Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 162157
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
156 PM PST Sat Dec 16 2017

.SYNOPSIS...
A trough of low pressure moving through Southern California today
will drop south of the region tonight. Gusty westerly winds will
turn back offshore and strengthen as surface high pressure builds
south over the Great Basin. Very dry air will return, along with
gusty northeast winds through and below some passes and canyons. The
winds will weaken sunday evening, with more fair, dry and mild
weather on tap for Monday and Tuesday. For Wednesday, another low
pressure system moving through the Great Basin will bring strong
onshore flow again, with cooling, more clouds, and higher humidity
with strong and gusty winds over the mountains and deserts. Moderate
to strong offshore flow may return again late Thursday into Friday.

&&

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

Skies were partly cloudy at midday with NWS radar indicating a few
light showers just off the San Diego County coast and over the
Coachella Valley. The mid-level front responsible for the morning
sprinkles and snow showers has moved off to the east. Surface
pressure gradients were turning back offshore 6 MBS SW NV to KSAN,
but winds were mostly light inland, with some gusty southerly winds
reported along the coast through midday.

The showers off the coast will continue to develop and drift south
through the evening and may brush extreme SW San Diego County as low
pressure strengthens aloft and drifts SE. The shower over the
Coachella Valley will likely end soon as difluence aloft ends over
that area. Elsewhere, skies will be clear to partly cloudy. North to
northeast winds will be increasing below canyons and passes this
evening over northern portions of the CWA and continue through
Sunday.

The shortwave dropping SE today will briefly cut-off a low pressure
center over the AZ/MX border area on Sunday before heading east.
This will support increasing offshore flow over SoCal as surface
high pressure builds into the Great Basin. Strongest winds are
expected over northern portions of the CWA where a Wind Advisory is
in effect starting this evening, and a Red Flag Warning is in effect
for Sunday. Please see Fire Weather Discussion below for details.

A more vigorous shortwave trough will drop SE into the Great Basin
midweek. Until then...high pressure aloft will continue fair and
mild weather through Wednesday. There are still model differences in
the handling of the trough, but the models have been fairly
consistent with the track and intensity through Wed, then
differences develop late in the week. After Thursday, the 12Z ECMWF
is more progressive with the wave, while the 12Z GFS hangs back an
upper low, implying better upper-level support for offshore flow at
the sfc over SoCal Fri/Sat. Either way, we will end up with a dry
offshore pattern, and the winds could be quite strong in some areas.

A greatly amplified, blocking pattern persists along the West Coast
into the final week of the year with a deep, cold vortex over
eastern Canada and the Great Lakes. There are hints of change as
both models push the core of the ridge into the high latitudes,
opening the door for some under-cutting, Pacific flow to break
through around 30 N Latitude. The ECMWF looks more bullish with the
break through with the 12Z runs.

&&

.AVIATION...
162130Z...VFR will prevail with a few patches of stratus with bases
2000-2500 FT MSL through early this evening. There is a slight
chance of -SHRA, mainly near KSAN through the remainder of the
afternoon.

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. Winds will increase out of the NE tonight with the highest
winds of 15-20 KT over the offshore waters towards San Clemente
Island.

&&

.FIRE WEATHER...
Offshore flow will return late tonight and Sunday as surface high
pressure builds south over the Great Basin. Northeast winds 15 to 30
MPH with gusts 40 to locally 55 MPH are expected to return across
portions of San Bernardino...Riverside...and Orange Counties late
tonight. Strongest winds are expected on Sunday morning. Farther
south across San Diego County...winds will generally be weaker with
peak gusts 35 to 40 MPH on the wind-prone coastal slopes on Sunday,
peaking Sunday evening. RH will drop to around 15% on Sunday
resulting in marginal Red Flag conditions for several hours, mainly
northern areas. RH values will drift higher Sunday evening into
Monday as the winds ease.

Another round of moderate to strong northeast winds is possible for
Thursday into the weekend with low inland humidity.

&&

.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...jad
AVIATION/MARINE...Gregoria



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