Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 242004

Area Forecast Discussion
National Weather Service San Diego CA
104 PM PDT Sun May 24 2020

A warming trend will prevail inland through midweek as high pressure
builds over California. The ridge will peak about Thursday, sending
midweek temperatures well above average inland and over the deserts.
Near the coast, a shallow marine layer will likley keep beaches much
cooler with areas of low clouds and fog at times. Onshore flow will
strengthen Friday into the weekend, sending cooler air and marine
clouds farther inland as the strong ridge drifts east.



Skies were clear over SoCal at midday, except for some patchy low
clouds along the southern San Diego County coast and offshore. At
Noon PDT, may areas were 2-7F warmer than yesterday. Winds were
mostly light and from mixed directions due to weak gradients.

The longwave trough over the West will lose it`s grip over the
region tonight and Monday as high pressure builds aloft in advance
of a cut-off low 1100 miles WSW of San Diego. Continued subsidence
in the atmosphere should thin the marine layer overnight another
1000 FT or so for some fog on the higher coastal terrain, and set up
inland areas for a much warmer Memorial Day. Some warming will occur
near the Coast as well, but the beaches should remain cool. There is
less confidence that any offshore will influence temperatures
Mon/Tue, except near the foothills. For Tuesday, the marine layer is
flattened further, increasing the likelihood for more widespread fog
along the coast and offshore, with the onshore sfc pressure
gradient prevailing.

The operational models have the ridge peaking over SoCal on
Thursday, followed by gradual 500H falls as the cut-off approaches
the Coast. Expect very warm to hot days inland, and especially over
the deserts where the sea breeze will have little effect. An
Excessive Heat Warning is in effect for these areas Tue-Fri. There
remain GFS/ECMWF path/timing differences with the incoming cut-off
low, but still prefer the slower and more northerly path of the
ECMWF runs. The unyielding ridge to the east forces the low NE
toward the PacNW. This should still set up strong onshore flow
across SoCal as the sfc gradient tightens between the ridge inland
and the low Fri/Sat.

Until then, the main forecast uncertainty is with the depth of the
marine layer, and the persistence of any low cloud deck beneath a
very strong inversion. Since it is May, we have to expect more
clouds/fog to linger at some beaches after Tuesday, with perhaps
only partial clearing, and afternoon onset of low clouds over
coastal areas as well some days.

For early next week, with the cut-off low out of the picture, the
ridge could build back over SoCal for more heat inland, but the
pattern still supports solid onshore flow at the surface, amplified
by a strong thermal gradient.


242000Z...Coast/Valleys...Some patches of low clouds will continue
to stick around the immediate coast through this afternoon, but most
areas will be clear through this evening. Low clouds will move back
into the coastal areas after 06Z, with slightly lower bases, around
1000 ft MSL. Low clouds should cover most coastal areas overnight,
but likely won`t extend more than 10 miles inland. Vis restrictions
and fog are likely on higher coastal terrain overnight, possibly
dense at times.

Mountains/Deserts...Mostly clear with unrestricted vis through


Patchy dense fog could impact the coastal waters nights and mornings
starting Monday and continuing through much of the week, with
visibility less than 1 nautical mile at times. Otherwise, no
hazardous marine weather is expected through Friday.


A long-period (16-18 seconds) south-southwest swell will build in on
Tuesday and continue through much of the week, bringing elevated
surf of 3 to 5 feet to the beaches and high rip current risk. Some
sets to 6 feet are possible in Orange County.


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


CA...Excessive Heat Warning from noon Tuesday to 7 PM PDT Friday for
     Apple and Lucerne Valleys-Coachella Valley-San Diego County
     Deserts-San Gorgonio Pass Near Banning.



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