Flash Flooding and Debris Flows in California, March 14 to 15, 2023

Area Forecast Discussion
National Weather Service San Diego CA
145 PM PDT Tue Mar 14 2023

.SYNOPSIS...
Rain will continue spread from north to south this afternoon,
becoming heavy at times late this evening through early Wednesday
morning. Local flooding will be possible from the coastal slopes
of the San Bernardino Mountains, south and westward across
portions of the Inland Empire and Orange County. The rain will
decrease Wednesday evening, with a dry day in store for Thursday.
Chances for rain return over the weekend, with mostly light rain
expected. It doesn't look to dry out anytime soon, with more storm
systems impacting the West next week.

&&

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

At 1 PM, light rain was continuing to move into SoCal, with the
heaviest rain still remaining to our north and west. Gusty
southwest to west winds have started up in the mountains and
deserts, with the strongest winds occurring this afternoon and
evening. Peak gusts to 55 mph are expected in those locations.

A low pressure system, currently situated off the coast of
northern California, and atmospheric river moisture with the low
pressure system will be continuing to make its way down the
California coast today. Rainfall will become more widespread and
heavy this evening and overnight. The highest rainfall rates are
expected between midnight and 6 AM Wednesday morning.

Forecast Rainfall Totals Today through Wednesday:

Orange County Coast/inland: 2.5 to 3.5 inches
Inland Empire: 1.5 to 3 inches, highest in northern IE
San Bernardino County mountains: 3 to 5 inches
High Desert: 0.25 to 0.60 inch
Riverside County mountains: 2 to 4.5 inches
San Diego Coast/valley: 1.25 to 2.50 inches, highest in northern
SD County.
San Diego Mountains: 2 to 4.5 inches
Coachella Valley/San Diego deserts: 0.15 to 0.30 inch

High resolution model guidance puts the highest rainfall rates
across Orange County and the coastal slopes of the San Bernardino
County mountains from midnight to 6 AM Wednesday. With periods of
high hourly rainfall rates, flooding will be possible, especially
late tonight into Wednesday morning. A Flood Watch remains in
effect for all of Orange County, the Inland Empire, and the
Riverside and San Bernardino County mountains from this evening into
Wednesday afternoon.

In addition to the rain, winds will also be gusty today and
tonight, especially along the desert mountain slopes and into the
High Desert, with peak gusts to 55 mph. A Wind Advisory remains in
effect through 9 PM Wednesday.

Rain will gradually taper off from northwest to southeast
Wednesday afternoon and evening, though some light showers will be
possible into Wednesday night in the mountains.

The storm will push east later on Wednesday, with Thursday looking
to be a dry day with overall zonal flow prevailing. Beyond
Thursday, it doesn't look to be a totally dry weekend. Weak storm
systems moving through the mean flow could result in some
additional precipitation between Friday and Sunday. Any
precipitation that does fall looks to be light, with rainfall
totals less than 0.25 inch. Timing is still uncertain on when
exactly we'd see the rainfall, but ensembles and the NBM indicate
the possibility of light rainfall at times Saturday and Sunday.

It doesn't look to dry out anytime soon, with models and ensembles
showing precipitation chances entering the forecast again mid to
late next week.

&&

.AVIATION...
142015Z...Coast/Valleys...CIGS 800-1500 ft MSL spreading across the
region from northwest to southeast. Vis reduced to 2-4 SM in RA BR
through tonight. Local CIGS down to 500 ft MSL with vis below 2SM in
heavier rain this evening and tonight. Expect CIG and VIS impacts at
all TAF sites through tonight.

Mountains/Deserts... Incoming BKN/OVC clouds and rain will bring
mountain obscurations through tonight with areas of VIS 1-3 miles.
Mostly unrestricted VIS in the desert areas. West winds with gusts
25-40 kt will continue at times through tonight, mainly from the
mountain crests east through the desert foothills and east of
passes, along with MOD up/downdrafts and local LLWS.

&&

.MARINE...
No hazardous marine conditions are expected today. Wind gusts to 25
knots will produce hazardous boating conditions late tonight through
Wednesday night. Check the Small Craft Advisory for details. Heavy
rain will reduce visibility at times tonight through Wednesday
morning. No additional marine hazards are expected Thursday through
Saturday.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch from 5 PM PDT this afternoon through Wednesday
     afternoon for Orange County Coastal Areas-Orange County
     Inland Areas-Riverside County Mountains-San Bernardino
     County Mountains-San Bernardino and Riverside County
     Valleys-The Inland Empire-San Gorgonio Pass Near Banning-
     Santa Ana Mountains and Foothills.

     Wind Advisory until 9 PM PDT Wednesday for Apple and Lucerne
     Valleys-Riverside County Mountains-San Bernardino County
     Mountains-San Gorgonio Pass Near Banning.

PZ...Small Craft Advisory from 2 AM Wednesday to 2 AM PDT Thursday
     for Coastal Waters from San Mateo Point to the Mexican
     Border and out to 30 nm-Waters from San Mateo Point to the
     Mexican Border Extending 30 to 60 nm out including San
     Clemente Island.

&&

PUBLIC...Connolly
AVIATION/MARINE...PG