Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 221015

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

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,
increasing the threat of wildland fires. Autumn officially begins
this afternoon at 1:02 PM PDT.



Satellite imagery showed areas of clouds along the foothills
and coastal slopes of the mountains, but over the coastal basin,
they were mostly scattered. The onshore sfc pressure gradient had
weakened considerably over the past 24 hours, just turning offshore
SW NV-KSAN, but there were still a few gusty SW wind reports at 25
to 45 MPH in the San Diego County Mts at 3 AM PDT.

The deep, meridional trough covering much of Canada and the Western
U.S. still has a few short waves to rotate through, delaying the
passage of the trough axis until Saturday afternoon. Heights
gradually rise early next week, however the latest operational
GFS/ECMWF runs still have substantial differences in the height
field over the SW. The 00Z ECMWF develops a secondary closed low on
the back side of the trough (over southern NV) by 12Z Wed. This low
drifts SW to SE CA on Thu. The 00Z operational GFS develops a hang-
back closed low as well, but this one is centered over SW CO on Thu
morning. This results in large height/thickness differences mid
next week over SoCal.

There is some agreement early next week in the offshore pattern
developing, but confidence wanes as the week progresses, regarding
the strength and persistence of the offshore gradient due to the
mismatched closed low development. This lowers confidence in the
daytime temperature and wind forecasts. In either case, next week
looks dry and warmer. The drier air and lack of cloud cover will
allow the nights to cool more efficiently where winds die off. This
should make it feel even more Autumnal.


220845Z...Coast/Valleys/Coastal Slopes...Areas of mostly SCT-BKN low
clouds with bases 2500-3500 ft msl and tops 4500-5000 ft msl
remaining somewhat patchy overnight. Coastal slopes remaining
partially obscured with vis blo 3SM where clouds and terrain
intersect. Clouds becoming FEW-SCT 16-18Z. Patchy stratus with bases
mostly 2500-3500 ft redeveloping after 23/04Z.

Mountains/Deserts...West winds with sfc gusts 25-35 kt diminishing
through the morning. Otherwise mostly clear with unrestricted vis
through Saturday morning.


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 has been extended
through 1 AM Saturday. Lighter winds and smaller seas are expected
over the weekend.


With the expectation of offshore flow beginning on Sunday...RH will
drop 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 activation is not requested. However weather spotters are
encouraged to report significant weather conditions.



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.



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