Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 220358 AAA

Area Forecast Discussion
National Weather Service San Diego CA
858 PM PDT Sun May 21 2017

No major changes were made this evening. The main concern this
evening is with the potential for dense fog development overnight
near the coast. Earlier convection over the Southern Sierra
generated blowoff which is dropping south over SoCal this evening
and will likely disrupt dense fog development at least initially.
Will continue to monitor. The current forecast has it well handled
although a DFA may be needed overnight if and when dense fog
materializes. Temperatures on Monday will remain well above normal
well inland and across the deserts, but cooler temperatures are
likely near the coast and into the western valleys due to the
strengthening of the marine layer.


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.



(Discussion originally issued at 114 PM)

Skies were mostly clear again this afternoon over SoCal, with the
exception of the immediate coast and adjacent coastal waters where
a few patches of stratus/fog remained in satellite imagery at
midday. The offshore surface pressure gradient from NV had leveled
off and was almost neutral, while the gradient to the lower deserts
had increased to around 3 MBS onshore. Winds were mostly light, but
had turned onshore along the coastal slopes and foothills where they
were offshore yesterday.

High pressure aloft over the West Coast will continue fair and hot
days inland across SoCal through Tuesday, while a shallow marine
layer keeps coastal areas cooler with periods of low clouds/fog. A
weak shortwave trough sliding south along the east flank of the
ridge is helping to draw the marine air inland today, but as it
passes the onshore surface gradient will weaken into Tue, trapping
the higher dewpoint air under a strong, shallow inversion near the
coast. This favors the development of fog/low clouds along the
coastal strip and offshore. Areas of dense fog are possible over the
coastal waters and several miles inland through Mon morning, and
possibly again late Mon night into Tue morning.

Higher res models show the heat peaking today over the inland
valleys, with only slight cooling into Tue, while the deserts
continue to warm, peaking on Tuesday when max temps could reach 110
F in the lower deserts, and the upper 90s in the high desert
areas. The low deserts will have a moderate to elevated heat risk
level Mon/Tue.

Cooler weather will arrive on Wednesday and intensify through Friday
as a broad trough is carved out over the West. Increasing onshore
flow will build the marine layer inland and result in gusty winds
over the mountains and into the deserts, where cooling will finally
arrive late in the week. Weak ridging ensues for next weekend, which
should result in a return of fair and somewhat warmer weather.

Looking ahead into next week...The 12Z ECMWF/GFS runs are similar in
showing the evolution of a more energetic, lower latitude branch of
the westerlies across the EastPac. The GFS is much more aggressive
in developing a large cut-off low off the SoCal coast by late Tue/
Wed timeframe. This suggests the possibility of some precip near the
end of the month or perhaps to begin June.


220300...SKC, unrestricted visibility, and light winds will prevail
in all areas except for the coast through 06Z Tuesday. Areas of
dense fog are expected within 10 miles of the coast until 16Z
Monday. There is a high probability of dense fog at KCRQ during this
period, and a moderate potential for dense fog at KSAN and/or KSNA.
Visibility restrictions of 1 sm or less are expected along the
coastal mesas. Conditions will improve after 16Z Monday, but
additional areas of dense fog are possible from 03Z to 16Z Tuesday.


Dense fog will develop in the Southern California Bight tonight into
Monday morning and again Monday night into Tuesday morning. This
will produce visibility restrictions of 1 nm or less. Breezy
northwest winds in the afternoons and evenings will continue outside
of 20 nm through Friday, with peak gusts near 20 kts in the vicinity
of San Clemente Island.


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





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