Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 212014

Area Forecast Discussion
National Weather Service San Diego CA
114 PM 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 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.


211930...Mostly P6SM vis and SKC will prevail through 22/0300 UTC,
except for a few patches of low clouds at the beaches (producing SCT-
BKN cigs 200-500 ft agl). 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-to-moderate
confidence in timing. Elsewhere, P6SM vis and mostly SKC will
continue. Clearing of fog/stratus expected after 22/1500 UTC.


Dense fog is likely to develop tonight into Monday morning and again
Monday night into Tuesday morning, resulting in visibilities of 1 NM
or less over the coastal waters.

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


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





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