Area Forecast Discussion
Issued by NWS San Diego, CA

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377
FXUS66 KSGX 161208
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
408 AM PST Sat Dec 16 2017

.SYNOPSIS...
A trough of low pressure moving through Southern California today
will bring stronger onshore flow with cooling...more clouds...and with
gusty southwest to west winds in the mountains and deserts.
Surface high pressure over the Great Basin will bring offshore
flow with drying and with gusty north to northeast winds and lower
inland humidities for Sunday into Monday with the strongest winds
on Sunday. For Wednesday and Thursday...another low pressure
system moving southeastward through the Great Basin will bring
strong onshore flow with cooling...more clouds...and strong gusty
west winds in the mountains and deserts. Moderate to strong
offshore flow may follow for late Thursday into Friday with
periods of moderate to strong northeast winds.

&&

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

.SHORT TERM (Today through Monday)...
A trough of low pressure is moving through Southern California
early this morning. This will bring stronger onshore flow with
significant cooling...more clouds...and gusty southwest to west
winds in the mountains and deserts. For inland coastal areas...the
valleys...and the mountains...high temperatures today will be 15
to 20 degrees cooler than on Friday...10 to 15 degrees cooler
towards the coast and in the upper deserts...and 5 to 10 degrees
cooler in the lower deserts. A brief return of lower level
moisture will bring slight chances for light showers...mainly for
central and southern coastal areas of San Diego for late morning
into the afternoon with any rainfall a few hundredths of an inch
at most.

The flow will turn offshore for tonight and Sunday with stronger
and gusty north to northeast winds developing along and below
coastal mountain slopes and below passes and canyons for portions
of San Bernardino...Riverside...and Orange Counties for late
tonight into Sunday morning with gusts to 50 mph and isolated
gusts to 60 mph. Weaker northeast winds will spread into the
mountains and inland valleys in San Diego County on Sunday. Winds
will weaken and become more localized for Sunday night and
Monday. High temperatures for coastal and valley areas will warm
on Sunday while the mountains and deserts continue to cool. The
warming will spread inland on Monday.

&&

.LONG TERM (Tuesday through Friday)...
Warmer with weaker winds for Tuesday with weakening high pressure
aloft over California. A low pressure system off the Pacific
Northwest coast on Tuesday...will move into the Pacific Northwest
early Wednesday...and into the Great Basin late Wednesday and
Wednesday night. For Southern California...this will bring
stronger onshore flow with cooling for Wednesday into
Thursday...a possible return of the marine layer with night and
morning coastal low clouds and fog...and strong gusty west winds
in the mountains and deserts for Wednesday and Wednesday night.

For Thursday into Friday...upper level low pressure to the east
and surface high pressure over the Great Basin will bring periods
of moderate to strong northeast winds. There remain differences
with the location and strength of that upper level low pressure
among the global models along with the duration and strength of
the offshore flow. The ECMWF is weaker with the offshore flow
along with lesser duration of the offshore flow. The ECMWF also
advertises deamplification of the high amplitude pattern across
the mid latitudes of the eastern Pacific and North America for
early the week of Christmas. In between for next weekend...the
differences across Southern California are significant with some
global models maintaining closed upper level low pressure over
the southwest states and the ECMWF developing upper level high
pressure from the subtropical eastern Pacific into California.

&&

.AVIATION...
161100Z...Areas of stratus will develop in coastal areas and extend
into the valleys and foothills of San Diego County starting 13Z-15Z
today. There is also a slight chance of -SHRA from the coast to the
foothills in San Diego County. Bases will be 1200-2500 ft MSL with
tops to 4000-5000 ft MSL. Clearing will occur 18Z-20Z in Orange
County but might not occur until 03Z-05Z this evening in parts of
San Diego County. After 05Z tonight, most areas will be clear, but
locally gusty northeast winds 15-30 knots with gusts 35-45 knots
will develop along southwest mountain slopes and through/below
passes and canyons, continuing Monday morning. Lighter winds will
occur in San Diego County.

&&

.MARINE...
Combined seas could reach 8-9 feet in the outer waters tonight, just
below small craft advisory criteria, due to a WNW swell around 12
seconds. Otherwise, no hazardous marine weather is expected through
Wednesday.

&&

.FIRE WEATHER...
Surface high pressure over the Great Basin will bring a period
of offshore flow for late tonight into Monday. Gusty north to
northeast winds will strengthen for late tonight across portions
of San Bernardino...Riverside...and Orange Counties and spread
southward into San Diego County on Sunday. The strongest winds are
expected across portions of San Bernardino...Riverside...and
Orange Counties for late tonight through Sunday morning with
lowest humidities for late Sunday morning through Sunday
afternoon. Winds are not expected to be as strong for San Diego
County with stronger winds and lower humidities mostly confined to
the coastal slopes of the mountains and the inland valleys. Winds
will weaken for Sunday night and Monday.

Another round of moderate to strong and gusty northeast winds is
possible for Thursday into Friday with low inland humidities.

&&

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

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Fire Weather Watch from Sunday morning through Sunday evening
     for San Diego County Inland Valleys-San Diego County
     Mountains-Including The Palomar And Descanso Ranger
     Districts of the Cleveland National Forest.

     Red Flag Warning from 8 AM to 8 PM PST Sunday for Orange County
     Coastal Areas-Orange County Inland Areas-Riverside County
     Mountains-Including The San Jacinto Ranger District Of The
     San Bernardino National Forest-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-
     San Gorgonio Pass Near Banning-Santa Ana Mountains-
     Including The Trabuco Ranger District of the Cleveland
     National Forest.

PZ...NONE.

&&

$$

PUBLIC/FIRE WEATHER...17
AVIATION/MARINE...Maxwell



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