Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 181542
AFDSGX

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

.SYNOPSIS...
Fair and seasonally mild weather will continue through Tuesday under
high pressure aloft. Cooler with more clouds on Wednesday as a low
pressure system moving into the Great Basin, drags a cold front
south across the region. Ahead of the front, a few light showers
could break out along and west of the mountains, while strong
westerly winds develop over the mountains and deserts. A period of
moderate offshore will follow behind the front on Thursday, with
gusty northeast winds. Warmer with less wind on Friday.

&&

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

Skies were clear across SoCal this morning with generally mostly
light winds. Surface pressure gradients were weak but remained
offshore at 7 MBS (SW NV to KSAN). No marine inversion was present
with sub-freezing dewpoints all the way to the coast. A few valley
locations were reporting temps in the 30s, while most areas west of
the mts had 40s to low 50s at 7 AM PST.

Looks like the forecast is on track this morning. No changes
anticipated.

From previous discussion...

.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...
181500Z...There is a very small chance that fog could develop within
a few miles of the coast late tonight and early Tuesday. Otherwise,
FEW-SCT clouds AOA 20000 ft msl and P6SM vis through Tuesday
morning.

&&

.MARINE...
A cold front will bring strengthening northwest winds possibly
gusting to 25 kt in the outer waters Wednesday night, which would be
hazardous to small craft. Then, models indicate that gusty offshore
(northeast) winds to 25-30 kt could occur over the coastal waters
Thursday morning. However, confidence in the offshore winds being
hazardous is low at the moment. &&

.FIRE WEATHER...
Winds have diminished, but low RH will develop through the day with
values of 10 to 15% expected. Local east winds along the foothills
this morning will be mostly light and turn back onshore this
afternoon around 10 MPH. RH will increase tonight into Tuesday with
continued light wind.

Onshore flow will increase sharply on Wednesday, becoming strong and
gusty over the ridges and onto the desert slopes by afternoon/
evening at 30 to 45 MPH with local gusts 60 to 70 MPH. 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 lower RH would be below the
Cajon Pass and in inland Orange County...mainly 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...jad/17
AVIATION/MARINE...Harrison



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