Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 150313

Area Forecast Discussion
National Weather Service San Diego CA
813 PM PDT Sat Oct 14 2017

High pressure aloft over Southern California will bring a warming
trend through Monday, when temperatures will be in the nineties
from near the coast to the mountains. Surface high pressure over
the Great Basin will bring periods of gusty northeast winds near
the coastal foothills and below passes and canyons...mostly
through Sunday morning. Weak onshore flow will return on Tuesday
and strengthen on Thursday and Friday. Slow cooling will begin on
Tuesday and Wednesday with greater cooling on Thursday and Friday
along with a chance of precipitation and periods of stronger and
gusty west winds for the mountains and deserts.




Extremely low dewpoints lie across inland locales away from the
sea breeze, which has made it into western portions of the Inland
Empire. Dewpoints where the sea breeze has passed are in the 50s
and 60s, while ahead of the sea breeze, insanely low dewpoints in
the single digits and teens prevail. In the mountains, dewpoints
are even lower in the -10s to -30s! The San Diego sounding this
evening shows -20s to -30F dewpoint at the level of the ob
(~8000FT), so looks reasonable. The extremely dry air will remain
in place on Sunday.

Dewpoints are 60-65F along the coast with ACARS data showing a
surface inversion developing. This presents the potential of
patchy fog developing along the coast, so have added this to the
forecast. Confidence is very low on dense fog. Fog could not form
at all, but given a possibility, patchy fog mention near the
coast is prudent.

...Previous Discussion (Issued at 152 PM PDT Sat Oct 14 2017)...

Mostly clear this afternoon, except for a few high clouds. Sfc
pressure gradients are now -11.3 mb SAN-TPH, and the offshore
winds are surfacing in the I.E. So far today, the strongest gust
reported is 45 mph at Highland Springs last hour. Highland Springs
is at 2233 ft, a few miles south of Beaumont. The driest
conditions west of the mtns are in the northern I.E. where
relative humidities are less than 10 percent.

A ridge of high pressure aloft will strengthen over Southern
California into Monday with surface high pressure over the Great
Basin. This combination will bring a warming trend with very dry
conditions and periods of gusty northeast to east winds. The
offshore winds will continue through Sunday morning before
weakening Sunday Afternoon. The strongest winds will be near the
coastal foothills and below passes and canyons where gusts to 45
mph will be possible. Monday will likely be the warmest day, with
90+ degree temps from within 1-2 miles of the coast inland to the
foothills. The very warmest locations for inland coastal areas
into the western valleys could have high temperatures near 100 on

For Tuesday and Wednesday...the high pressure aloft over Southern
California will begin to weaken in response to a low pressure
trough moving in from the northwest. This will bring a return of onshore
flow which slowly strengthens. Temperatures will begin to decrease
near the coast and spread inland with night and morning coastal
low clouds and fog returning and spreading farther inland later in
the week.

For Thursday and Friday...Temperatures continue to decrease and
onshore flow strengthens as the approaching trough deepens off the
California coast. The increasing onshore flow could bring periods
of gusty west winds to the mtns and adjacent desert areas.
Medium-range deterministic models show a chance for some precip
on Fri. This is dependent on whether or not moisture from the sub-
tropics gets entrained as the trough absorbs an upper level low
currently located at about 35N 140W.

Chances of precip should end next Saturday as the trough axis
moves east.


150300Z...Areas of east to northeast winds 15-25 KT will continue
through 18Z Sunday with local surface gusts 30-40 KT through/below
passes and canyons of the San Bernardino and Riverside County
Mountains and Santa Ana Mountains producing MDT-STG UDDFS and LLWS
over and west of the mtns, including near KONT and KSBD. Winds will
be mostly below 25 KT after 18Z Sunday.

Near the coast, there is a small chance of fog with visibilities
below 1 mile 05Z-13Z Sun. This could occur near KSAN and KCRQ though
confidence is low. Otherwise, skies will be clear with unrestricted
vis through Sun evening.


No hazardous marine weather is expected through Thursday.


High pressure aloft will strengthen over Southern California
bringing a warming trend through Monday. Mostly weak offshore flow
will continue into Sunday. Winds will then weaken late Sunday
into Monday. Lowest daytime humidities will be 5 to 10 percent
each afternoon through Monday...with Sunday being the driest day.
Overnight recovery will be poorest near the coastal foothills
where mixing from winds continues overnight. Periods of gusty
northeast to east winds near the coastal foothills and below
passes and canyons will continue at times through Sunday morning.
The stronger winds are expected to be in the Inland Empire below
the Cajon and Banning Passes and near the coastal foothills of the
Santa Ana Mountains. The strongest gusts will be mostly around 45


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


CA...Red Flag Warning until noon PDT Sunday for Orange County Inland
     Areas-San Bernardino County Mountains-Including The
     Mountain Top And Front Country Ranger Districts Of The San
     Bernardino National Forest-San Bernardino and Riverside
     County Valleys  -The Inland Empire-Santa Ana Mountains-
     Including The Trabuco Ranger District of the Cleveland
     National Forest.



PUBLIC...Gregoria (Update)/PG(Previous Discussion)
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