Area Forecast Discussion
Issued by NWS San Diego, CA

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982
FXUS66 KSGX 202153
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
200 PM PST Fri Jan 20 2017

.SYNOPSIS...
A strong Pacific storm will bring periods of rain, heavy at times,
strong winds, mountain snow and a chance of thunderstorms this
afternoon into this evening, tapering off gradually late tonight and
Saturday. Flooding will be possible in some areas. A brief dry
period will occur late Saturday through early Sunday before the next
storm bring more heavy rain, strong winds, mountain snow with
another chance of thunderstorms and flooding. Showers will linger
Tuesday, with fair weather likely starting Wednesday. Temperatures
will average below normal through the middle of next week, then
increase some the second half of next week.

&&

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

Widespread rain with snow in the mountains has been occurring, with
a snow level around 5000 feet. There was also convection, especially
from the coastal waters into Orange County, though only one
lightning strike has been recorded so far, near Cajon Pass a little
after 1 PM. Strong winds were occurring as well, including a 99 MPH
gust at Burns Canyon, and numerous gusts over 40 MPH near the coast
along with some damage to trees.

A 190-knot jet was over the NE Pacific, with San Diego in the left
front quadrant, favorable for convection. Most of the lightning was
still west of Point Conception where the colder air aloft was.
Strong low-level wind shear and moderate CAPE values will help aid
thunderstorm development this afternoon into this evening, with a
small chance of waterspouts or even a brief tornado over land this
afternoon. Damaging winds is a possible threat due to 50+ knot winds
at 850 MB over parts of the coast from synoptic-scale dynamics plus
added mountain wave influence to the winds in the deserts and desert
slopes of the mountains. Heavy rains in thunderstorms could also
produce flash flooding, and orographic precip will be significant
with the strong W-WSW flow combined with saturation to near 600 MB.
Local rainfall amounts of an inch per hour will be possible, with 2-
4 inch total accumulations on some of the coastal slopes for this
afternoon through tonight. Heavy snow, with accumulations locally
over one foot from this afternoon through tonight, will occur above
about 5000 feet, along with dense fog in most mountain areas. A lot
of moisture lingers overnight, so the showers likely will be
numerous most of tonight before tapering off Saturday morning.

The break in the precipitation should occur late Saturday through
very early Sunday morning, though a lot of clouds will likely linger
due to moisture both below 850 MB, helped by the wet ground, and
some moisture above 600 MB. The next storm will move from northwest
to southeast Sunday through Monday and bring substantial rainfall
again along with strong winds and a possibility of thunderstorms.
Strong warm air advection in the 850-700 MB layer will bring the
snow levels up to 7000 feet or higher Sunday, so this will cut back
on snowfall amounts, and high humidity in the mountains will cause
some snowmelt to occur, in addition to the rain. There will
definitely be a flash flood threat Sunday through Monday, especially
as soils will be saturated. Cold air comes in behind a front Sunday
night, so snow levels will fall to 3500-4000 feet Monday and
continue Tuesday under a broad cold low pressure system. This could
result in substantial snow accumulations as saturation reaches 500-
600 MB at times. Fair weather will not return until at least
Wednesday, but models are consistent at showing a high pressure
ridge late next week for fair warmer weather.

&&

.AVIATION...
202053Z...Multiple BKN-OVC cloud decks within the 1000-15000 ft msl
layer over and west of the mountains, and in the 5000-15000 ft msl
layer over the deserts, will continue for the most part through
21/1500 UTC, in addition to areas of vis 2-5 sm, local vis AOB 1 sm
under heavier showers, and mountain obscuration. Widespread moderate
to heavy showers and scattered thunderstorms likely through 21/0300
UTC. Thunderstorms may produce strong downdrafts of 45-55 kt, hail,
and funnel clouds. CB tops to 20000 ft msl. 21/0300-1500
UTC...thunderstorm threat lessens and showers gradually become SCT
mainly over the mountains.

Sustained winds 20-30 kt with gusts to 40 kt between 18-23Z
impacting all coastal and valley TAF sites. SW to W wind gusts of 50-
70 kt over the mountains and down the desert mountain slopes and
into adjacent deserts today will result in severe uddfs/llws/rotors
over and east of the mountains.

&&

.MARINE...
Another stronger Pacific storm will move through the region today
and then again Sunday and Monday. Both storms will bring the threat
for thunderstorms and waterspouts, in addition to potential gale-
force winds and very large seas (greater than 10 feet). For the
storm today, strong to perhaps severe thunderstorms will be
possible, with thunderstorms potentially producing gusts to 50 kt,
frequent downpours and lightning, large hail, and waterspouts, and
so a Marine Weather Statement has been posted. Meanwhile, a Gale
Warning continues through Saturday afternoon.

&&

.BEACHES...
A High Surf Warning is in effect through Tuesday. A very large
moderate period west swell (from 280 degrees) will combine with a
moderate sized long-period west-northwest swell (from 285 degrees)
to bring significant wave heights of around 13-17 feet to the
coastal waters tonight through Saturday, likely resulting in very
large surf rising to 15 feet by early Saturday. This surf could
cause damage, beach erosion, powerful rip currents and coastal
flooding. Early morning high tides will only be at around 5 ft, so
coastal flooding magnitude will likely be minor.

The swells will diminish a little on Sunday but high surf conditions
are still likely to continue into Tuesday. It is not until Wednesday
that surf and swell conditions will lower below high surf
thresholds.

Also, there is an enhanced lightning threat at the beaches today
from the late morning through the early evening, so a Beach Hazards
Statement has been issued. &&

.HYDROLOGY...
A wet and stormy pattern will continue today and get a boost
this afternoon with heavy rain rates and a chance of
thunderstorms. Rain rates are almost certain to create urban
flooding problems and may produce flash flooding. Check the flash
flood watch for details. Showers will gradually taper off tonight
and Saturday. Heavy rainfall will return late Sunday and continue at
times through Monday with occasional showers lingering through at
least Tuesday. Snow levels will mostly be around 5000 feet today.
Snow levels will rise up to 7500 feet Sunday before dropping to as
low as 3500 feet by Monday afternoon. Grand totals for this wet
period from today through Tuesday:

Coast and Valleys: 2-5 inches
Mountains: 4-10 inches, with isolated 12+ inches
Deserts: 0.50-2 inches

CNRFC models indicate the San Diego River at Fashion Valley will
reach 10.3 feet by Monday, one foot below the flood stage of 11.3
feet and well above the monitor stage of 7.5 feet.

&&

.SKYWARN...
Skywarn will continue to be activated through this evening.
Skywarn will likely be activated Sunday through Monday. The main
threats are flooding, damaging winds, heavy snow, and high surf.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning until 6 AM PST Saturday for Riverside
     County Mountains-San Bernardino County Mountains-San Diego
     County Mountains.

     Flash Flood Watch through this evening 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 Diego County
     Coastal Areas-San Diego County Mountains-San Diego County
     Valleys-San Gorgonio Pass Near Banning-Santa Ana Mountains
     and Foothills.

     High Wind Warning until 10 PM PST Saturday for San Diego County
     Deserts.

     Wind Advisory until 10 PM PST Saturday for Apple and Lucerne
     Valleys-San Gorgonio Pass Near Banning.

     High Wind Warning until 10 PM PST this evening for Orange County
     Coastal Areas-Orange County Inland Areas-San Bernardino and
     Riverside County Valleys-The Inland Empire-San Diego County
     Coastal Areas-San Diego County Valleys-Santa Ana Mountains
     and Foothills.

     Wind Advisory until 10 PM PST Saturday for Coachella Valley.

     High Surf Warning until 10 PM PST Tuesday for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Gale Warning until 4 PM PST Saturday 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/HYDROLOGY...Maxwell
AVIATION/MARINE/BEACHES...Harrison



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