Flash Flood in California, September 30, 2018

Area Forecast Discussion
National Weather Service San Diego CA
318 PM PDT Sun Sep 30 2018

.SYNOPSIS...
Weakening tropical cyclone Rosa is expected to move across
northern Baja Monday, and Arizona Monday night and Tuesday. For
Monday through early Tuesday, Rosa will bring showers and a slight
chance of thunderstorms, mainly for the mountains and deserts with
lesser chances to the west. On Wednesday, a Pacific storm will
move inland across southern California, bringing a chance of
showers and slight chance of thunderstorms, mostly along and west
of the mountains. Dry and a little warmer for Thursday into next
weekend.

&&

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

.SHORT TERM (Tonight through Tuesday)...

Mid-high level moisture is streaming into Southern California from
Hurricane Rosa. As lower level moisture continues to increase
tonight, a few showers could affect the mountains and lower
deserts, especially after midnight but with minor rainfall
amounts of less than a tenth of an inch. The bulk of the moisture
associated with Rosa will move into Arizona Monday-Monday night.
SoCal will be on the western periphery with scattered to numerous
showers, and possibly a thunderstorm, focused over the eastern
San Bernardinos to the San Jacinto-Idyllwild area extending south
to the San Diego County mountains as well as the deserts
(especially lower). Rainfall amounts across these mountain areas
are forecast to reach 0.50-1.0 inches. Near a quarter of an inch
on average for the deserts. Less than a tenth of an inch west of
the mountains. Confidence is too low to issue a flash flood watch
for the Cranston Fire burn scar. There will be extensive cloud
cover and this will inhibit full instability from being realized
and should limit rainfall intensity below thresholds for all burn
scar locales through Tuesday.

&&

.LONG TERM (Wednesday through Saturday)...
A slow moving closed low pressure system off the California coast
early Wednesday is expected to move inland across southern
California Wednesday and Wednesday night. The latest 12Z model
runs still show some timing differences with the GFS on the faster
end of the guidance and the ECMWF on the slower end. However,
both models bring the brunt of the rainfall with this system from
the coast, valleys to the coastal mountain slopes during the
Wednesday afternoon-evening period. GFS has trended weaker with
the bulk of the rain occurring west of our area. However, the
ECMWF shows more and is slower, keeping rain around through the
night on Wednesday. Given the cold air aloft with the system,
isolated thunderstorms are also possible with the front.

Rainfall amounts are forecast to range from 0.50-1.0 inch along
the coastal slopes of the San Bernardinos, around a half an inch
from Orange County to the Inland Empire to a quarter of an inch
or less further south to San Diego. Less than a tenth of an inch
for the lower deserts, but slightly more for the high deserts.
We will trend these numbers up or down in time, depending on the
evolution of this system.

The Pacific storm system will weaken and move inland by Thursday
with weaker low pressure troughing for Thursday and Friday across
the southwest states with drying and slow warming. For next
weekend, strong high pressure will develop over the eastern
Pacific along 140-145W with a highly amplified low pressure system
developing downstream across the western states. There remain
some differences among the global models with the placement of the
trough axis. For those that are a little farther east, this
trough should be dry for Southern California. For those a little
farther west, there could be enough of an over water trajectory to
bring some showers along and west of the mountains for late
Saturday and Sunday. The current forecast is kept dry for now.

&&

.AVIATION...
302020Z...Patchy marine stratus over the coastal waters and locally
inland late tonight into Monday morning with bases near 1000 FT MSL
and tops to around 1500 feet MSL, otherwise SCT-BKN cloud layers
above FL120 increasing from the south. The risk of OVC low clouds
tonight at KSAN is low, so the confidence in the TAF forecast of
little to no low clouds is moderate-high at KSAN.

San Bernardino County Mountains/Deserts...Areas of south
to southwest winds 25-35KT over the ridges and on the desert slopes
with moderate up/downdrafts.

&&

.MARINE...
Isolated thunderstorms are possible later on Wednesday. Otherwise,
no hazardous marine weather is expected through Wednesday.

&&

.BEACHES...
South swells from Tropical Cyclone Rosa will increase overnight and
peak on Monday between 8 and 10 FT before subsiding on Tuesday.
Elevated surf will affect most beaches with the largest surf to be
on south facing beaches of Orange and far northern San Diego
Counties. Surf of 5-8 FT, with biggest sets 9-10 FT will likely
impact these areas late Sunday through Monday. Expect strong rip and
long shore currents to impact all beaches.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 6 AM PDT Tuesday for Orange County
     Coastal Areas.

     Beach Hazards Statement through late Monday night for San Diego
     County Coastal Areas.

PZ...NONE.
&&

PUBLIC...Gregoria
AVIATION/MARINE...Small