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Forecast Discussion for San Diego, CA

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FXUS66 KSGX 200930

Area Forecast Discussion
National Weather Service San Diego CA
230 AM PDT Fri Sep 20 2019

Fair and warmer weather is expected over the weekend with some
light offshore flow and higher pressure. A low pressure system
will bring partly cloudy skies Sunday and Monday, and cooler
weather Monday. Fair and slightly warmer weather Tuesday and
Wednesday will turn cooler the latter half of next week.



Low clouds cover areas of the coastal basin early this morning,
with greater coverage over inland valleys. The brisk onshore flow
we had last evening is beginning to turn offshore and weaken.
Strong westerly winds have been weakening and a few spots are
turning northerly. Very localized wind gusts through and below
Cajon Pass will reach 25-30 mph. That weak offshore flow this
morning will clear our clouds rather quickly, leaving us with a
sunny and warmer day. Weak offshore flow continues Saturday and
Sunday, with an easterly component. While winds will be weak, that
offshore flow will bring dry interior air into the valleys and
close to the coast. The compression of air descending in elevation
will dry and warm the air, so a pretty decent warming trend is
forecast this weekend. On Sunday lots of 90s should appear not far
from the coast and in the valleys, and in the lower desert. And
it will be a relatively dry heat. Coastal low clouds will be
patchy at most during nights and mornings, and confined to the
coast. A low pressure trough digs into the West from northwest to
southeast late Sunday and Monday (this is the system that had us
thinking recently it might bring rain to SoCal). At the same time
some moisture from what will be remnants of hurricane Lorena will
migrate north. The two will be on a collision course that will
bring showers and thunderstorms close, but not quite into
southwest California. Several runs of several models have depicted
this scenario similarly, so confidence is growing we`ll get no
precip out of this low and its interaction with the remnant
tropical moisture. A serious rain event for Arizona on Monday is
looking more likely, however. The shortwave trough drops into
Arizona and weakens Tuesday and Wednesday. That could set up SoCal
for some offshore flow with the upper support on the west side of
the low, but there`s no cold air in the interior West to produce
steep offshore pressure gradients to drive winds at the surface.
Some weak offshore winds look possible for Tuesday, but nothing
more. Skies will be generally clear Tuesday and Wednesday, with
onshore flow returning Wednesday. Thursday and beyond, a trough
of low pressure amplifies along the West Coast and current
guidance has it unseasonably deep. It should rebuild the marine
layer and possibly deepen it enough to produce showers from the
coast to mountains Friday and Saturday. It would also bring strong
onshore winds. But you know the caveat: that`s too far out in the
forecast to put a lot of confidence in any solution. Consistency
isn`t bad, however, so plan on more clouds, breezes and a decent
cooling trend.


200850Z...Coast/Valleys...Areas of BKN stratus with bases 1800-2500
ft MSL and tops to 4000 ft MSL west of the mountains. Most CIGS will
be intermittent, especially away from the inland valleys. VIS
unrestricted below the cloud bases, but lower coastal slopes of the
mountains will be obscured. Clouds clearing 15-17Z. Patchy stratus
with bases near 1000 ft MSL possibly developing after 21/09Z, but
confidence is low.

Mountains/Deserts...Mostly clear with unrestricted vis through
Saturday morning. W/NW surface winds gusting to 25-35 kt through the
passes and locally in the deserts gradually weakening through the
morning. Pockets of MOD UDDFS and LLWS possible over/east of the
mountains through about 12Z.


No hazardous marine weather expected through Tuesday.


Weak offshore flow this morning will mainly bring local gusts to
around 30 MPH below Cajon Pass and a few mountain crests. Minimum
humidity will mostly be 15-20 percent in the far inland valleys
and mountains, but this will mostly occur after the peak of the
wind. Min humidity in the valleys will be in the teens over the
weekend, but with only light breezes.


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






Previous Forecast Discussions may be found at
NWS San Diego, CA (SGX) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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