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Area forecast discussion
National Weather Service San Diego California
920 am PDT Thu Sep 21 2017

Synopsis...
a trough of low pressure will drop into Southern California today
and linger through Saturday. Today it will bring cooler and
cloudier weather, some showers from the coast to the mountain
crests, and areas of strong winds in the mountains and deserts. As
the trough departs over the weekend, offshore flow will develop
to bring clear skies and a warming trend next week.

&&

Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
counties...

...

Water vapor loop shows a trough digging south across California. A
narrow moisture axis ahead of this trough will move south across
Southern California today. Precipitable water on the morning San Diego sounding
was 0.90 inches, up from 0.76 inches last evening. Precipitable water could rise
to near 1 inch this afternoon, so nothing significant but enough
low level moisture below about 6,000 feet for shower development
ahead of the trough in upslope flow.

Light showers are developing and hi-res models show showers
continuing to develop and focusing along the coastal mountain
slopes as well as near the coast. Have adjusted probability of precipitation upward and
made a slight uptick in quantitative precipitation forecast. Rainfall amounts will be light...less
than a tenth of an inch, except 0.25-0.60 inches along the
coastal slopes with maxima along the San Bernardino coastal
slopes. Crestline has received 0.12 inches of rain so far this
morning, the maximum in our service area so far.

Winds are on the increase out ahead of the trough. Max gusts are
in the 35-45 mph range and expect this to continue. Wind prone
locales such as Whitewater likely will hit 50+ mph this
afternoon/early this evening before winds taper off tonight. A
Wind Advisory continues in effect from the mountain crests to the
desert floor. Patchy blowing dust and sand, creating restrictions
in visibility, will be found across the deserts.

..previous discussion (issued 230 am PDT Thu Sep 21 2017)...

A big trough of low pressure, quite unseasonal for the last day
of Summer, will drop into Southern California today. Clouds have
already filled the coastal basin pretty much to the brim, that
is, to the mountain crests. A couple spots on the coastal slopes
have already begun to measure a few hundredths of rain. Later this
morning, a more organized band of legit showers will form along a
surface wind discontinuity over the ocean, meaning some uplift
will be generated along a line where southerly winds meet with
westerly winds. This band of showers will bring light rainfall:
below a tenth inch for most everywhere, but some foothill
locations such as near Crestline could harvest a third inch or
more. Winds strengthen this afternoon and evening. Strongest winds
will occur through mountain passes and east into adjacent deserts
with top gusts over 45 mph. Temperatures will be in the low 70s
for most of the low elevations west of the mountains today, 15-20
degrees below average for the inland valleys. Residual moisture
and onshore flow will keep skies partly cloudy and temperatures
well below normal for Friday. A secondary shortwave within the big
trough over the west will slow down the trough such that it won't
really move out until Sunday. The surface flow will gradually go
offshore and that will help clear the area of clouds over the
weekend. Northeast winds will get started Sunday and appear each
morning through at least Tuesday. Local gusts below Cajon Pass and
in the Santa Ana Mountains could exceed 40 mph each morning, but
overall this will not be a widespread or strong offshore event.
This Fall weather comes right on schedule as if the atmosphere
knew it was time to start acting like fall. Temperatures will
climb each day next week, topping 90 in a few spots by Monday,
with 90s more common in the following days. Clouds will be hard
to come by next week. The clear skies and dry air will allow temps
to drop at night, providing some relief from the hot days.

&&

Aviation...
211530z...coast/valleys/coastal slopes...widespread stratus over the
coast, valleys, coastal slopes and mountain passes, with highly
variable bases of 2500-4000 ft msl, tops of 5000-6000 ft msl, areas
of -dz/isol -shra producing local vis/cigs of 2-5 sm/1000-1500 ft
msl, and mountain obscuration of coastal slopes below 6000 ft msl,
will continue for the most part through 22/0000 UTC, although cloud
coverage may become broken-scattered after 1800 UTC with -dz/-shra likely
ending by 2100 UTC. 22/0000-1500 UTC, broken-overcast stratus filling the
coast, valleys and coastal slopes, with bases 2500-3500 ft msl, tops
4500-5000 ft msl and mountain obscuration of coastal slopes below
5000 ft msl.

Mountain ridges/desert slopes/deserts...mostly sky clear with p6sm vis
through tonight. Through 1800 UTC, west winds 15-25 kt with gusts 35
kt with mod-stg uddfs/llws. Winds will strengthen after 1800 UTC
with gusts to 45 kt possible below the passes, on desert slopes, and
near the desert foothills, resulting in stg uddfs/llws as well as
surface wind gusts at kpsp and ktrm possibly reaching 35 kt at
times. Blowing dust & sand this afternoon and evening in the deserts.

&&

Marine...
a Small Craft Advisory will be in effect for the outer coastal
waters this afternoon through late tonight as a trough of low
pressure creates moderate to strong onshore flow. Northwest wind
gusts of 25 kt are likely in the outer waters, while the inner
waters will see gusts mainly in the 15-20 kt rang (which is below
Small Craft Advisory criteria). At the same time, there will also be
short-period seas of 4-7 ft. Although winds will decrease early
Friday morning, another increase in winds is expected late Friday
afternoon with wind gusts in the outer waters mainly around 20 kt.
Lighter winds and smaller seas are forecast over the weekend.

&&

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




&&

Sgx watches/warnings/advisories...
California...Wind Advisory until 1 am PDT Friday for apple and Lucerne
valleys-Coachella Valley-Riverside County mountains-San
Bernardino County mountains-San Diego County deserts-San
Diego County mountains-San Gorgonio Pass near Banning.

Pz...Small Craft Advisory until 1 am PDT Friday for waters from San
Mateo Point to the Mexican border extending 30 to 60 nm out
including San Clemente Island.

&&

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