Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 221605
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
905 AM PDT Fri Sep 22 2017

.SYNOPSIS...
A low pressure trough covering the West will continue the cool
weather through Saturday. The trough will weaken early next week,
allowing offshore flow to develop, bringing much warmer and drier
days, and clear, cool nights. Northeast winds could get gusty at
times below offshore wind-prone passes and canyons at times.
Autumn officially begins this afternoon at 1:02 PM PDT.

&&

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

Partly cloudy skies were present at sunrise from the mountain
crests westward this morning. The marine inversion is very weak,
so the cloudy pattern will be more unstable today. That means
cumulus clouds will be forming and dissipating for much of the
day, basically partly cloudy. We have a big and unseasonable
trough of low pressure over the west today, unseasonable for
summer that is, since it`s not fall until this afternoon. At 102
pm today it will become "a deep early season trough." Speaking of
the equinox, if you step outside at that minute and look at the
sun with special glasses...wait a minute. Anyway, the big early
season trough will continue over the West this weekend, bringing
partly cloudy skies west of the mountains and cool weather. Bring
in the cats and close those windows the next couple nights; we`ll
be dropping into the 50s for most areas. Models are letting the
big trough linger longer next week than previous models have done.
But the offshore flow will develop on schedule Sunday, and all
clouds will be swept away. Areas of northeast winds in the
offshore pattern will develop early Sunday in foothills and
locally into valleys, primarily in and below the Cajon Pass.
That`s because the offshore pressure gradient from Nevada is
northerly, so San Diego County will not be in a favorable position
for strong winds. Monday and Tuesday look like the strongest wind
days overall. Temperatures will slowly and gradually rise each
day as high pressure aloft slowly and gradually builds. Then comes
the uncertainty as some model guidance features a weak cut off
low to settle over the Southwest around the middle of next week.
That would change the temperature forecast next week, but the dry
offshore flow and the absence of the marine layer and associated
clouds remain more certain.

&&

.AVIATION...
221600Z...Coast/Valleys/Coastal Slopes...SCT-BKN025-035 will
continue this morning mainly over San Diego County coast/valleys.
Clouds becoming FEW-SCT 16-18Z. VFR prevail through the day. Patchy
stratus with bases mostly 2500-3500 ft redeveloping after 23/04Z.

Mountains/Deserts...West winds with sfc gusts to 25 kt diminishing
through the morning. Otherwise mostly clear with unrestricted vis
through Saturday morning.

&&

.MARINE...
Northwest winds gusting to 15-20 kt will strengthen again this
afternoon and evening with gusts 20-25 kt. This will be accompanied
by combined seas of 5-8 ft and steep waves, resulting in hazardous
boating conditions. The Small Craft Advisory continues through 1 AM
Saturday. Lighter winds and smaller seas are expected over the
weekend.

&&

.FIRE WEATHER...
Offshore flow will develop Sunday, dropping RH considerably, and
periods of gusty northeast winds are likely along the coastal
slopes and foothills, peaking through and below favored passes and
canyons of San Bernardino, Riverside, and Orange Counties
Mon/Tue. At this time, it looks like local winds of 25 to 35 mph
with gusts topping 40 mph are possible, especially Mon/Tue
mornings through midday.

Several hours of near critical fire weather conditions are possible
in these foothills areas, mainly north of San Diego County Mon/Tue
as RH falls to 10-15% and winds exceed 25 mph.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory until 1 AM PDT Saturday for Waters from San
     Mateo point to the Mexican Border Extending 30 to 60 nm out
     including San Clemente Island.
&&

$$

PUBLIC/FIRE...MM
AVIATION/MARINE...Gregoria


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