Flood and High Wind in California, December 21 to 22, 2016

Area Forecast Discussion
National Weather Service San Diego CA
307 PM PST Wed Dec 21 2016

...Locally Heavy Rainfall Possible Overnight-Thursday for San
Diego County, especially Southern Areas...

...Significant Storm to Impact SoCal Friday Evening-Saturday...

.SYNOPSIS...

An upper level low will move slowly northeast to near San Diego
by Thursday morning. This system will bring numerous showers and a
slight chance for thunderstorms through Thursday with locally
heavy rainfall possible across San Diego County, especially
southern areas overnight. Friday will be a mainly dry day, then a
major winter storm will drop into SoCal from the NW beginning
Friday night, continuing through Saturday night, bringing
widespread rain to the region, with heavy snow possible in the
mountains. Travel conditions will be hazardous Friday evening
through Saturday. Christmas Day into early next week is expected
to be slightly milder and dry.

&&

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

Winter officially began at 4:44 AM this morning and sure is
starting on an active note here in Southern California! An
impressive upper low spinning off Baja will move to near San Diego
by Thursday morning. Precipitable water will increase further
(record levels 1.15-1.30") as this low approaches tomorrow morning
and upper level divergence will increase. A slightly destabilizing
layer aloft could even produce a couple of elevated thunderstorms
with the risk greatest towards the international border. Lightning
strikes were detected east of the upper low off mainly southern
Baja.

Parameters look to be coming together for more rainfall than
originally thought. Prior runs of the wrf and cansac-wrf showed a
bullseye of 2"+ over San Diego overnight-Thu morning. However,
the latest WRF runs now has heavier rain over the ocean and east
over the mountains (1-2"). So while confidence is low in where
heavier rain will fall, it's increasing that locally heavy
rainfall of 1"+ will fall somewhere. So, POPS and rainfall
amounts were increased significantly, especially from San Diego
to the southern mountains. Overall average rainfall is 0.5-1.0"
but isolated higher totals will be possible where convection
occurs.

Winds in the lower levels will turn easterly tonight-Thursday
while strengthening with gusty offshore winds on the coastal
mountain slopes and into the valleys. This will dry the lower
levels a tad which is a negative for higher rainfall totals.
However, believe moisture will be significant enough above the
lower levels to allow for localized heavier rainfall to occur
underneath convective cells.

A break in the action will occur Thu night-Friday between storm
systems. Then all eyes turn to the northwest as a sharp trough
amplifies southward and crashes into SoCal Friday evening. Global
models shows a healthy band of precip along the cold front. Am
anticipating a band of heavy rain and gusty winds to sweep down
the coast from NW to SE across the area, impacting holiday
travelers with rain-slicked roads and a period of poor
visibility. Wind gusts of 30+ mph are forecast for the
metropolitan areas Fri night, while mountain ridge tops and
coastal desert slopes could see gusts increasing to 60+ mph.

This will be a cold storm system digging down with H85 temps
projected to fall below zero to San Diego Saturday afternoon. Snow
levels will fall fast, down to about 3500 FT Saturday afternoon.
However, we need to closely assess this as we get closer to the
event as this looks like a vigorous cold trough, perhaps capable
of lowering snow levels further. Timing of the precip and cold
air will dictate how much snow falls at higher elevations. While
the bulk of the heaviest precip looks to occur before the coldest
air, enough moisture lingers through the day Saturday to produce
appreciable snowfall. The snow forecast will be fine tuned in
time, but right now areas such as Big Bear are projected to
receive 6"+ with the highest peaks a foot plus.

The storm will wind down/pull out Saturday night with nice but
chilly weather forecast for Christmas Day.

Have kept the forecast dry for early next week, but will have to
monitor the progression of another potential cut-off upper level
low as GFS brings this close by, though the EC keeps it further
away. Gradually moderating temps are expected early next week
after a cold weekend.

&&

.AVIATION...
212130Z...BKN-OVC cloud bases mostly 4000-7000 ft MSL, layered to
15000 ft MSL or higher, will continue through Thursday, with local
cloud bases to 2500 ft MSL mainly tonight. Scattered showers will
occur this afternoon mainly in San Bernardino County, with showers
increasing tonight with a slight chance of thunderstorms mainly late
this evening through early Thursday. Some vis could lower to 3-5
miles in heavier showers, including both the deserts and areas west
of the mountains, and some mountain terrain will be obscured at
times in clouds and fog, especially tonight and Thursday morning.
Showers will very slowly dissipate Thursday afternoon.

&&

.MARINE...
Periods of light to occasionally moderate showers will continue
through Thursday as a low pressure system moves up from the south.
There is a small chance of thunderstorms over the coastal waters
tonight through thursday morning as well. These storms may produce
brief heavy rain, gusty winds, and cloud-to-water lightning. A
stronger Pacific storm system will bring heavier showers, possible
thunderstorms, strong gusty WNW winds, and rough seas late Friday
through late Saturday.

&&

.BEACHES...
There is a small chance of thunderstorms at the beaches late this
evening through early Thursday morning.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

PUBLIC...Gregoria
AVIATION/MARINE...Maxwell

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN DIEGO CA
748 PM PST WED DEC 21 2016

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0205 PM     DEBRIS FLOW      10 S AGUANGA            33.30N 116.88W
12/21/2016                   SAN DIEGO          CA   LAW ENFORCEMENT

            DEBRIS (MUD/DIRT/ROCKS) ON ROADWAY. PLOW IS
            NEEDED TO CLEAR LANES.

&&

PG