Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 220437
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Thu Sep 21 2017

.SYNOPSIS...
A deep trough of low pressure will continue through Saturday and
bring below normal temperatures and a deep marine layer with areas
of low clouds. The trough will gradually move east early next week,
and offshore flow will develop and bring locally gusty northeast
winds mainly below passes and canyons. There will be a return to
warmer weather next week, with temperatures rising to around 5
degrees above normal west of the mountains by Tuesday.

&&

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

Skies were partly to mostly cloudy with patchy fog in the mountains.
The marine inversion was 5000 feet MSL on the 5 PM NKX sounding and
almost 10000 feet on some of the AMDAR soundings out of KONT
earlier. The inversion had weakened to less than 2 deg C. Thus,
expect kind of sporadic clouds through Saturday from the mountains
west to the coast as the marine layer will continue to be deep, with
greatest cloud cover inland. Temperatures again will mostly be in
the lower to mid 70s west of the mountains Friday as the profile
aloft will be rather cool under the broad deep West Coast trough,
and with the trough axis shifting only slightly east Saturday, it
will only be slightly warmer then. Lower desert max temps will
mostly just be in the 80s through Saturday.

The upper low will gradually shift east Sunday as a ridge builds
over the East Pacific. With that pattern, north flow aloft will
dominate and surface offshore flow will develop as a surface high
builds from the Great Basin into the northern Rockies. The offshore
flow does not look too strong over So-Cal as the surface pressure
gradients will be spread out between So-Cal to Nevada and Utah, and
the source region of the air coming out of the Great Basin will not
be all that cold. Right now, it looks like we will have local wind
gusts exceeding 45-50 MPH through/below some of the passes and
canyons, though possibly a little stronger around Tuesday. Mountain
wave potential does not look that great as there is no indication of
a strong stable layer/inversion at/above mountain crests, but that
could change somewhat. We have a good warming trend, with temps Tue-
Thu about 5 deg F above normal west of the mountains and near normal
elsewhere. The marine layer should be absent most of next week.

&&

.AVIATION...
220345Z...Coast/Valleys/Coastal Slopes...Areas of mostly SCT-BKN low
clouds with variable bases 2500-3500 ft msl and tops 4500-5000 ft
msl, slowly increasing in coverage through tonight. Cigs blo 3000 ft
likely at times for all TAF sites west of the mtns, but timing is
uncertain. Expect higher terrain to be obscured with vis blo 3SM
where clouds and terrain intersect. Clearing to the coast expected
by 18Z Fri.

Mountains/Deserts...Mostly clear with unrestricted vis through
Friday. Strong west winds 25-35 kt with gusts 45 kt will continue
producing stg uddfs/llws over and east of the mtns tonight,
including vcnty KPSP and KTRM. Blowing dust & sand will also be
possible in the deserts.

&&

.MARINE...
Northwest winds with gusts to 25 kt are likely in the outer waters
through midnight, while the inner waters will see gusts mainly in
the 15-20 kt range. This will be accompanied by combined seas of 4-7
ft with steep waves and rough conditions. Although winds will
decrease early Friday morning, another increase in winds is expected
late Friday afternoon with wind gusts in the outer waters mainly
around 20 kt. Lighter winds and smaller seas are expected through
the weekend.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory until 1 AM PDT Friday for Apple and Lucerne
     Valleys-Coachella Valley-Riverside County Mountains-San
     Bernardino County Mountains-San Diego County Deserts-San
     Diego County Mountains-San Gorgonio Pass Near Banning.

PZ...Small Craft Advisory until 1 AM PDT Friday for Waters from San
     Mateo point to the Mexican Border Extending 30 to 60 nm out
     including San Clemente Island.

&&

$$

PUBLIC...Maxwell
AVIATION/MARINE...PG



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