Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 211506

Area Forecast Discussion
National Weather Service San Diego CA
806 AM PDT Sun May 21 2017

High pressure aloft will maintain fair skies, and warm to hot days
across the region through Tuesday, except along the immediate coast,
where a shallow marine layer will keep the days cooler, with areas
of fog and low clouds at times. The ridge breaks down midweek as a
broad low pressure trough develops over the West, strengthening
onshore flow and deepening the marine layer with progressively
cooler weather far inland. Noticeably cool on Friday as the trough
peaks and daytime temperatures top out well below average inland.
Then, warmer again next weekend as the ridge slowly rebuilds.



Skies were clear again this morning over SoCal, with the exception
of the immediate coast and adjacent coastal waters where patches of
stratus/fog were evident in satellite imagery early this morning.
The 12Z Miramar sounding had a strong 10 degree C sfc-based
inversion, peaking around 2500 FT MSL. Some cooling was noted below
1500 FT, with several degrees of warming above, through 15K FT.
Unlike yesterday morning, easterly flow below 10K FT was quite
limited and weaker. The offshore surface pressure gradient from NV
was weaker as well at only 2 MBS to KSAN at 7 AM PDT, while the
onshore to the lower deserts had strengthened slightly to 3.5 MBS.

Based on the sounding, sfc gradient trends, and the observed marine
clouds/fog along the coast, suspect it will be cooler today over
coastal areas, but not sure how far inland that will translate.
Perhaps not more than a few miles. The local WRFEMS shows better
sea breeze strength this afternoon and coastal cooling, which may
help limit the afternoon temp rise in the valleys as well, so max
temps could be similar to yesterday, otherwise it could be up to 3
degrees F hotter before sea breeze onset. No forecast changes this

From previous discussion...

.SHORT TERM (Today through Tuesday)...
Satellite imagery shows a few patches of stratus developing just
off the coast with greater coverage beyond the offshore islands.
Higher resolution models show some increase in stratus coverage
just off the coast past sunrise with the stratus extending locally
inland near the coast. Aircraft soundings Saturday evening showed
a very shallow and strong marine inversion for Orange and Los
Angeles County coastal areas with the marine layer a little deeper
for southern San Diego County with the inversion base around 1000
feet. Any stratus near the Orange County coast early this morning
could be in the form of dense fog with that a little less likely
along the central and southern San Diego County coast.

Weak onshore flow has returned for coastal areas. Slow deepening
of the marine layer the next few days will result in greater
stratus coverage each night with the stratus spreading a little
farther inland each night. The stratus is expected to spread a
little farther inland into coastal areas tonight into Monday
morning...and into portions of the western valleys Monday night
into Tuesday morning. Cooling near the coast today...will spread
slowly inland into portions of the valleys for Monday and Tuesday.

Farther inland...strengthening high pressure aloft along the
California coast will bring continued slow warming into Tuesday
with high temperatures for Tuesday for inland areas 10 to locally
15 degrees above average. High temperatures in portions of the
lower deserts on Tuesday could be around 110...close to meeting
excessive heat warning criteria for portions of the Coachella


.LONG TERM (Wednesday through Saturday)...
Slow cooling is expected for all areas on Wednesday as high
pressure aloft begins to weaken. Continued slow deepening of the
marine layer will allow coastal low clouds and fog to spread a
little farther inland into the western valleys on Tuesday night
into Wednesday morning.

For Thursday and Friday...low pressure over the interior western
states will bring stronger onshore flow with significant
deepening of the marine layer for Thursday into Friday. Coastal
low clouds are expected to spread into the far inland valleys on
Thursday morning...and onto the coastal mountain slopes on Friday
morning. Greater cooling is expected for Thursday into Friday with
Friday high temperatures for inland areas around 10 degrees below

For next weekend...high pressure aloft will return near the West
Coast with a warming trend for inland areas. The marine layer will
keep coastal high temperatures near average with the marine layer
becoming shallower through the weekend with night and morning
coastal low clouds and fog not extending as far inland.


211418Z...Mostly P6SM vis and SKC will prevail through 22/0300 UTC,
except for fog/low stratus at the beaches (producing cigs 200-500 ft
agl and vis AOB 1 sm) which should clear by 1700 UTC. 22/0300-1500
UTC, dense fog/very low stratus likely to gradually develop within
about 3-8 sm inland, resulting in areas of vis 1/4 sm or less, cloud
bases 100-400 ft msl and tops around 500-600 ft msl. For KSAN, KCRQ
and KSNA, confidence in dense fog occurrence is moderate-to-high
with low confidence in timing. Elsewhere, P6SM vis and mostly SKC
will continue.


Patchy dense fog will continue over the coastal waters this morning
before clearing/lifting this afternoon. Widespread dense fog is
likely to develop tonight into Monday morning and again Monday night
into Tuesday morning.

Also, 20 kt northwest wind gusts will be possible over the outer
waters near San Clemente island each late afternoon and early
evening through Thursday.


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





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