Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 231055
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
355 AM PDT Thu Sep 23 2021

.SYNOPSIS...
A low pressure system developing over northern Baja will bring a
cooling trend into the weekend with chances for afternoon showers
and thunderstorms in the mountains, deserts, and inland areas
through Saturday. The cooling trend will continue into the middle
of next week as another low pressure system moves inland to the
north. Coastal low clouds will return the next few nights and
spread farther inland into portions of the inland valleys early
next week. Smoke from fires to the north will spread into portions
of southern California today and continue into Friday.

&&

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

.SHORT TERM (Today through Saturday)...
A weak closed low pressure system will develop over northern Baja
on Friday and not move much through Saturday. This will bring a
cooling trend into the weekend with high temperatures today for
the coast and valleys 5 to 10 degrees cooler and a few degrees
cooler for the mountains and deserts.

There is a slight chance of showers and thunderstorms for the
mountains into the deserts for this afternoon. Uncalibrated output
for the ECMWF ensembles for Big Bear are quite dry with the
wettest member only having rainfall of around one tenth inch.

Most of the ECMWF ensemble members have at least some measurable
rainfall for Big Bear on Friday afternoon, but with greater spread
in the amounts. The general spread in amounts doesn`t chance much
for Saturday but with not as many members having measurable
rainfall.

Beyond the cooling and precipitation chances, smoke from fires to
the north will spread southward into portions of southern
California and continue into Friday.

Satellite imagery shows increasing coverage of coastal low clouds
beyond the offshore islands, but still very patchy over the
coastal waters. Coverage should increase somewhat near the coast
over the next few hours with greater coverage the next two nights
over the coastal waters and adjacent coastal areas, possibly
extending into portions of the western valleys.

&&

.LONG TERM (Sunday through Wednesday)...
On Sunday, the low pressure system over northern Baja will begin
the move east as a low pressure system moves across the eastern
Pacific toward the West Coast. There are some relatively minor
differences in the eastward movement of the Baja low out of the
area on Sunday with small remaining precipitation chances on
Sunday afternoon for the slower solutions.

That trough of low pressure to the north will move inland around
Tuesday with some model differences in the timing and amplitude.
For southern California, this will continue the cooling trend into
the middle of next week with high temperatures on Wednesday around
5 degrees below average to as much as 10 degrees below average
for some mountain and inland valley locations.

Onshore flow will be stronger across southern California with
night and morning coastal low clouds and fog spreading into
portions of the inland valleys.

&&

.AVIATION...
230900Z...SCT-BKN100-200 through this evening. AFT 20Z ISOLD
-SHRA/-TSRA with bases FL100, mainly near the mountains. Coastal
areas AFT 06Z FRI...SCT-BKN007 with LCL VIS 3-5SM BR.

&&

.MARINE...
No hazardous marine weather through Sunday. Wind gusts to 20 kts
over the Outer Waters late this afternoon.

&&

.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...17
AVIATION/MARINE...JAD


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