Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 201736
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 AM 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 today,
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 mostly moderate rain was moving through coastal, valley
and mountain areas at mid-morning with snow levels averaging around
or just below 5000 feet, with a little higher values in San Diego
County versus farther north. Hourly rainfall rates were mostly below
1/10 inch, but a few locations have had rates to 1/4 inch per hour,
mainly on orographic mountain slopes.

A 190-knot jet was over the NE Pacific, with San Diego in the left
front quadrant, favorable for convection. Some lightning was
occurring this morning upstream with the nearest strikes just off
Point Conception. Strong low-level wind shear and moderate CAPE
values will help aid thunderstorm development today, with a small
chance of waterspouts or even a brief tornado over land this
afternoon. HRRR and local WRF have the convection starting around
noon, with peak timing at mid-afternoon. Damaging winds is a
possible threat due to 50+ knot winds at 850 MB over parts of the
coast due to synoptic-scale dynamics, especially along a likely band
of convection early this afternoon. In addition, mountain wave
activity will bring winds in the mountains and deserts, with gusts
over 70 MPH in a few areas.  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 today
through tonight. Heavy snow, with accumulations locally over one
foot from today through tonight, will occur above about 5000 feet,
along with dense fog in most mountain areas. A lot of moisture
lingers tonight, so the showers likely will be numerous most of
tonight before gradually tapering off Saturday.

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 7500 feet or higher Sunday, so this will cut back
on snowfall amounts, and high humidity in the mountains will cause
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...
201643Z...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 light to
moderate showers will continue through 1900 UTC, mainly over and
west of the mountains and high deserts, then heavier showers and
scattered thunderstorms likely 1900-21/0300 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 goes into effect at 10 AM and 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 10.5 feet by Monday, a little less than one foot below the
flood stage of 11.3 feet and well above the monitor stage of 7.5
feet.

&&

.SKYWARN...
SKYWARN will be activated from late morning 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 from 10 AM this morning to 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...Small Craft Advisory until 10 AM PST this morning 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.

     Gale Warning from 10 AM this morning to 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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