Flash Flood and Flooding in California, September 24, 2021

Area Forecast Discussion
National Weather Service San Diego CA
934 AM PDT Fri Sep 24 2021

.SYNOPSIS...
An upper level low pressure system will remain across the area
through Saturday, then move slowly eastward and out of the area
on Sunday and Monday. Some showers and thunderstorms over the
mountains and deserts this afternoon may move westward to the
coast. Decreasing chances for afternoon showers and thunderstorms
for the mountains, deserts, and inland valleys will continue into
Sunday. A slow cooling trend will continue through Sunday with
Monday a little warmer. Another low pressure system moving inland
to the north will bring more cooling for Tuesday and Wednesday
followed by warming on Thursday.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

.Morning Update...
Our upper level low has arrived and is already bringing some
showers and storms to the San Diego County deserts at this hour.
Some of these storms over the eastern mountains and deserts may
also produce heavy rain that could lead to flash flooding, hence
our issuance of the Flash Flood Watch through this evening.
HREF/WRF/HRRR guidance continues to keep this area active through
the evening with the possibility of some storms pushing westward
toward the western valleys and even the coastal areas. We will
continue to look at the forecast today for this potential. Storm
chances shrink a bit tomorrow into Sunday as the system pushes off
to the east by early next week.

.Previous Forecast as of 442 AM today...

.SHORT TERM (Today through Sunday)...
Upper level low pressure centered over northern Baja will move
slowly westward today drawing moisture into southwestern
California in easterly flow aloft. Higher resolution models focus
development of showers and thunderstorms this afternoon in the
mountains in Riverside and San Diego Counties, some that could
move west of the mountains toward the coast during the afternoon.
HREF LPMM shows the potential for isolated heavier rainfall to
around 1.5 inches, mainly in the mountains. That easterly flow
will also push smoke out of the area during the late afternoon and
evening.

On Saturday, with the upper low centered off the northern Baja
coast, steering level flow becomes southeasterly and weakens.
Development of afternoon showers and thunderstorms is focused
mainly over the mountains from the San Bernardino Mountains
southward with coverage and rainfall potential less than today.

For Sunday, the center of the upper low will move slowly eastward
into northwest Mexico and southwest Arizona. Moisture will be
decreasing, but a few thunderstorms are possible in the mountains
on Sunday afternoon.

The marine layer has deepened to around 1500 feet with satellite
imagery showing widespread coastal low clouds extending inland
into the westward San Diego County valleys and spreading
northward across the Orange County coastal plain. Night and
morning coastal low clouds and fog are expected to spread farther
into the valleys over the weekend.

&&

.LONG TERM (Monday through Thursday)...
Weak high pressure will move across southern California as the
weekend upper low moves eastward and a large low pressure system
centered off the British Columbia coast moves toward the West
Coast.

Monday may be a little warmer for inland areas followed by slow
cooling for Tuesday and Wednesday as that low pressure system
moves inland to the north strengthening the onshore flow across
southern California.

&&

.AVIATION...
241613Z...Coast and offshore...OVC008-012 with tops near 1500 FT MSL
becoming SCT010-015 by 18Z. 18Z-03Z SCT-BKN100 with LCL VIS 5SM in
ISOLD SHRA/-TSRA and CB bases near 6000 feet MSL. After 03Z...Areas
of BKN010 redeveloping through 06Z.

Valleys/Mountains/Deserts...SCT-BKN100 with SCT SHRA/TSRA forming
over the mountains, some of which may drift west over the valleys
with bases 060-100 and tops to FL400. The stronger thunderstorms
will briefly lower to BKN-SCT around 6000 feet MSL in +RW that will
severely restrict visibility, along with gusty and erratic winds.

&&

.MARINE...
There is a high probability of no significant weather over the
coastal waters Friday through Monday. However, a small chance does
exist for a few showers and thunderstorms, in association with a
developing upper-level low pressure system off the Baja Ca Coast
from late Friday through Sunday morning.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch until 7 PM PDT this evening for Coachella
     Valley-Riverside County Mountains-San Diego County Deserts-
     San Diego County Mountains.

PZ...NONE.
&&

PUBLIC...APR
AVIATION/MARINE...Small