four Corners/Southern Rockies region. This keeps us under south-
southeasterly flow which supplies the monsoon moisture. GFS and
ECMWF ensemble means show PWATs at or above 1.25" through next week.
Thus, above-normal rain chances and seasonal temperatures are
expected to persist through the period.
&&

.AVIATION...For Harry Reid...A moist southeasterly flow will
continue to bring abundant cloud cover across southern Nevada.
Easterly winds will diminish, becoming light and variable. SCT-BKN
clouds 15-20k feet lowering to 10-15k feet this afternoon.
Thunderstorm chances increase this afternoon with storm motion from
the southeast to the northwest. Possible impacts include brief heavy
rain, gusty outflow winds, and lightning. Storms are expected to
dissipate by sunset.

For the rest of southern Nevada, northwest Arizona and southeast
California...Isolated to scattered thunderstorms are expected to
develop this afternoon in the Las Vegas Valley and in the Colorado
River Valley impacting TAF sites with brief heavy rain, gusty
outflow winds, and dangerous lightning. Storm movement today will be
towards the north-northwest. Cloud bases 10-12 kft MSL. Low storm
chances at KBIH and KDAG today but an isolated cell or two
developing is not out of the question.
&&

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

DISCUSSION...Woods
AVIATION...Salmen

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