Flash Flooding and Damaging Wind in California, September 2 to 5, 2025

Area Forecast Discussion
National Weather Service Las Vegas NV
412 AM PDT Tue Sep 2 2025

.KEY MESSAGES...

* Monsoonal moisture surges into the forecast area today, resulting
  in widespread shower and thunderstorms capable of producing flash
  flooding.

* Moisture remains in place through midweek before slowly scouring
  out of the region, shifting precipitation chances further east
  each day.

&&

.DISCUSSION...Today through Monday.

The ridge of high pressure over the Desert Southwest will shift
slightly eastward today, as a result of a shortwave approach off the
California coast. This eastward shift will aid in monsoonal moisture
advection into our CWA today, due to the resultant southerly flow.
PWATs will range from 1.25 to 1.50 inches across our southern zones,
including San Bernardino County, Clark County, and southwestern
Mohave County. In addition to this moisture, an easterly wave will
rotate around the aforementioned high pressure to combine with the
incoming California shortwave. As this occurs, the resultant
vorticity lobe will push northward through southeastern California
this afternoon, providing necessary forcing for widespread
thunderstorms. Convection will initially favor the higher terrain,
as cloud cover may inhibit lift from daytime heating in valley
locations. With ample moisture and forcing, the primary threats
today will be moderate-to-heavy rain, frequent lightning, and flash
flooding. Best chances of widespread flash flooding exist across San
Bernardino County and Death Valley, where Flash Flood Watches are in
effect from 9 AM this morning through 1 AM PDT tonight.

Today's vorticity lobe will weaken somewhat on Wednesday as it
shifts over south-central Nevada. Moisture will continue to inch
further north on Wednesday, with PWAT values across our southern
zones increasing to 1.50 to 1.75 inches. As such, expect widespread
showers and thunderstorms once again on Wednesday, with best chances
spreading further east. Thunderstorms on Wednesday will be
productive rain-makers, with high rain rates and instances of flash
flooding expected once again. Confidence regarding the location of
these storms is low at the moment, so the Flood Watch has not been
extended into Wednesday at this time. Moisture slowly scours
northwest-to-southeastward out of the region through the remainder
of the work week, with precipitation chances becoming more and more
confined to the eastern zones each day. Expect below-normal
temperatures through the forecast period, as a result.
&&

.AVIATION...For Harry Reid...For the 12Z Forecast
Package...Thunderstorms are possible this afternoon, starting
anytime after 18Z. The highest chance for convection and impacts at
the terminals will be 23Z-03Z when strong, erratic winds form
outflows, lightning, moderate to heavy rain, and CIGS to 10kft
possible. Convection should slowly wane tonight, with thunderstorms
moving away around 06Z. It should be dry tonight into Wednesday
morning, however isolated showers may develop and move through the
valley Wednesday morning after 12Z. Outside of convection, winds
should remain light and follow typical wind trends. Low confidence
in winds this evening as wind direction and speeds will depend on
what outflows move through in the evening, however light southwest
to west winds should return at some point overnight.

For the rest of southern Nevada, northwest Arizona and southeast
California...For the 12Z Forecast Package...Thunderstorms are
expected across the region this afternoon and evening, with the
highest threat and coverage of convection in San Bernardino and Inyo
counties. Thunderstorms today in all locations could result in
erratic gusty winds, heavy rain, lightning, and CIGS down to 7000ft.
 Convection should wane this evening and overnight, however there is
a low chance that showers and thunderstorms could persist tonight in
San Bernardino County. Outside of the convection, winds should
remain light today and follow typical patterns. &&

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

&&

DISCUSSION...Soulat
AVIATION...Nickerson

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