Area Forecast Discussion
Issued by NWS San Diego, CA

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000
FXUS66 KSGX 211056
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
356 AM PDT Wed Oct 21 2020

.SYNOPSIS...
There will be a cooling trend into the weekend with night and
morning coastal low clouds and fog spreading farther into the
valleys. A weak low pressure system from the west could bring some
light precipitation from the coast to the mountains for Saturday
into early Sunday. A colder low pressure system from the north
could bring additional precipitation into early Monday with much
cooler and well below average high temperatures for Sunday and
Monday. A period of moderate strength Santa Ana winds is possible
for late Monday into early Tuesday as the low pressure system
moves east.

&&

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

.SHORT TERM (Wednesday through Friday)...
Through Friday, high pressure aloft will slowly weaken with a weak
coastal eddy at times. The marine layer will slowly deepen with
night and morning coastal low clouds and fog spreading farther
inland into the valleys and onto the lower coastal mountain slopes
by the weekend. Cooling will continue to spread inland with
Friday high temperatures around average for the mountains and
deserts and around 5 degrees below average for the coast and
valleys.

&&

.LONG TERM (Saturday through Tuesday)...
A weak low pressure system from the west will move inland across
far southern California and northern Baja some time late Saturday
into early Sunday. This will bring a chance of mainly light
precipitation at times from Saturday morning through early Sunday
from the coast to the mountains.

A cold low pressure system moving southward through the western
states may become a closed low pressure Monday into early Tuesday
somewhere in the southwest states with the deterministic ECMWF and
ECMWF ensemble mean having the center over southern California on
early Tuesday. Over the past 24 hours, the guidance has trended in
that direction and the large spread in the guidance has narrowed
somewhat over the past 24 hours.

The cooling should continue into Sunday or Monday. High
temperatures on those days should be well below average, as much
as 15 to 20 degrees below average for some mountain, inland
valley, and high desert locations. With the cooling, low
temperatures on early Tuesday morning could fall into the 40s in
the valleys, 30s in the high desert, and 20s in the mountains.

A track more to the west as that low pressure system moves
southward through the western states would have greater potential
to entrain moisture and bring additional precipitation for Sunday
into early Monday while a track farther to the east could keep
southwestern California largely dry for late Sunday into early
Monday.

As that low pressure system begins to move east, there is the
potential for a period of moderate strength Santa Ana winds for
late Monday into Tuesday. Of the 50 ensemble members of the ECMWF,
about 20 percent support moderate to strong winds with about 20
percent supporting weak winds.

&&

.AVIATION...
210915Z...Coast/Valleys...Low clouds with bases 1200-1600 feet MSL
and tops around 2000 feet will continue move into parts of the
inland valleys through 13Z. Vis 0-5SM between those elevations on
higher coastal terrain. Scatter out 16-19Z, though locally BKN
clouds may persist near the coast through the afternoon. Clouds will
begin to push back inland after 01Z this eve with similar
bases/tops/vis spreading farther inland tonight into Thursday.

Otherwise...mostly clear skies today and tonight.

&&

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

&&

.FIRE WEATHER...
With onshore flow and slow deepening of the marine layer, cooling
and higher coastal humidity will spread inland into the weekend. A
weaker low pressure system from the west will bring a chance for
mostly light precipitation from the coast to the mountains for
Saturday into Sunday. For Sunday into early Monday, a colder low
pressure from the north will bring more cooling for Sunday and
Monday and chances for additional precipitation. As that low
pressure system begins to move east, a period of moderate strength
Santa Ana winds is possible for late Monday into Tuesday. There
are much smaller chances for weak winds or moderate to strong
winds.

&&

.HYDROLOGY...
There are increasing chances for at least light measurable
precipitation some time during the weekend from the coast to the
mountains, but with less confidence on the timing and relative
amounts. Any amounts for Saturday into early Sunday are expected
to be light. A cold low pressure system from the north could bring
additional precipitation for Sunday into early Monday but with a
wider spread in whether the precipitation would occur and in the
amounts if it did.

&&

.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/HYDROLOGY...17
AVIATION/MARINE...MM


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