Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 170526
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
926 PM PST Mon Jan 16 2017

.SYNOPSIS...
High pressure aloft will continue dry weather through Wednesday
with weak offshore flow Tuesday, becoming onshore Wednesday. For
Thursday into early early next week...a series of Pacific storms are
expected to bring periods of strong onshore winds...rain...and
mountain snow which could be heavy at times.

&&

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

Patchy low clouds are developing along the San Diego County coast
this evening and will continue through Tuesday morning. Otherwise
clear skies will prevail. Surface pressure gradients remain weakly
offshore to the high desert and nearly neutral to the low desert.
Wind gusts across the forecast area are generally less than 25
MPH. Locally breezy northeast winds will continue below the passes
through mid-morning Tuesday.

A couple more days of benign weather is in store for Tuesday and
Wednesday with highs a few degrees below normal. Some low clouds
and fog is possible along the coast into the far western valleys
Tuesday night into Wednesday morning. Increasing cloud cover
through the day Wednesday ahead of the first in series of Pacific
storms.

The first storm arrives late Wednesday night with showers
continuing into Thursday night. There is a slight chance of
thunderstorms during the day Thursday as cold air aloft helps
destabilize the atmosphere. Precipitation totals with this first
system will range from about one to two thirds inch west of the
mountains, three-quarters to locally 1.5" in the mountains, up to
one-quarter inch in the high deserts, and 0.15" or less in the low
deserts. Snow levels will fall to around 5000-5500 ft Thursday
with snowfall totals around 4-8". Gusty Southwest to West winds
will accompany this system, with the strongest winds along the
desert slopes of the San Diego County mountains.

A second stronger system arrives Friday. Deep layer moisture will
help bring appreciable rainfall across the forecast area,
including the deserts. Storm totals for this system will be around
1.25-1.5" west of the mountains, 2-3" in the mountains, and
0.25-0.50" in the deserts. The heaviest precipitation will occur
Friday morning through the afternoon, with lighter showers
lingering into Saturday afternoon as a weaker wave moves through.
Snow levels will remain near 5000-5000 ft during the heaviest
precipitation, falling to 4000-4500 ft by Saturday morning.
A foot or more of snow is likely above 5500 ft. There will be a
brief break late Saturday into Sunday morning.

Models are coming into better agreement with the strongest storm,
which is set to occur on Monday. A deep plume of subtropical
moisture with precipitable water in excess of one inch will head
for Southern CA. Based on current timing the heaviest precip will
occur Sunday evening through Monday morning. The deterministic 00Z GFS
is significantly faster with the departure of the trough, though
the 12Z ensemble mean is slower and more in line with the EC. The
latter solution will keep post-frontal showers around longer and
lead to even more snowfall, especially at the lower elevations.

With several days of moderate to heavy rainfall, the risk of urban
flooding, flash flood and debris flows in the burn areas, and
river flooding is substantial. Rain soaked grounds and breezy
conditions at times could lead to many shallow rooted trees
toppling. Significant snow in the mountains will make travel
extremely hazardous, if not impossible, at times. The good news is
that beyond Tuesday we should have at least a couple days of nice
weather again.

&&

.AVIATION...
170430Z...Coast/Valleys...Patchy BKN stratus will occur along the
coast/western valleys through 15Z Tue with bases 1200-1700 ft MSL and
tops to 2000 ft MSL. This will probably reach KSAN and briefly KCRQ
though confidence of timing is low. Local vis will be 2-4 miles in
fog through 15Z Tue. Otherwise, mostly clear skies and mostly
unrestricted vis will occur through Tue, with areas of stratus
developing late Tue evening.

Mountains/Deserts...Mostly clear with unrestricted vis through Tue
evening.

&&

.MARINE...
A Pacific storm will bring rain, strong southwest to west winds
gusting to 25 kt and large seas of 7-10 feet on Thursday, mainly in
the outer waters. These conditions would be hazardous to small
craft. Another storm system from Friday into Saturday has the
potential for winds reaching gale force in the inner and outer
waters and combined seas of 11 to 18 feet, with highest seas south
of San Clemente island. Winds and seas will diminish a little
Saturday night into Sunday but then pick up again late Sunday
as another storm system moves through the region.

&&

.BEACHES...
The moderate sized long-period west-northwest swell from around 285
degrees will gradually diminish through Tuesday and should not
create any high surf.

From Thursday through next weekend, a couple short to moderate
period large west-northwest swells (from 285 degrees) will impact
the coast. The highest surf will occur over the weekend when a very
large short-period WNW swell combines with a moderate sized WNW long-
period swell. During the peak of the swell event, surf could be
large enough (sets possibly reaching 15 ft) to cause coastal damage,
minor coastal flooding and moderate beach erosion.

&&

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

$$

PUBLIC...SS
AVIATION/MARINE/BEACHES...Maxwell



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