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Southwest California area forecast discussion...updated 
National Weather Service Los Angeles/Oxnard California
355 am PDT sun Apr 19 2015

..updated aviation and marine sections...

Synopsis...
expect a steady cooling trend through the middle of the week with
low clouds and fog in the coastal and valley areas. There will be a
slight chance of thunderstorms over some of the mountains early in
the week. Persistent low pressure off the coast will bring a slight
chance of showers to much of the area by Wednesday then persist into
Thursday or Friday.

&&

Short term (today-tue)...latest fog product imagery indicated low
clouds across most all coastal waters as well as the central coast
and the Santa Ynez Valley. Only patchy dense fog reported in the
Santa Ynez Valley at this time. For the coastal areas of Santa
Barbara and Ventura counties...low clouds were much patchier in
nature...and for now los angels County coast was clear as well as
all the valleys S of Point Conception. A localized north wind has
kept stratus off the coast...but would not be surprised if there
area some patchy low clouds developing over the la coast around
dawn.

Synoptically...a 568 dm cutoff low will be around 300 nm west of San
Francisco today. This upper low will pivot south sitting 300 nm west-southwest
of San Diego by Monday afternoon...then continue to move over
southern cal on Tuesday. This approaching upper low will bring some
instability across the local mountains today and tomorrow. There
will be limited middle level moisture today... which should allow for
just some afternoon cumulus build ups over Ventura/sba County
mountains...with only a few clouds over the la County mountains
otherwise...for today...the onshore flow will strengthen some from
yesterday which should allow for low clouds to linger across the
central coast into the afternoon hours. A bit tougher marine layer
cloud forecast for the la/vtu/sba coastal areas this morning...but
expect low clouds to persist off the coast today. Temperatures will
continue to trend cooler as a weak troughy pattern sets up over the
area and the continued onshore flow. Expect some gusty sub advisory
level winds across the Antelope Valley today.

By tonight...low clouds should become more widespread and deeper
ahead of the approaching upper low. Expect all coast and coastal
valleys to experience low clouds and patchy fog tonight into Monday
morning. The NAM/GFS and ec models continue to show a pooling of middle
level moisture across the Ventura/sba County mountain areas on Monday.
The instability is pretty impressive with lifted index values
between -3 and -5 over the Ventura County mountains...with NAM/WRF
sounding indicating cape of near 1600 j/kg. Quite impressive. Precipitable water
values will be around .70" which are not impressive. Also...newer
model runs now show a stronger southeast mean steering wind between 700-500
mb over the area. Therefore if any thunderstorms do develop...they
will likely not be sitting in one place too long which will help to
prevent any flash flooding. However...there could be some localized
Road flooding in the Ventura/sba County mountains...and small hail is a
possibility. It still looks fairly dry over la County...but there
should be better cumulus buildups over the eastern San Gabriel Mountains once
again...expect hi temperatures to only reach the 60s across coastal areas
while valleys are only in the lower to middle 70s in most inland
areas...except near 80 in the Antelope Valley.

Not much change for Tuesday in respect to night through morning low
clouds at the coast and lingering around the beaches in the
afternoon. And another afternoon of slight chance thunderstorm
possibilities for the Ventura /sba County mountains. Although the
models continue to keep the middle level moisture away from the la
County mountains...the position of the upper low might warrant a
slight chance for Tuesday afternoon. Will let later shifts look more
closely at this potential.

Long term (wed-sat)...
there were some significant changes to both the GFS and ec models
with respect of the positioning of the secondary upper low that was
expected to swing in over central and Southern California Wed/Thu.
It now pivots the low well off the coast and then turns in to Baja California
California. Therefore have backed down probability of precipitation to slight chance for the most
part and raised snow levels to above 5500 to 6500 feet. If any showers
do occur...they will be fairly light. The threat of thunderstorms
will not be an issue for coast and valleys as the cold core of the
upper low remain off the coast of so cal. However...there will
continue to be lingering middle level moisture over the mountains and
an isolated thunderstorms are not out of the question on Wed/Thu.
But kept them out for now. As the upper low pivots to the SW of Los
Angeles...some wrap around moisture will be possible...so Thursday will
be the best chance for showers across inland la/vtu counties. It now
looks like Friday/Sat will be less cloudy...but continue with partly
cloudy wording for both Fri/Sat. There will be a modest warm up
Fri/Sat...but still remain just below or around normal for this time
of year.

&&

Aviation...19/1100z.

At 0845z...the marine layer at klax was just under 1000 feet deep.
The top of the inversion was located at 2400 feet...and the
temperature at the top of the inversion was 18 degrees celsius.

Widespread stratus in coastal areas from vtu County northward...with
mostly IFR conds...except local vlifr conds in patchy dense fog.
Clouds with IFR conds may push into the Salinas valley for a few
hours around daybreak. Skies across l.A. County were mostly
clear...but some stratus with high IFR or low MVFR conds will
probably reach coastal sections by daybreak and persist through middle
morning. At this point...expect the valleys of l.A. County to remain
clear this morning. More widespread stratus is expected
tonight...with generally high IFR to low MVFR conds in most areas.

Klax...low confidence in the 12z taf. There is a 40 percent chance
that conds may remain VFR through this evening. There is a 20
percent chance that ceilings tonight may be in the IFR category for a
few hours.

Kbur...moderate to high confidence in the 12z taf. There is a 10 to
20 percent chance of IFR conds between 13z and 17z this morning.
There is a 20 percent chance that ceilings late tonight will arrive as
early as 07z.

&&

Marine...19/330 am PDT...
it is likely that northwest winds will remain below Small Craft Advisory levels across the
outer waters through at least Tuesday evening...although there may be
some local Small Craft Advisory level gusts through this evening. Winds will remain
below Small Craft Advisory winds across the inner waters through at least Tuesday.

&&

Lox watches/warnings/advisories...
none

&&

$$

Public...Kaplan
aviation/marine...db

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