Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 181224
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
424 AM PST Mon Dec 18 2017

.SYNOPSIS...
Offshore flow will continue to weaken today with locally gusty
northeast winds near the coastal foothills this morning. Dry
weather with weaker winds and near seasonal temperatures will
continue through Tuesday. A low pressure system moving into the
Great Basin Wednesday will bring stronger onshore flow with
cooling...more clouds...and strong gusty west winds in the
mountains and deserts. A weakening cold front will move through
far southern California late Wednesday afternoon into Wednesday
evening with small chances for very light precipitation along and
west of the mountains. A period of moderate offshore will follow
for Thursday with gusty northeast winds. Warming with weaker winds
for Friday.

&&

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

.SHORT TERM (Today through Wednesday night)...
High pressure aloft over California today will weaken on Tuesday
with a low pressure system off the Pacific Northwest coast on
Tuesday moving into the Great Basin on Wednesday. Offshore lower
level flow will continue to weaken today...become weakly onshore
on Tuesday...then much stronger onshore for Wednesday into
Wednesday evening.

Locally gusty northeast winds near the coastal foothills and
below passes and canyons will continue to weaken this morning.
Some patchy stratus could develop over the coastal waters tonight
and extend locally inland near the coast late tonight into Tuesday
morning. A greater stratus return is possible for Tuesday night
into Wednesday along with deepening of the marine layer. Stratus
could return Tuesday evening along the coast and extend into
portions of the inland valleys for Wednesday morning.

Warming that occurred toward the coast on Sunday from the
offshore flow will extend inland today and Tuesday. Warmer high
temperatures toward the coast will continue today...with slow
cooling spreading inland on Tuesday...with greater cooling
spreading inland on Wednesday with the stronger onshore flow.

The stronger onshore flow will bring strong gusty west to
northwest winds to the mountains and deserts for Wednesday into
Wednesday evening...strongest Wednesday afternoon and early
evening. The strongest winds are expected from mountain ridge tops
along desert mountain slopes into adjacent desert areas and in
the northern Coachella valley below the San Gorgonio Pass with
the stronger isolated gusts to around 70 mph.

This low pressure system will also push a weakening cold front
through southwestern California late Wednesday afternoon into
Wednesday evening. Some light precipitation is possible with the
front...mainly along and west of the mountains. Any precipitation
is not expected to be significant...a tenth of an inch or less on
the coastal slopes of the mountains to a few hundredths of an inch
or less towards the coast.

&&

.LONG TERM (Thursday through Sunday)...
As the low pressure system over the Great Basin moves east...a
brief period of moderate offshore flow is expected on Thursday. It
may not be too different from the offshore flow and gusty
northeast winds that occurred this past Saturday night into
Sunday...except the duration of the stronger winds may be a little
less.

For Friday into next weekend...greater differences among the
global models arise. The global models have a highly amplified
pattern with high pressure aloft near or just off the West Coast.
However...there are greater differences with the downstream cold
trough of low pressure. The GFS is farther east which would
result in warmer weather for Southern California with weaker winds
and near seasonal temperatures. The ECMWF and Canadian models are
farther west with this cold trough of low pressure which would
result in a cooler and windier pattern for Southern California.

The current forecast leans towards those cooler...breezier
solutions...but not as cool as the ECMWF. The period from next
weekend into the week of Christmas is one of increasing
uncertainty from the western states into the eastern Pacific...and
an exceedingly low confidence forecast for Southern California
with a large range of forecast possibilities.

&&

.AVIATION...
181000Z...Mostly clear with unrestricted vis today and tonight.

&&

.MARINE...
A cold front arriving Wednesday will boost winds in the outer waters
possibly strong enough to be hazardous for small craft. Otherwise,
no hazardous marine weather is forecast through Friday.

&&

.FIRE WEATHER...
Locally gusty northeast winds near the coastal foothills and below
passes and canyons will continue to weaken this morning. Inland
humidities will be lower today...especially in the mountains...but
winds will be weak this afternoon when humidities are lowest.
Onshore flow will bring increasing humidities for Tuesday and
Wednesday with stronger onshore flow on Wednesday bringing strong
and gusty west to northwest winds for the mountains and deserts
for Wednesday into early Wednesday evening. A brief period of
moderate offshore flow is expected on Thursday. However...similar
to this past Sunday...the duration and areal extent of near
critical fire weather conditions may be fairly limited. The most
likely areas to experience near critical fire weather conditions
from gusty north to northeast winds and decreasing humidities
would be below the Cajon Pass and in inland Orange County...mainly
for late Thursday morning into Thursday afternoon.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC/FIRE WEATHER...17
AVIATION/MARINE...MM


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