Severe Thunderstorm Warning 29, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
900 AM PDT Sun Aug 13 2023

.SYNOPSIS...A circulation along the Southern California Coast
will bring a disturbance over our region today leading to an
increase in thunderstorm activity. Monsoonal moisture will persist
across the region through much of this coming week leading to a
chance of thunderstorms each day...especially over the mountains.
Temperatures will remain near normal through Monday and then climb
to a few degrees above normal by mid week.
&&

UPDATE...Active day on track as upper low spins over southern
California and provides some dynamic forcing for fairly widespread
convection today. The 12z VEF sounding sampled 500mb temps of -11C
which is rather cool for mid-August, and morning satellite imagery
depicts mostly clear skies to start the day across the Mojave Desert
and Northwest Arizona. Meanwhile, a band of morning showers and
embedded thunderstorms associated with a deformation zone stretches
from southern Utah into the southern Great Basin and Sierra.

With the clear start to the day across our central and southern
zones, and steep lapse rates in play, convection will fire quickly.
This mornings HRRR and HREF guidance suggest central and southern
Mohave County, the southern half of Clark County, and eastern San
Bernardino Counties will be favored for the strongest updrafts
through the late afternoon, with activity tending to shift
southwestward into western and southern San Bernardino county later
in the evening. With cool temps aloft and some moderate forcing, a
few stronger storm cores may produce small hail, perhaps to the size
of a penny. DCAPE values across the desert terrain are also in the
1500 j/kg range which will also be sufficient for locally strong
wind gusts, potentially exceeding 50 mph.

Made some updates to the PoPs to blend in some convective guidance
but otherwise things are on track.

-Outler-

&&

.SHORT TERM...Today through Monday.

Today looks to be a fairly active day for thunderstorm development
even though moisture is somewhat marginal with precipitable water
values generally 0.7-1.0" across most of our area along with surface
dewpoints in the mid to upper 40s. The difference maker will be a
broad vort max rotating around the SoCal closed low lifting up over
southern Nevada and surrounding areas bringing some upward forcing
due to vorticity advection and slight cooling near 500 mb. This
will combine with good surface heating across Clark, Mohave and San
Bernardino counties. The latest HRRR and other high res models
indicate scattered convective cells rapidly breaking out 18-21Z
across these counties and moving toward the west-northwest at 15-20
mph in the cyclonic flow around the upper low. The main threat with
these storms aside from lightning will be widespread winds gusting
over 35 mph supported by DCAPE values 1500-1700 J/kg and inverted V
RAP forecast soundings. The HREF has a large bullseye of 70 percent
probabilities for winds greater than 35 mph over the aforementioned
area.

A mid-level deformation zone will persist across Lincoln, central
Nye, Esmeralda and northern Inyo counties between the SoCal low and
a high over the Pacific Northwest extending down to northern CA/NV.
This will keep considerable cloud cover going over these areas this
morning followed by embedded thunderstorms developing this afternoon
and moving quickly toward the west. Strong winds will be the primary
threat for these areas also. Cannot completely rule out localized
flash flooding due to training cells for any place particular site,
but it is not as prevalent of a signal as strong winds.

Convection will diminish this evening and overnight then Monday
looks to be a down day as the closed low retrogrades off shore and
brings drier and more stable air over most of our region. This is
reflected by the NBM in a significant reduction in PoP values  over
our forecast area with 15-30 percent chances confined to eastern
Mohave and northern Lincoln counties.

.LONG TERM...Tuesday through Saturday.

Ensemble guidance remains in good agreement that the upper low over
southern California over the weekend will have retrograded further
west off the southern California coast Tuesday. This will allow
ridge to our east to expand westward allowing temperatures to warm
back up to several degrees above normal by Thursday.

As the high builds, at least the GEFS shows an increase in PW values
from the south-southeast into the Mojave Desert/western Arizona.
PWAT anomalies stay in the range of 100-175% of normal during the
period. NBM paints 20-50% PoPs across the eastern half of the CWA,
all the way into next weekend.
&&

.AVIATION...For Harry Reid...Variable winds start to turn to the
northeast later this morning with speeds of less than 10 knots.
Thunderstorm activity returns to southern Nevada once again this
afternoon, including along higher terrain around The Valley. Gusty
and erratic outflow winds may come with these storms. The greatest
potential for outflow winds at the terminal will be in the late
afternoon to early evening, but earlier outflows cannot be ruled
out. Outside of storms, winds remain around 10 knots out of the
southeast in the afternoon, decreasing and turning to the southwest
in the evening.

For the rest of southern Nevada, northwest Arizona and southeast
California...Another round of showers and thunderstorms is expected
across most of the forecast area today. Along with thunderstorm
activity possible within the vicinity of each terminal, outflow
boundaries from area storms may result in periods gusty and erratic
winds. Outflow activity should be the most active between the mid
afternoon and early evening, but outflows earlier in the day cannot
be ruled out. Outside of storms, most areas will maintain a diurnal
wind pattern. The exception is the Colorado River Valley sites,
which should see a push of northerly winds during the day.
&&

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

SHORT TERM...Adair
LONG TERM...Pierce
AVIATION...Meltzer

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