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Southwest California area forecast discussion...updated 
National Weather Service Los Angeles/Oxnard California
1012 PM PST Sat Dec 27 2014

Updated aviation section


High pressure over the region will keep a dry weather pattern in
place through Tuesday. A brisk and cold weather pattern will
move into the area as a trough of low pressure originating from
Canada drops into the region for middle-to-late next week.



The latest water vapor imagery indicates upper-level dirty ridging
over the eastern Pacific Ocean. The ridge...centered about 900
miles southwest of Los Angeles...will remain in place through at
least Tuesday. At the surface...a northerly surface pressure
gradient across the area will keep a cold air mass in place.
With freeze and frost headlines in effect later tonight...there
is some concern that high level moisture upstream could inhibit
radiative cooling processes overnight. With several locations
cooler than this time last is likely that locations in
the warning and advisory areas will see temperatures hit criteria.
At this changes are planned for frost advisories and
freeze warnings. An update will be issued to increase cloud cover
and tweak temperatures slightly.


Previous discussion...

Short term...models in pretty good agreement with this next
system, tracking it mostly over land through interior Oregon and
California before settling in so cal Tuesday night and early
Wednesday. Moving over land it will be starved for moisture, but a
very cold air mass will likely enable light showers to develop
just about anywhere, though most likely over the mountains downsloping
winds will likely reduce chances for measurable rain for most
areas except the eastern portion of la County where there will be some
limited upslope flow. Areas south of la County will stand a better
chance of getting measurable precipitation from this event due to the
topography so am leaving probability of precipitation in the 20-30 range for the most
part. Despite the cold air aloft with 500 mb temperatures near -30c across
the eastern part of the forecast area it doesn't look particularly
unstable at this time. It's a very cold air mass from top to
bottom and lifted indice's remain above zero for the duration of the event so
will not be adding thunderstorms at this time. Tuesday night looks
like our best chance at precipitation with decent pva, but again very
limited moisture. Snow levels likely down to around 2500' but
minimal accumulations expected. Still could pose some hazards over
the Grapevine as basically any accumulations there cause problems.

Long term...the upper low starts to exit the area Wednesday
morning and by 18z most of the action is south and east of our
area. Could still be some wrap around moisture from the north into
the mountains and Antelope Valley but not seeing a whole lot of
dynamics to support anything more than just some isolated showers.
Gusty Santa Ana winds likely for Ventura and la counties as a
decent offshore gradient develops behind the departing upper low.

With clearing skies and mostly light winds new years morning should
be very cold and likely will need freeze warnings for many of the
valleys and central coast and possibly frost advisories for the
la/Ventura coast. Those planning on camping out in Pasadena for the
rose parade should plan accordingly for a very cold night with lows
possibly in the low 30s.

Fairly quiet weather then through next weekend as weak high pressure
builds in for a slow warming trend. Still quite cold overnight
through at least Saturday morning.



At 0555z...there was no marine inversion at klax.

VFR conditions are expected throughout the period...except for a
chance of LIFR to IFR conditions at terminals north of Point
Conception between 11z and 16z.

Klax...VFR conditions are expected throughout the period except
for a 10 percent chance of vlifr conditions between 13z and 16z.
Any east winds will remain less than 5 knots.

Kbur...VFR conditions are expected throughout the period. No wind
issues are expected at this time.


Marine...27/900 PM PST.

No significant weather is expected through Sunday. There is a
chance of Small Craft Advisory level winds across the outer
waters between Sunday night and Monday...with Small Craft
Advisory conditions likely between Monday night and New Years
Day. There is a chance of gale force winds and steep short-period
seas between Tuesday night and Wednesday across all coastal
waters as a cold trough of low pressure drops into the region.


Lox watches/warnings/advisories...
frost advisory (see laxnpwlox).
Freeze warning (see laxnpwlox).





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