Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 210511 AAA

Area Forecast Discussion
National Weather Service San Diego CA
1000 PM PDT Sun Jun 20 2021

Hot and dry conditions will continue from the foothills, across the
mountains and into the deserts this afternoon, while the marine
layer and onshore flow moderates temperatures farther to the west.
Low pressure approaching the Coast will amplify the onshore flow
through Monday and gradually push cooling farther inland through the
first half of the week. Areas of low clouds and patchy fog will
continue along the coast at times. On Tuesday into Wednesday,
widespread high clouds from the remnants of Dolores will arrive. As
those clouds part, a re-building ridge aloft will bring hotter
weather again Friday into next weekend.



Evening Update:

No significant updates were needed this evening. Today was another
hot one for the inland areas, with peak  temperatures almost 120
degrees in the lower deserts. Winds have also been gusty in the
deserts this evening with peak gusts as high as 45 mph in Palm
Springs. These winds will diminish after midnight,
however look for another diurnal resurgence of winds in the passes
and deserts for Monday afternoon through Monday evening.

Remainder of previous discussion, issued at 100 pm pdt:

Heat stress and wildfire potential are the main concerns for the
mountains and deserts. Look for wind to increase this afternoon and
evening, and continue through the night. Overnight minimums will be
elevated in windier areas. Along the Coast, the marine layer will
keep it much cooler with periods of low clouds and patchy morning
fog much of the week.

High pressure over the Southwest today will drift eastward through
midweek as a weak Pacific trough edges closer to the California
Coast. The repositioning of these weather features will bring
increasing onshore winds over SoCal, driving cooling inland through
about midweek. The ridge is forecast to drift back westward later in
the week along 30N, as the trough slowly weakens, maintaining
westerly flow aloft, but with rising heights/thickness and
consequently, warming over SoCal. The picture gets muddied a bit by
next weekend with differing model solutions, but warming is favored
through the weekend when temperatures are currently forecast to be
back above normal most areas and perhaps excessively so once again.

Until then, there will be some moisture from the remnants of
Tropical Cyclone Dolores to monitor for Tue/Wed. For now, it looks
substantial, aloft anyway, and that should bring plenty of high
clouds on Tuesday, which will help to moderate peak surface
temperatures even beyond the increased onshore influences. Initially
this looks to be occurring in a very stable atmosphere. So other
than a few sprinkles over the mountains, no precipitation is
expected. For Wednesday though, a weak easterly wave is drawn
northward and could clip us with enough instability to draw on that
moisture for some showers. For now, the trajectory of that feature
looks to be too far east to affect us, so there are no forecast POPS.

The Excessive Heat Warnings all expire this evening. It will remain
quite warm on Monday far inland. wind Advisories may be needed
tonight into Monday, but for now the threat is marginal enough to
not issue. Cooling will be more noticeable far inland on Tuesday,
with continued cooling through midweek. Daytime temperatures may
actually be a few degrees below average this period.


210400Z...Coasts/Valleys...Stratus with bases 900-1400 ft MSL will
fill in along the coast through 06Z, then spread 10-15 mi inland
overnight. Bases may locally lower to around 700 ft MSL with 4-6 SM
vis in BR/HZ, mainly on higher coastal terrain. Clouds clearing back
to the coast 15-18Z, though areas near the immediate coast may
remain BKN-OVC through much of the afternoon, including near KSAN
and KCRQ. Clouds will slowly spread back inland after 22/02Z with
similar bases.

Mountains/Deserts... West winds with surface gusts 25 to 35 kt
will continue across the desert mountain slopes and locally into the
deserts through about 08Z, weakening overnight. Winds will
strengthen again Monday afternoon with similar strength gusts,
including gusts near 30-35 kt at KPSP. MOD-STG up/downdrafts and
areas of LLWS possible over and east of the mountains.


Breezy northwest winds will occur each afternoon and evening Monday
through Wednesday across the outer waters with gusts approaching 20
kt, with the strongest winds Tuesday and Wednesday. Otherwise, no
hazardous marine weather is expected through Friday.


A south swell from 200 degrees will build Monday, peaking at 3 ft/18
sec. The swell will persist into Wednesday before slowly fading.
This will generate surf of 3-6 ft along the south facing beaches of
far northern San Diego County into Orange County through Wednesday,
as well as strong rip and longshore currents.




Extreme heat, low afternoon RH of 5-15%, and gusty westerly winds
will maintain elevated fire weather conditions along the desert
slopes, through the passes and into some desert areas through
Monday. Strongest southwest to west winds of 25 to 35 mph with gusts
40-50 mph and locally higher are likely in these areas at times,
especially near the passes. Near critical fire weather conditions
will develop in these areas for several hours today and again Monday

The extreme heat today, coupled with high mixing heights and very
dry fuels in the mountains, will increase the potential for plume-
driven fire growth as well.

Some cooling will begin on Monday. Better cooling and slightly
higher humidity will work a little farther inland Tuesday and
Wednesday when clouds and high elevation moisture will arrive.
Westerly winds will remain gusty in the usual onshore favored areas.


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





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