Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 201558

Area Forecast Discussion
National Weather Service San Diego CA
858 AM PDT Fri Oct 20 2017

A trough of low pressure will move through the West Coast today,
bringing cool weather, along with some light showers mainly in the
coastal mountain slopes. Locally strong west winds will continue
in the mountains and deserts through this evening. 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.



The marine layer has some muscle to it this morning, with a low
pressure trough moving through the northwest states. Low clouds
are not your typical flat stratus, but the lumpy stratocumulus
that are producing scattered light showers. Most of the showers so
far have dropped less than a tenth inch, and mostly in the
foothills. These will gradually decrease during the day from
northwest to southeast. Winds are gusting to over 40 mph mainly in
the mountains and locally into deserts, and those will continue
through this evening. We even have some breezy winds 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. 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 Tuesday morning 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 90+
degrees 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 and nights
to cool efficiently. 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. At some point later next week
the onshore flow would bring back some coastal clouds.


201600Z...Coast/Valleys...Mostly BKN-OVC stratus with variable cigs
2000-4500 ft MSL and tops to 6000 ft MSL will continue through 18Z,
then very slowly decrease through this afternoon. Isolated -SHRA
will occur through 18Z. Most vis will remain above 5 miles, except
where higher terrain is obscured. Only patchy stratus is expected
tonight, mainly San Diego County, with bases mostly above 2000 ft
MSL. Otherwise, SCT clouds above 20000 ft MSL will continue.

Mountain/Deserts...Terrain obscurations in clouds/fog/isolated -SHRA
will continue below 6000 ft MSL on coastal slopes through 19Z. 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 today.


Small Craft Advisory continues through 1 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. 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.


A 2-3 ft/14-15 second south swell from 200 degrees will continue to
subside today. 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.


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. A Fire Weather Watch remains in
effect from Saturday for the San Bernardino mountains, Inland
Empire and Santa Ana mountains, then starting Sunday for the rest
of Orange County, the valleys and mountains of San Diego County
and the mountains of Riverside County, with all areas continuing
through Tuesday.


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


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 Diego
     County Inland Valleys-San Diego County Mountains-Including
     The Palomar And Descanso Ranger Districts 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

     Fire Weather Watch from Saturday morning through Tuesday
     afternoon 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 for winds until 1 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



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