Flood Watch 2, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
830 AM PST Sun Jan 8 2023

.SYNOPSIS...
Mostly dry and cool conditions continue through Monday evening.
Another in a series of Pacific storm systems brings widespread
rainfall, heaviest in the San Bernardino Mountains, as well as
strong winds and high elevation snow to the region late Monday
night and Tuesday. Another break in the active weather occurs
Wednesday through early Friday, with potential for another storm
next weekend.

&&

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

High clouds were the main feature in satellite imagery this morning.
They were thickest over central California, where moderate to
locally heavy rain has been falling overnight. There is a
significant break just offshore, so there will be periods of sun
today, and it will be mild. Our 12Z Miramar sounding remains very
dry (PW of 0.28") with little change in the temperature profile, but
winds have backed a bit more southwesterly and increased in speed
above 5K FT. The sfc pressure gradients are turning onshore now,
albeit weak. Strongest wind reports were west or southwest at around
15-20 MPH, mostly near the mountains, with some high elevation gusts
of 30 mph or more.

Expect a quiet day here today, with some passing high clouds. For
Monday, clouds will be lowering and thickening from north to south.
There is a chance for some light rain to break out over northern
areas as well in the afternoon, while San Diego County remains
mostly dry. No forecast updates are planned this morning.

From previous discussion...

A potent upper trough aided by a strong Pacific jet will amplify
and move inland over California on Monday Night and Tuesday
morning, becoming negatively tilted in the process. The
amplification and strengthening of this system will lead to strong
southerly winds across much of the region, particularly across
the San Bernardino Mountains and into the High Deserts. A High
Wind Watch has been issued for these areas, and trends will need
to be monitored for more widespread Wind Advisory conditions on
Tuesday.

The enhanced southerly flow will also lead to more warm air and
moisture advection across southern California. EFI guidance
continues to indicate that 60-80% of ensemble members favor a
heavier and more widespread precipitation event than the re-
analyzed model climate, with these members also favoring a
relatively significant event per SOT guidance. These trends are no
longer confined to just the mountains, but now much of Southern
California from the mountains westward. Heavy rain will be a major
concern for the upslope-favored coastal slopes in the San
Bernardino Mountains. More moderate to very briefly heavy rainfall
will occur for other mountain ranges as well, but lower-level
winds will not be nearly as favorable for upslope flow.

With a dynamic system providing plenty of atmospheric lift,
widespread PW values near or exceeding 1 inch, much cooler air
aloft arriving Tuesday afternoon, as well as at least some progged
instability per global ensembles, there will be chances for
isolated thunderstorms across much of Southern California as the
brunt of the storm moves through the region. The convective nature
of this precipitation will lead to some localized higher totals,
but also potentially some drastically lower totals in some areas.
The primary area of uncertainty remains San Diego County where
some hi-res guidance, while still a bit far in advance, shows a
very sharp gradient in rainfall totals compared to areas farther
north. See the Hydrology section below for more details.

The enhanced warm air advection leads to snow levels starting off
fairly high again for this event, generally around 8,000 feet. By
Tuesday afternoon, snow levels will drop to around 6,000 feet and
then down to near 5,500 feet Tuesday night. By the time
precipitation ends late Tuesday night, a few inches of snow will
be possible generally at resort areas with light accumulations in
most of the mountain communities.

A break in the action occurs Wednesday through Friday morning,
during which dry and cool conditions are anticipated. The active
and stormy weather pattern looks to persist over the Eastern
Pacific through at least next weekend, per model cluster analysis
which shows a persistent theme of a stronger trough over the
Pacific. There remains the potential for another, stronger, and
colder storm system to occur next weekend, though details in the
timing and amplitude of the upper trough, and any subsequent
impacts, among ensemble members remain very dispersive.

&&

.AVIATION...
081620Z... Coast/Valleys... SCT-BKN clouds AOA 20000 ft MSL with
unrestricted VIS today. -SHRA possible in portions of northern
Orange County and Inland Empire through the day, but most likely
after 08Z Mon. Cigs could lower to 2000 ft with vis restrictions of
3-5 SM in passing showers. South to southeast winds are expected to
prevail at coastal TAF sites through much of the day Mon ahead of a
passing front.

Mountains/Deserts... High clouds AOA 20000 ft MSL today. South winds
will increase after 12Z Mon over the mountains and into the deserts.
Gusts of 30 to 40 kt expected, over the San Bernardino mtns into the
High Desert. Areas of LLLWS and mod turbulence expected over the
east of the mtns. Winds expected to strengthen Mon PM into Tue.

&&

.MARINE...
No hazardous marine conditions are expected through Monday
afternoon. An approaching storm will likely bring hazardous boating
conditions in the form of gusty winds and hazardous seas late Monday
into Tuesday. Peak gusts of 25-30 kt expected with waves 9-11 ft
with a 12-14 second period. The storm could also produce isolated
thunderstorms over the coastal waters on Tuesday.

&&

.BEACHES...
A west swell will continue to generate surf of 4-6 feet at the San
Diego County beaches through this afternoon. Another uptick in surf
is expected Wednesday due to an incoming west-northwest swell from
280 degrees at 14 seconds. Highest surf will occur at west-facing
beaches particularly in San Diego County.

On Tuesday, an incoming storm could produce thunder and lightning at
area beaches.

&&

.HYDROLOGY...
Widespread precipitation appears on track to affect much of
Southern California Monday Night through much of Tuesday.
Strengthening southerly flow will lead to increasing moisture
advection ahead of the system as well as upslope flow in the San
Bernardino Mountains. As the brunt of the storm moves through on
Tuesday, ample moisture will be in place with widespread PW values
at or exceeding 1 inch. As cooler air aloft enters the region
Tuesday afternoon, some instability will develop and lend itself
to creating some areas of convective rainfall, with thunderstorms
possible leading to rainfall rates between 0.75-1.00”/hr.
Preliminary rainfall totals are listed below:

Coasts: 0.75-2.00 inches, lowest amounts south and highest north
Valleys: 1.00-2.50 inches, localized amounts of 3.00 inches
San Bernardino Mountains: 3-8 inches
Other Mountains: 1-3 inches
High Deserts: 0.50 to 1.50 inches, highest near the foothills
Low Deserts: 0.20 to 0.50 inch

There is potential for additional changes and further refinements
to these amounts given the convective nature of Tuesday
afternoon’s rainfall. Recent runs of hi-res guidance have
suggested totals anywhere from around 0.20 inch to upwards of an
inch are possible in southern San Diego County, lending
uncertainty to the event total precipitation. Will opt to remain
closer to WPC/RFC/ensemble QPF numbers which have remained steady.

The CNRFC maintains the Santa Margarita and San Diego Rivers
reaching Monitor stage. Soils remain fairly saturated, leading to
enhanced runoff potential. Urban flooding and ponding of water
will be most likely on Tuesday, and some small stream flooding is
possible as well, especially in northern parts of the forecast
area.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement until 2 PM PST this afternoon for San
     Diego County Coastal Areas.

     High Wind Watch from Monday morning through late Tuesday night
     for Apple and Lucerne Valleys-San Bernardino County
     Mountains.

PZ...None.

&&

PUBLIC/HYDROLOGY...10/Adams
AVIATION/MARINE/BEACHES...CO