Flash Flooding and Flooding in California, November 19 to 21, 2019

Area Forecast Discussion
National Weather Service San Diego CA
418 AM PST Tue Nov 19 2019

.SYNOPSIS...
A low pressure system tracking into and across Baja today into
tonight will bring cooling, increasing clouds, and a chance of
showers for this afternoon and evening. For tonight and Wednesday,
a cold low pressure system from the north will bring increasing
chances for showers along with a slight chance for thunderstorms
on Wednesday. Periods of showers will continue into Thursday and
could linger into early Friday. A weak low pressure system from
the northwest could bring a few showers for late Friday into early
Saturday. Drier and a little warmer for Sunday.

&&

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

.SHORT TERM (Today through Thursday)...
A low pressure system centered over the eastern Pacific to the
west of central Baja is moving toward the northeast and will move
across northern Baja this afternoon and evening, accelerating and
weakening as it moves into southern Arizona and New Mexico late
tonight and Wednesday. The most significant moisture and greatest
instability with this system will be directed into the eastern
deserts of southern California into Arizona with greater potential
for thunderstorms with locally heavy rainfall in these areas.

That low pressure system will also bring increasing clouds today
and could spread some showers northward into portions of
southwestern California for this afternoon and evening. The
return of onshore flow will spread cooling inland today with high
temperatures in the deserts around 5 degrees cooler than Monday
with some coastal areas 10 to 15 degrees cooler.

A cold low pressure system moving quickly southward along the West
Coast will reach northern California this afternoon and southern
California Wednesday. It will bring increasing showers for tonight
into Wednesday with periods of showers continuing into Thursday.
Some showers could linger into early Friday.

Through Thursday, rainfall for Orange, southwestern San
Bernardino, and western Riverside Counties is expected to range
from one to two thirds of an inch near the Orange County coast and
in the western Inland Empire to 1 to 1.5 inches in the mountains
with locally greater amounts, with mostly one quarter inch or
less in the deserts.

In San Diego County, rainfall is expected to range from around 1
inch near the coast, to 1.5 to 2 inches on the coastal slopes of
the mountains with locally greater amounts.

The most significant rainfall is expected on Wednesday along with
a slight chance for thunderstorms. Rainfall rates could exceed
one- half inch per hour in stronger showers and isolated
thunderstorms resulting in local flash flooding in poorly drained
urban areas and near recent burn scars.

The snow level will lower to around 6000 feet on Wednesday and
5500 to 6000 feet for Wednesday night into Thursday. A few inches
of snowfall is possible in the mountains, mostly above 6500 feet
with around one foot possible on the highest peaks.

Wednesday will be significantly cooler with high temperatures 15
to 25 degrees cooler than today, with high temperatures in the
50s and lower 60s in coast, valley, and high desert areas, 30s
and 40s in the mountains, and 60s in the lower deserts. Thursday
will be a few degrees warmer, but still 10 to 15 degrees below
average.

&&

.LONG TERM (Friday through Monday)...
A few showers could linger into early Friday. Less certain are the
details across the eastern Pacific into the West Coast for Friday
through Monday. The global models do take a low pressure system
move quickly across the Pacific and develop it into a closed low
pressure system somewhere off the California coast for late
Thursday and Thursday night. Given recent performance, WPC does
not favor the more western placement of this upper low by the
ECMWF. A track farther to the east, similar to the 00Z and 06Z
deterministic GFS runs would be close enough to southern
California to bring a return of showers for portions of southern
California for late Friday and early Saturday before it moves
farther off the coast.

Another low pressure system moving quickly into the southwest
states in fast northwest flow aloft around Monday could bring
cooling and strong gusty winds. If the track is far enough to the
west, it could entrain some moisture and bring some
precipitation. The current forecast weakly follows the GFS trends
until the situation better resolves itself.

&&

.AVIATION...
190435Z...SCT-BKN high clouds at/above 10000 ft MSL through Tuesday
evening. Lower clouds with bases 2000-3000 ft MSL and tops to 4000
ft MSL developing along and west of the mountains Tuesday evening
with scattered -SHRA, mainly after 20/03Z.

&&

.MARINE...
No hazardous marine weather is expected through Tuesday. Stronger
south to west winds with gusts near 20 kt are expected Tuesday night
through Wednesday, with showers at times Tuesday night through
Thursday. Thunderstorms are also possible Wednesday, producing heavy
showers and cloud-to-water lightning.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from this evening through late Wednesday night
     for Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Coastal Areas-San Diego County
     Mountains-San Diego County Valleys-Santa Ana Mountains and
     Foothills.

     Winter Storm Watch from Wednesday morning through Thursday
     evening for Riverside County Mountains-San Bernardino
     County Mountains.

PZ...NONE.

&&

PUBLIC...17
AVIATION/MARINE...MM