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Area forecast discussion
National Weather Service San Diego California
139 PM PDT Tue Jun 27 2017

Synopsis...
stronger onshore flow will bring a cooling trend through Thursday
with high temperatures returning to near average for Wednesday and
Thursday. Coastal low clouds and fog will return to coastal areas
tonight...and spread into some valleys late tonight into Thursday
morning. Slightly warmer Friday under weak high pressure, with
slow cooling over the weekend followed by a slightly warmer
Fourth of July.

&&

Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
counties...

Temperatures are a few degrees lower at 1 PM today than they were
yesterday at this time, except for the Lower Desert. A brand new
marine layer has come off the factory floor (along the central
coast), and this shiny new marine layer will drive into the
Southern California showroom tonight. This appears to be in
response to a shortwave moving by to our north, which has also
flattened out the monster ridge responsible for our recent heat.
The relaxed intensity of the high will help the intense heat take
a chill pill in the interior away from the marine layer, and the
marine layer cruising into Southern California will take care of
the cooling west of the mountains. Daytime temperatures will be
noticeably lower Wednesday and Thursday, especially west of the
mountains. This new low cloud deck is not a dense fog deck as was
its predecessor at the coast this morning. As the new coastal
clouds move ashore, there might be dense fog briefly on coastal
mesas and hills, but true fog with low visibility will not be an
issue at the coast itself. As it moves ashore this evening it
should deepen overnight enough to spread into western valleys. For
the coming week, the stagnant summertime pattern will have only
very minor fluctuations between the influence of a weak ridge or a
weak trough over Southern California. Right now we see relatively
cool Wednesday and Thursday followed by a slightly warmer Friday
and Saturday, a cooler Sunday and Monday, and a warmer Fourth of
July. The coastal clouds will make it into some of the westernmost
inland valleys during the nights.

&&

Aviation...
271930z...coast...clear with unrestricted vis this afternoon.
However, widespread low clouds are likely to develop along the coast
between 03z-06z Wed with bases near 500 ft mean sea level. Low clouds will
slowly spread 10-15 miles inland by 12z Wednesday morning, with
bases 500-800 ft mean sea level. Expect areas of fog with vis 3-5 sm. Vis may
be reduced locally to 1sm or less on the coastal mesas and over
higher coastal terrain. Coastal taf sites will likely be impacted,
although the timing is uncertain. Scattered clouds 1000 ft or lower with
restricted vis will be possible at kont around 12z Wed but
confidence is low. Otherwise, clear with unrestricted vis through
Wed morning.

Valleys/mountains/deserts...clear skies with unrestricted vis will
prevail through Wed morning. West winds gusting 30-40 kt will occur
over and east of the mountains through tonight. This will produce
low level wind shear and MDT up/downdrafts over and east of the mountains. Winds
will weaken early Wed morning.

&&

Marine...
stronger west northwest winds with local gusts 20-25 kt will develop
this afternoon/evening near San Clemente Island. Areas of fog are
likely tonight into Wednesday morning but visibility should remain
above 1 mile. Otherwise, no hazardous marine conditions are expected
through Friday.

&&

Fire weather...
single-digit humidity will continue for the deserts and desert
slopes of the mountains of Riverside and San Diego counties
through Wednesday with a slight recovery Thursday. Stronger
onshore flow will bring periods of strong gusty west winds along
the desert slopes of the mountains into the deserts of Riverside
and San Diego counties through late tonight. Critical fire weather
conditions will continue through Wednesday night, however they
will become more localized and mostly in the desert where there is
little if any fuel to burn if a fire were to start. The red flag
warning remains in effect through 1 am Wednesday.

&&

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

&&

Sgx watches/warnings/advisories...
California...red flag warning until 1 am PDT Wednesday for Coachella Valley-
including the Palm Springs South Coast desert district-
Riverside County mountains-including the San Jacinto Ranger
district of the San Bernardino National Forest-San Diego
County deserts-including the Anza Borrego Desert State Park-
San Diego County mountains-including the Palomar and
Descanso Ranger districts of the Cleveland National Forest-
San Gorgonio Pass near Banning.

Pz...none.

&&

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