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Forecast Discussion for San Diego, CA

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034
FXUS66 KSGX 230950
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
250 AM PDT Mon Sep 23 2019

.SYNOPSIS...
A low pressure system will bring a little cooler weather today.
The low will stall over northern Baja Mexico Tuesday and
Wednesday. This will bring some offshore winds to foothills and
adjacent valleys Tuesday, and hot weather to the Inland Empire.
It will also bring some moisture for a small chance of showers
Wednesday through Friday. A deeper trough of low pressure sets up
this weekend for much cooler weather.

&&

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

Welcome to fall. The weather for the first week of the season is
very complex and messy, with a lot of chaotic moving parts. A low
pressure trough is dropping south through eastern California
today. It will cut off and separate from the jet stream tonight
and Tuesday, dropping into northern Baja California. There it will
sit and spin through Wednesday before finally deciding to weaken
and gradually lift out Thursday and Friday. The marine layer is
the first to be influenced by this low as low clouds are spreading
farther inland this morning, but with somewhat patchy coverage.
Onshore winds will increase as the low settles over our region
with some mountain/desert gusts over 30 mph. The counter clockwise
flow around this compact low will bring offshore flow Tuesday.
Northeast winds look strongest in the San Bernardino Mountains
where gusts will exceed 30 mph. Those offshore breezes will crank
up temps to near 100 in the northern Inland Empire. San Diego
County will not feel the effects of the offshore flow and heat as
much. With the low in northern Baja Tuesday, it will tap into some
of that tropical remnant moisture from Sonora and Arizona. That
will bring some clouds and an increasing chance of showers
Wednesday. Even a small chance of thunderstorms to mountains and
lower deserts. The moisture drifts across the entire region
Wednesday night and Thursday, so there`s a small chance of showers
in all areas then. There is enough residual moisture Friday as
onshore flow increases, leading to a small chance of showers from
the coast to the mountains on Friday. Over the weekend a very deep
and unseasonable trough of low pressure amplifies along the West
Coast, bringing much cooler weather with temps 10-20 degrees below
average. That`s assuming the trough develops the way models
depict today. The trough will also boost onshore winds quite a bit
in the mountains and deserts. I told you it would be messy.
Adding to the mess is the level of confidence in showers happening
for any particluar time at any particular location: it`s low.
We`ve got a little more confidence in winds and temperatures. Stay
tuned to forecast updates and don`t be surprised if the forecast
changes from day to day with all the chaos going on.

&&

.AVIATION...
230850Z...Coast/Valleys...Patchy stratus with bases 600-1000 ft MSL
will continue to form along portions of the coast and up to 15 mi
inland this morning. VIS locally reduced to 2-4 SM where clouds near
higher terrain in the valleys. Clouds clearing to the coast 16-18Z,
then spreading back inland after 24/01Z with bases near 1000 ft MSL.
Otherwise, a few high clouds and unrestricted vis through Tuesday
morning.

Mountains/Deserts...A few high clouds and unrestricted vis through
Tuesday morning.

&&

.MARINE...
No hazardous marine weather is expected through Friday.

&&

.FIRE WEATHER...
Onshore breezes this afternoon and evening will coincide with some
low RH values where the mountains drop into the deserts. Offshore
flow will develop Tuesday to produce dry northeast winds in the
San Bernardino Mountains and in all foothill areas. By Tuesday
afternoon, RH could fall into the single digits with temps
approaching 100. Winds will decrease as the RH decreases Tuesday.
Onshore winds will return later this week, possibly becoming
strong during afternoons and evenings Friday, Saturday and Sunday.
This trough will bring higher RH from the mountains westward.

&&

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

&&

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

&&

$$

PUBLIC/FIRE WEATHER...MM
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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