Flash Flood Warning 143, NWS Las Vegas, NV

Area Forecast Discussion
National Weather Service Las Vegas NV
1213 PM PDT Wed Sep 13 2023

.SYNOPSIS...Scattered showers and storms today across southern
Nevada and northwest Arizona. Activity expands into Inyo and San
Bernardino Counties tomorrow. Dry conditions return Friday and
persist into early next week as temperatures remain near normal.
&&

.SHORT TERM...through Thursday.

Low-amplitude troughing persists over the western US today, leaving
our area under quasi-zonal flow ahead of an approaching shortwave.
Latest RAP shows very few (if any) vort maxima passing through this
afternoon, so convection is likely going to be terrain-driven.
Additionally, we'll be in the left entrance region of a 250mb jet
streak, inducing subsidence and producing an inversion around 500mb.
However, temperatures above the inversion have cooled slightly,
resulting in CAPE values similar to what we saw yesterday, even
despite the slight decrease in moisture. 12z HREF mean shows 500-750
J/kg of SBCAPE this afternoon, primarily across Lincoln, Clark, and
Mohave Counties. This lines up well with modeled convection being
largely limited to southern NV and northwestern AZ this afternoon.
Dry low levels are expected to limit heavy rain potential, but up on
the high terrain where sub-cloud evaporation is mitigated, heavy
downpours will be possible. The HREF depicts this well, highlighting
the high terrain of Clark County and the AZ Strip with a 30% chance
of seeing 1" of rain. Gusty winds and lightning will be the main
concerns for the lower elevations. Later this evening and tonight,
another round of convection is possible across our northern areas as
the base of the aforementioned shortwave moves in. Given the limited
moisture and instability after sunset, impacts are expected to be
minimal at best.

Tomorrow, the shortwave moves through the area, sparking scattered
convection across much of the area, generally north of I-40. With
moisture continuing to decrease, so too will instability. Ensemble
mean SCAPE values are forecast to be less than 500 J/kg except for
Mohave County where they get up to 750 J/kg. In terms of potential
storm impacts, tomorrow looks fairly similar to today. Heavy rain
possible on the high terrain, gusty winds and lightning the main
concerns for the lower elevations. Temperatures both today and
tomorrow expected to remain slightly below normal.

.LONG TERM...Friday through Tuesday.

After the shortwave passes through on Thursday, drier air will begin
to filter into the region. As a result, PoPs drop below 10% for
pretty much the entire CWA and remain that way into early next week.
The only exceptions to this may be the Spring Mountains on Friday,
and the Sierra crest this weekend. As ridging develops over the
western US, temperatures creep up to slightly above-normal values
over the weekend. By next week, the ridge axis is forecast to shift
eastward, with troughing developing upstream. This will result in
decreasing temperatures and possibly increasing winds. How well the
northern and southern branches of the jet stream can phase will
determine how cool/windy we get.
&&

.AVIATION...For Harry Reid...Another day with mountain showers and
storms is expected from 18z through around 02z. Some of this
activity may drift off the higher terrain to the west and into the
Las Vegas Valley, but confidence is quite low. Still, gusty outflow
winds could affect KLAS in the late afternoon/early evening.
Otherwise light winds 10 knots or less are expected, first from the
northeast, turning slightly southeast after 20z. Winds to turn back
to the northeast tonight, becoming variable after 07z. Similar
conditions are expected again tomorrow.

For the rest of southern Nevada, northwest Arizona and southeast
California...Mountain showers and thunderstorms may bring impacts,
namely gusty outflow winds, to KVGT/KHND between 22z and 02z.
Otherwise generally light diurnal winds 10 to 15 knots or less can
be expected most terminals. The one exception will be KDAG, where a
few gusts to 20 knots can be expected late this afternoon into this
evening.
&&

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

DISCUSSION...Woods
AVIATION...Austin

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