Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 202030
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
130 PM PDT Fri Oct 20 2017

.SYNOPSIS...
A trough of low pressure will move through California tonight,
with isolated showers ending and mountain and desert winds
decreasing. Offshore flow will develop this weekend and continue
through at least Tuesday, bringing hot and dry weather along with
occasional gusty northeast winds, especially through and below
passes and canyons. Weak onshore flow is forecast to develop
Wednesday and Thursday for a modest cooling trend.

&&

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

Our marine layer has enough fortitude to present isolated light
showers, mainly in San Diego County. These are decreasing so
measurable rain will be very isolated. A few mountain spots
received up to two tenths inch, lower elevations a few hundredths
or only a trace. Winds are gusting to over 45 mph mainly in the
mountains and locally into deserts, and those will continue
through this evening, then decrease overnight. It will even be
breezy along the coast. The trough axis moves east tonight and
high pressure aloft begins to build this weekend. The trough will
leave behind cold air into the Great Basin, which will lead to
surface high pressure and an offshore pressure gradient aligned
toward Southern California. That will force offshore winds
through and below mountain passes starting Saturday morning mainly
through and below Cajon Pass and into eastern Orange County. For
more on anticipated critical fire weather conditions, see fire
weather discussion below. These locally breezy offshore winds
continue Sunday, then increase in coverage and strength Monday
through Tuesday. Top gusts Monday and Tuesday mornings will
exceed 40 mph in, and adjacent to, foothills. With strong high
pressure aloft combining with dry offshore flow, temperatures will
soar each day. The hottest days Monday and Tuesday will feature
some areas exceeding 100 degrees west of the mountains and even
exceeding 90 along parts of the coast. It will be wise to avoid
strenuous outdoor activity during those days, but saying that, it
will be a dry heat, enabling the human cooling system to cool
efficiently. Nights will cool efficiently for most areas, except
those that remain breezy during the nights. Temps there will stay
in the 70s both nights. The offshore flow diminishes Tuesday
night and the strong high pressure ridge begins to break down
Wednesday and Thursday. That will bring a tepid cooling trend, but
daytime temps will continue above average. Onshore flow will try
to return, which would eventually bring some coastal clouds and
fog, but another offshore push may develop instead.

&&

.AVIATION...
202030Z...Coast/Valleys...Mostly BKN stratocumulus with variable
cigs 2000-4500 ft MSL and tops to 6000 ft MSL will continue through
this afternon. Isolated -SHRA will occur through 22Z. Most vis will
remain above 5 miles, except where higher terrain is locally
obscured. Stratocumulus will gradually become more patchy tonight,
but linger longest in San Diego County, with bases mostly above 2000
ft MSL. Wind shear from northeast winds will develop vcnty KSNA,
KONT, KRAL, KSBD Sat after 12Z with local surface winds 15-30 kt
with gusts to 35-45 kt.

Mountain/Deserts...Areas of terrain obscurations in
clouds/fog/isolated -SHRA will continue below 6000 ft MSL on coastal
slopes through 23Z. Windy conditions will continue from the mountain
crests east into the deserts through this evening with west winds 15-
30 kt and local gusts 40-50 kt. This will bring mod-stg uddfs/llws
over and east of the mtns. Local BLDU vcnty PSP and TRM will be
possible this afternoon. In the San Bernardino County Mountains,
winds will become northeast after 10Z Sat 15-30 kt with gusts to 35-
45 kt.

&&

.MARINE...
Small Craft Advisory was extended through 4 AM Saturday. A low
pressure system with a weakening cold front moving in from the
northwest will bring strong northwest winds to the coastal waters
today. Wind gusts of 25-30 kt with steep waves will result in
hazardous boating for small craft. A combination of the wind waves
and a longer period WNW swell to bring seas around 10 feet to the
outer waters. Highest seas and strongest winds will be near and
south of San Clemente Island. Conditions will slowly improve late
tonight and Saturday morning. Lighter winds and calmer seas are
expected next week.

&&

.BEACHES...
A 2-3 ft/14-15 second south swell from 200 degrees will continue to
subside this afternoon. A short-period northwest swell from 290
degrees and a long-period northwest swell from 295 degrees will
build late today through Saturday. This will bring another round of
elevated surf from 3-6 ft and an increased rip current threat.
Although surf lowers somewhat on Sunday, rip currents will still be
a concern. A Beach Hazards Statement is in effect through Sunday
afternoon.

&&

.FIRE WEATHER...
Critical fire weather conditions develop Saturday and continue
through Tuesday. Biggest concern is for Monday through Tuesday as
temperatures peak and widespread daytime humidity in single digits
develops away from the coast. The concern is more of the long
duration of the hot, dry, and windy conditions through certain
areas and corridors, although Red Flag conditions won`t
necessarily be widespread each day. Current guidance has strongest
winds Monday and especially Tuesday in the favored passes/canyons
and coastal mountain slopes. Consult the Red Flag Warning for
details on timing and location of critical conditions.

&&

.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 Tuesday afternoon
     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 Diego County Inland Valleys-San
     Diego County Mountains-Including The Palomar And Descanso
     Ranger Districts of the Cleveland National Forest-Santa Ana
     Mountains-Including The Trabuco Ranger District of the
     Cleveland National Forest.

     Wind Advisory until 9 PM PDT this evening for Apple and Lucerne
     Valleys-Riverside County Mountains-San Bernardino County
     Mountains-San Diego County Deserts-San Diego County
     Mountains-San Gorgonio Pass Near Banning.

     Excessive Heat Watch from Monday morning through Tuesday evening
     for Orange County Coastal Areas-Orange County Inland Areas-
     San Bernardino and Riverside County Valleys-The Inland
     Empire-San Diego County Coastal Areas-San Diego County
     Valleys.

     Red Flag Warning from 10 AM Saturday to 6 AM PDT Sunday for 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.

     Beach Hazards Statement through Sunday evening for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...Small Craft Advisory until 4 AM PDT Saturday for Coastal Waters
     from San Mateo Point to the Mexican Border and out to 30 nm-
     Waters from San Mateo point to the Mexican Border Extending
     30 to 60 nm out including San Clemente Island.

&&

$$

PUBLIC/FIRE...MM
AVIATION/MARINE/BEACHES...Maxwell



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