Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 241103

Area Forecast Discussion
National Weather Service San Diego CA
400 AM PDT Sat Oct 24 2020

It will be cooler today with a mix of clouds and limited sun as a
weak trough off the coast is pulled inland ahead of a colder and
deeper trough dropping south over the Great Basin. The marine layer
will continue to deepen as gusty westerly winds increase with these
systems. Some light precipitation will break out along and west of
the mountains tonight, with the best chance on Sunday into Sunday
night. Moderate to locally strong Santa Ana winds are expected
Monday into Tuesday morning, with fair and warmer weather to follow
next week.



Satellite imagery still showed a considerable amount of high clouds
over SoCal at 2 AM PDT. This was blocking any monitoring of lower,
marine clouds, but sfc observations indicated they were widespread
west of the mts as well. The sfc pressure gradients were fairly
weak, but west wind reports still had gusts to 40+ MPH through and
below the desert passes. Expect a mix of low clouds and high clouds
and limited sun today, which will keep daytime temperatures a bit
below those observed yesterday.

A weak trough off the coast will be drawn inland today and tonight,
along with an increase in atmospheric moisture. Prompting this
advance is a stronger and colder trough moving south over the Great
Basin. This system will continue to evolve through Monday night,
with a closed low pressure center forecast over SW AZ Tue morning.
The low will force strong NE flow over SoCal, especially along and
below the transverse mtn ranges. This has prompted a High Wind Watch
over northern portions of the SGX forecast area including the I.E.
and O.C. Cold and dry continental air will be forced over the mts,
and southwestward toward the coast on Monday, resulting in very low
RH. The combination of dry air and gusty winds has prompted the
issuance of a Fire Weather Watch for these areas as well. See Fire
Weather discussion below for more detail. Ahead of the dry push from
the NE will be sufficient moisture and lift for scattered, light
precipitation along and west of the mts from tonight through Mon
morning. See Hydrology discussion below for detail.

The storm center over AZ will drift east next week as high pressure
aloft over the EastPac expands over CA. This will bring an extended
period of warm days and cool nights into next weekend. A shallow
marine layer will slowly rebuild along the coast after midweek,
moderating the relative humidity over coastal areas and returning
some patchy night and morning low clouds and fog.


240830Z...Coast/Valleys/Mountains...Low clouds with bases 2500-3000
feet MSL will continue to fill into the entire coastal basin this
morning. Vis at terminals will remain 6+ miles, but vis could be
reduced to 3-5 miles where clouds and terrain intersect. Very slow
clearing expected today, with valleys clearing around 19-20Z and
coastal areas after 21Z. Low clouds will likely stick around some
coastal areas through the afternoon.

Deserts...SCT-BKN high clouds at/above 12,000 feet MSL through today.


No hazardous marine conditions are expected through Sunday.
Northeast winds could locally gust to 25 knots Monday and Monday
night which could bring hazardous small craft conditions.


...Fire Weather Watch late Sunday night through Tuesday...
Increasing onshore flow and RH will ease fire weather concerns this
weekend. Some light precipitation will help the effort as well, and
may exceed one-tenth of an inch in some areas of San Diego County
along and west of the mountains.

Sunday night into Monday, an abrupt wind shift will steer gusty
northeast winds across the area from north to south. Northeast winds
of 30 to 45 MPH with isolated gusts 60 to 70 MPH will drive very dry
air toward the coast with RH of 5-10%. Strongest winds will be over
the Santa Ana Mts and canyons, in the Inland Empire below the Cajon
Pass, and below the Morongo Pass. the winds are expected to ease on
Tuesday but will still be strong and gusty in the morning with
little overnight RH recovery.


A weak low pressure system from the west will bring scattered
showers from the coast to the mountains late Saturday night into
Sunday as the marine layer deepens. At this time, it appears amounts
will be less than one-tenth inch from the coast to the foothills.

Greater energy from the north could spark heavier showers along and
west of the mountains late Sunday into Monday morning. Amounts look
to be light, but greater instability could generate a few heavier
showers. Right now the flow favors San Diego County with a greater
share of rain. For areas west of the mountains and north of San
Diego County, rainfall totals are forecast to be one-tenth inch or
less, limiting any meaningful runoff. For San Diego County, totals
are forecast to range form one-tenth to one-quarter inch, with up to
one-half inch possible on the coastal mtn slopes in spots.


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


CA...Fire Weather Watch from late Sunday night through Tuesday
     afternoon for Orange County Inland Areas-San Bernardino
     County Mountains-Including The Mountain Top And Front
     Country Ranger Districts Of The San Bernardino National
     Forest-San Bernardino and Riverside County Valleys  -The
     Inland Empire-Santa Ana Mountains-Including The Trabuco
     Ranger District of the Cleveland National Forest.

     High Wind Watch from late Sunday night through Monday afternoon
     for Orange County Inland Areas-San Bernardino County
     Mountains-San Bernardino and Riverside County Valleys-The
     Inland Empire-Santa Ana Mountains and Foothills.




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