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232
FXUS66 KSGX 190415
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
915 PM PDT Wed Sep 18 2024

.SYNOPSIS...
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A low pressure system from the northwest will bring chances for
light precipitation tonight into Friday with more clouds from the
coastal waters to the mountains. A ridge building in from the west
will bring dry and much warmer weather this weekend into the
middle of next week with high temperatures returning to a few
degrees above normal.
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&&

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

Low clouds have rapidly developed across portions of the coastal
areas and valleys and will continue to spread inland overnight,
reaching the coastal mountain slopes by Thursday morning. The 00Z
KNKX sounding shows the cloud deck around 1,000 ft deep. This is
forecast to thicken overnight as an upper level low continues to
dig south along the Central CA coast. With weak difluence aloft
and a deepening moist boundary layer, there is a 20-30% chance of
light showers for the coastal areas, valleys, and coastal slopes
of the mountains Thu morning. Any accumulation will generally be
less than one-tenth inch on Thu. Otherwise it will be a chilly day
with highs 10-15 degrees below normal inland.

The center of the upper low slowly moves east from near Point
Conception to Riverside/San Bernardino Counties Thursday evening
into Friday afternoon, with the main trough axis moving through
our forecast area during the day Friday. This will generate
another round of light to moderate showers across the area. As
colder air aloft with the center of the low moves over,
instability will increase. Both the EC and GEFS ensembles have
surface based CAPE across portions of Riverside and San Bernardino
Counties Friday afternoon, and NBM thunderstorm probabilities are
around 10-20% in these areas. Latest runs of hi-res guidance do
show heavier showers in these areas with isolated rain rates of
0.30-0.50 inch/hr. Showers will come to an end by Friday evening
as the low moves out of the area.

Gradual warming takes place this weekend as we are wedged between
weak troughing across the interior West and upper level ridging
off the West Coast. Ensembles are in fairly good agreement through
about Monday, then uncertainty over the patter evolution begins,
which would mostly have implications on the temperature forecast
and marine layer depth/cloud cover extent. High temperatures will
gradually rise to near normal by Sunday, then around 3-6 degrees
above normal by Monday. NBM temperatures beyond Monday are
generally within 3 degrees of normal for the coast to the
mountains, and around 3-5 degrees above normal in the deserts.
This means highs will be back in the upper 70s along the coast,
80s for inland coastal areas/western valleys, low to mid 90s in
the inland valleys and high desert, 70s and 80s in the mountains,
and back over 100 degrees in the low deserts.
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&&

.AVIATION...
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190330Z...Coast to mountain ridges...Low stratus has filled into San
Diego County up to the coastal mountain slopes and into southern
Orange County at this hour, but will continue to gradually fill in
across the rest of Orange County and into the Inland Empire over the
next several hours. Bases are initially near 2500ft MSL (and topped
to 3000ft MSL), but will gradually rise through the overnight hours
to 3000-3500 ft MSL with tops to 3600-3800ft MSL. Local terrain-
based VIS obscurations to 1-3SM within that layer. Patchy light
rain/drizzle is possible along the coasts and up against the coastal
mountain slopes between 10-18z. By 16-18z Thursday morning, clouds
scatter for most of the inland areas but could linger along the
coasts and mountain slopes, bouncing between BKN-SCT at times, still
generally remaining above MVFR levels though. Any clouds that
cleared in the morning will fill back in Thursday evening with
similar bases/tops.

Mountain ridges to Deserts... Mostly clear skies with unrestricted
VIS through Thursday evening.
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&&

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

&&

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

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

&&

$$

PUBLIC...SS
AVIATION/MARINE...Munyan