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Southwest California area forecast discussion...updated 
National Weather Service Los Angeles/Oxnard California
440 PM PDT Monday may 4 2015

..updated aviation discussion...

Synopsis...

Night through morning low clouds and fog will continue for the coast
and valleys through midweek with onshore flow and weak low pressure.
Another low will move in Thursday and Friday with showers possible.
Temperatures will remain below normal through the week. High
pressure will briefly bring fair and warmer conditions for the
upcoming weekend.

&&

Short term (tue-thu)...
broad troughing over the western U.S. Will keep below normal
temperatures across the area through this period and prior to a
deeper low that dives south out of British Columbia and into central
calif later on Thursday. A deep marine layer between 2500 and 3000
feet brought about by moderate to strong onshore flow will keep
overnight low clouds through coastal and valley areas each night.
Expect the sba South Coast to also fill in the next couple of nights
given offshore pressure gradients slacken just enough. Skies should
clear out the beaches leading towards each midday period...however
some areas may struggle to do so...especially areas south. Another
result of this weather pattern will be continued gusty afternoon and
evening southwest winds across the Antelope Valley. Temperatures
will gradually cool over most areas the next few days to help make
Thursday the coolest day of the forecast. High temperatures will be much
below normal...as much as 15-20 degrees or more below normal some
areas. Only far northern slo County will experience highs over 70 on
Thursday.

Models agree on dropping the closed upper low just north of the area
by 5 PM Thursday. Slight chances of precipitation start Thursday
morning over Los Angeles County and north into the Ventura County
mountains as models indicate some instability with the approach of
cooler middle level temperatures. Precipitation chances increase slightly these areas
Thursday afternoon. Have also added a slight chance of
thundershowers for the Ventura County mountains Thursday afternoon
with model lifted indicies of -3 to -4 degree c indicated. Negative
lifteds are also indicated for valley areas...the forecast model
sounding for lax valid Thursday 5 PM indicates a -4 lifted...but could
not quite pull the trigger for any thunderstorm- down in the valleys. And as
with any low that takes this path right along or just inland of the
coast...moisture will be difficult to come by so not expecting much
in the way of measurable precipitation through Thursday night.

Long term (fri-mon)...
the best chances of precipitation appear at this time to be Thursday
night into Friday morning. These include scattered areas with
chances north of Point Conception when the low is at its closest
proximity to the area and most areas will benefit from some lift
with the low. Increased probability of precipitation a bit from the previous forecast but
kept below likely for now. Just not seeing any strong enough lift
indicators in the models quite yet.

The closed low should be situated over Vegas by midday Friday with
height rises over our area. Fine tuned the precipitation
probabilities for Friday...keeping best chances (although still not
too high) over l.A. County Friday and eliminating them north of pt
Conception. This is contrary to the European model (ecmwf) which indicates higher
probability of precipitation across most of the area due to a vorticity lobe it swings around the
low during the day Friday on top of the fact it keeps the low a
little further west...but ensemble precipitation forecasts do not
favor this scenario so went with a north to south drying trend. The
GFS..which is further east...also appears to be more consistent over
its last several runs.

Do not expect any significant precipitation totals with this system
at this time. Generally any area that does receive precipitation
should not get any more than a quarter of an inch with under a tenth
or even less more common.

A shortwave ridge moves in for Saturday and Sunday to bring warming
and drying each day. By Sunday high temperatures are expected to be
closer to just above normal most areas...meaning 70s coasts and low
80s inland valleys. Extended models each show some degree of
troughing to our west on Monday but at this time not close enough to
bring any cooling outside of possibly a few degrees cooling northern
slo County.

&&

Aviation...04/2335z...

At 2330z...the marine inversion was based at 2300 feet. The top of
the inversion was 4200 feet with a temperature of 14 degrees celsius.

Strong onshore flow with the deep marine layer is expected to bring
MVFR ceilings to coastal/valley taf sites overnight. Confidence in current
timing of ceiling arrival is moderate with 00z taf package. Trending towards
a slower clearing time on Tuesday...with a chance that ceilings could linger
across some coastal taf locations through entire afternoon. Gusty
southwest winds will continue at kpmd and kwjf through Tuesday.

Klax...high confidence in MVFR ceilings returning at some point
tonight...but only moderate confidence in timing of ceiling arrival.
Expecting slower clearing time on Tuesday...with a 20 percent chance
that ceilings could linger through entire afternoon.

Kbur...high confidence in MVFR ceilings returning at some point
overnight...but only moderate confidence in timing of ceiling arrival.
Expecting slower clearing time on Tuesday.

&&

Marine...04/205 PM...

For the outer waters...moderate confidence in winds increasing to
small craft levels late this evening. High confidence in in Small
Craft Advisory levels winds from Tuesday through Thursday.

For the inner waters...moderate confidence in west to northwest
winds remaining below Small Craft Advisory levels through the week.
There is a chance of winds increasing to Small Craft Advisory levels
across the western sections of the inner waters Tuesday through
Thursday.

A long period (18-20 second) southerly swell will continue across
the coastal waters through Tuesday...then slowly subside the second
half of the week.

&&

Lox watches/warnings/advisories...
high surf advisory (see laxcfwlox).
Beach hazard statement (see laxcfwlox).
Small Craft Advisory (see laxmwwlox).

&&
$$

Public...Jackson
aviation...gomberg
marine...rat
synopsis...sirard

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