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Southwest California area forecast discussion...updated 
National Weather Service Los Angeles/Oxnard California
1025 PM PDT sun Sep 21 2014

Updated aviation section


Building high pressure will bring a warming trend and a decrease
in night through morning marine layer cloud coverage through middle
week. A trough of low pressure will then bring cooler weather and
an increase in night through morning low clouds and fog for late
in the week. Offshore flow and a warming trend are possible by
next Sunday.



The latest water vapor imagery indicates a trough of low pressure
moving northeast across Nevada. A ridge of high pressure...centered
about 570 miles southwest of Los Angeles continues to March east
this evening. The ridge axis just sits off the California coast.
A warming trend should continue for the coming days...with the
warmest days likely occurring on Tuesday and Wednesday when the
ridge center is directly over Southern California. A deep marine
layer in place the last couple of days will shrink later tonight.
The latest amdar sounding indicate a marine layer depth near 1100
feet this evening...thinning about 700 feet than earlier this
evening. An update will issued shortly for a few tweaks to
temperatures for Monday and marine layer coverage tonight.


Previous discussion...

Short term...NAM shows a little middle level moisture moving into
the area from the remnants of Polo but at this point it's still
too dry and stable at lower layers to get more than just some ac.

Long term...the ridge will quickly be replaced by a rather
impressive trough along the West Coast for the second half of the
week. Models differ with respect to how deep the trough will get
with the European model (ecmwf) by far the deepest. However, it doesn't show it
tapping into any moisture source so at best we'd be looking at a
deepening marine lyr, possibly some coastal/valley drizzle, and
much cooler temperatures. We should be turning the corner Saturday as the
trough axis slides east and ridging returns from the west. GFS shows
some offshore flow developing Sunday so temperatures may jump
considerably. GFS had at one time shown offshore flow for this
weekend so will keep the forecast rather conservative at this
time, but most areas should see at least a few degrees of warming
with the ridging.


Widespread stratus with low MVFR to high IFR conds on the central
coast and in the Santa Ynez Valley this evening. Expect stratus to
push into most remaining coastal and valley late tonight...with
mostly MVFR conds...except local IFR conds in the valleys. Skies should
clear by late morning Monday in most areas...although ceilings may
linger near the coast into the afternoon. Expect widespread low MVFR
to high IFR conds in all coastal and valley areas Monday night.

Klax...moderate to high confidence in the 06z taf. There is a 20
to chance that ceilings will not arrive until at least 12z. There is a
20 percent chance that MVFR ceilings will persist through Sunday

Kbur...moderate to high confidence in the 06z taf. There is a 20
percent chance that conds will be IFR between 12z and 16z.


Marine...21/800 PM.

Low-end small craft winds are affecting the southern outer waters
near/S of Point Conception this evening. There is a chance of 25 knots
gusts in this area again Monday and Tuesday evenings but the gusts may be
localized. A strong low pressure system crossing the Pacific basin
will deepen and approach the Pacific northwest Monday into Tuesday.
Swell generated by this storm will move into the SW California
waters Wednesday evening...peak Thursday and linger through at least
Friday. Models continue to show northwest winds increasing
Thursday...with gales possible over the two southern outer zones by
Friday. Hazardous seas will likely exist due to the combination of
large northwest swell (8 to 10 feet) and steep wind seas.


Lox watches/warnings/advisories...
Small Craft Advisory (see laxnpwlox).





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