Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 230410
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
910 PM PDT Tue Aug 22 2017

.SYNOPSIS...
Low pressure centered off the Central Coast will drift southeast and
weaken through midweek. The low will keep the pleasant August
weather going through Wednesday, then a warming trend will begin,
pushing temperatures back above average by the weekend and into next
week. Areas of nocturnal marine clouds and patchy fog will spread
into the western valleys each night through Thursday, then decrease
as high pressure builds and the marine layer thins. Temperatures
could be well above average early next week.

&&

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

Satellite imagery showed marine stratus extending inland up to 12
miles over coastal San Diego County, with little development into
Orange County. A few cumulus were over the deserts, and some patches
of low-level clouds were seen drifting NNW over the inner coastal
waters. But over most of the CWA, skies were clear. The 00Z Miramar
sounding had a 10.5 degree C inversion based near 2K FT. The winds
had increased to 15-20 KTS and backed SE in the 8-10K Ft layer.
There was also an increase in PW to 1.11 inch. Potential instability
ticked up a bit, but there was still a second weak inversion near
12K FT. Surface pressure gradients were 5-7 MBS onshore KSAN to the
local deserts, little changed from yesterday.

Based on the 00Z sounding trend, moisture availability, approaching
shortwave, and small low-level cloud streets just offshore, suspect
we will be seeing more low to mid-level cloud development overnight
and early Wed morning as a dry front aloft pushes back toward the
Coast. Latest hires models are dry, but POP chances are non-zero for
the CWA and especially the coastal waters until later on Wed when
the shortwave passes and the atmosphere dries. For now, convective
parameters do not support POPS of more than 5 or 10 percent onshore,
but offshore, close to the elevated dewpoint front, the boundary
should offer slightly better support. With POPS just below
threshold, No forecast changes this evening.

From previous discussion...

For Wednesday and Thursday...the weak low pressure system off the
central California coast will weaken as it moves inland through
Southern California. This will bring slight cooling with high
temperatures for inland areas a few degrees below average. Areas
of night and morning coastal low clouds and fog will extend into
the western valleys. Some of the higher resolution models show an
area of elevated instability moving across portions of
southwestern California from the south late tonight and early
Wednesday...however moisture aloft remains quite limited. This may
bring some mid level clouds into our area...but any chances for
precipitation are quite limited. Otherwise...there may continue to
be some afternoon clouds near the mountains into the upper
deserts the next few days.

Friday through Monday... High pressure aloft over northwest
Mexico on Friday...will expand into the southwest states and Great
Basin for Sunday and Monday. High temperatures for inland areas
will warm to near average on Friday...then warm to 5 to locally 10
degrees above average. The marine layer will become shallower
with areas of night and morning coastal low clouds and patchy fog
not extending inland much past the coastal areas. Mid level
moisture may increase somewhat during the weekend into early next
week with more afternoon clouds for the mountains, however the
global models are keeping precipitation mostly to the east.

Some medium-range deterministic models are indicating the
possibility of more substantial monsoonal moisture returning to
the region by next Tue, but that`s mostly speculative at this
time.

&&

.AVIATION...
230300...Coast/Valleys...Low clouds will continue to advance
reaching up to 25 SM inland by 14Z. Cloud bases will be near 1300 ft
msl with tops around 2300 ft msl. Expect to find visibility
restrictions of 2 SM or less near higher terrain. Clearing to the
coast should occur between 15Z and 18Z Wednesday. Light westerly sea
breezes, SCT low clouds over the beaches and coastal waters, and SKC
inland will prevail between 18Z Wednesday and 00Z Thursday. Low
clouds will increase in coverage near the coast after 00Z Thursday.

Mountains/Deserts...FEW afternoon cumulus and unrestricted
visibility through Wednesday evening.

&&

.MARINE...
A light shower is possible overnight into Wednesday morning. No
hazardous marine weather is expected through Saturday.
&&

.SKYWARN...
Skywarn activation will not be needed on Wednesday.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC...JAD/PG
AVIATION/MARINE...Albright


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