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Forecast Discussion for San Diego, CA

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000
FXUS66 KSGX 250427
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
927 PM PDT Thu Sep 24 2020

.SYNOPSIS...
Seasonal weather will continue through Saturday, with areas of
night and morning low clouds along the coast and into some western
valleys. High pressure aloft will strengthen Sunday and continue
most of next week and bring hot weather to the region. On Sunday,
locally gusty northeast to east winds will occur over the
mountains and into some of the valleys, especially in San Diego
and Riverside Counties. Low humidity will continue over inland
areas for at least the next week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

...Update...

Low clouds lie along the San Diego County coast this evening and
could develop northward overnight. Some patchy fog is possible
where the low clouds intersect terrain on the mesas. Otherwise,
tranquil weather will prevail.

The main weather story will be the heatwave which will impact
SoCal beginning on Sunday as an upper ridge builds into the region
and winds turn offshore. The peak of the heat will be Monday-
Wednesday next week. With this comes fire weather concerns as the
airmass will be hot and dry. The coast will continue to see
humidity moderated with onshore winds there, but the inland
valleys and mountains will see minimum relative humidity next
week 10-15% in a hot airmass. Offshore east-northeast winds will
be strongest in the Sunday-Monday timeframe and perhaps strongest
Monday morning per EC. How strong is uncertain but gusts to at
least 30 mph will be possible through passes and canyons then.

...Previous Discussion (Issued at 144 PM PDT Thu Sep 24 2020)...

At 1 PM, skies were mostly clear across SoCal. Temperatures were
at or near where they were at this time yesterday. Low clouds will
redevelop later this evening and overnight, covering most coastal
areas by daybreak.

Fair weather will continue through Saturday, with temperatures
within a few degrees of normal for Friday and Saturday. A trough
of low pressure to the north will strengthen onshore flow and
deepen the marine layer through Saturday. Low clouds will be
mainly confined to coastal areas, but could reach into portions of
the western valleys. Clouds will become more patchy Sunday
through much of next week as high pressure builds into the region.

The big story for the next week is the upcoming heat and dry
conditions. High pressure will begin to strengthen and expand
across the West on Sunday and continue through at least the middle
part of next week. Northeast to east winds will develop over the
mountains and foothills Sunday through Monday morning, with gusts
of 20 to 30 mph. Some locally higher gusts are possible in the
windiest locations. Relative humidity will plummet in the deserts,
mountains and portions of the inland valleys on Sunday, with
minimum daytime humidity of 10 to 15 percent for much of next
week. The heat will begin to build in on Sunday, peaking Tuesday
and Wednesday, with temperatures 10 to 15 degrees above normal for
most locations away from the coast. Coastal areas will still have
somewhat of a marine influence, but temperatures should be around
5 to 10 degrees above normal.

Some differences exist between models and their ensembles on the
placement and strength of the high for the end of next
week. Temperatures may cool a few degrees late next week,
depending on the placement of the high, but still remaining above
normal through all of next week.


&&

.AVIATION...
250340Z...Coast/Valleys...Areas of low clouds along the coast with
bases 700-900 ft MSL will gradually spread north and up to 15 mi
inland overnight, possibly lifting to 800-1100 ft MSL after 10Z.
CIGS likely reaching KSAN and KCRQ by 05Z and KSNA by 10Z. Areas of
3-5 SM VIS in BR/HZ likely on higher coastal terrain, locally down
to 1 SM or less. Clouds and vis restrictions clearing 15-17Z.
Stratus will begin to spread back inland after 26/00Z with bases
near 1000-1400 ft MSL.

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

&&

.MARINE...
No hazardous marine conditions are expected through Tuesday.

&&

.FIRE WEATHER...
Lowest daytime humidity in the deserts and the desert mountain
slopes will fall to 10 to 15 percent each day through Saturday.
Combined with the west to southwest winds locally gusting around
25 to 30 mph, there likely will be a few hours of elevated fire
weather conditions in the deserts onto the desert slopes of the
mountains each day from mid afternoon through early evening.

High pressure over the western states will bring significant
warming for Sunday through at least the middle part of next week
with high temperatures 10 to 15 degrees above normal. Humidity
will fall to 10 to 15 percent each day in the mountains, deserts,
parts of the Inland Empire and far inland San Diego County
valleys. Gusty northeast to east winds will occur Sunday through
Monday morning in the mountains and far inland valleys, with
gusts mostly 30 mph or less, but higher gusts are possible in the
windiest locations. The combination of low humidity and gusty
winds will result in near critical fire weather conditions Sunday
and again Monday, mainly in the mountains. Elevated fire weather
conditions will exist for most of next week, due to the hot
temperatures and low relative humidity.

&&

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


&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

$$

Update...Gregoria
Previous Discussion/Fire Weather...Connolly
Aviation/Marine...SS

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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