Scientific Forecaster Discussion
Southwest California area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
1000 PM PST sun Dec 28 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. This will bring a
chance of showers to the area as well as low elevation snow for
the mountains and Antelope Valley.
A cold air mass remains in place as a strong north-northwesterly
flow aloft starts to develop ahead of trough of low pressure over
the Pacific northwest and a developing Omega block ridge of high
pressure to the west. A frost advisory is in effect for most
valley areas and central coast...with a freeze warning for the
Ojai and heritage valleys in Ventura County. Take precautions now
to protect crops and sensitive plants. Bring pets and livestock
An onshore flow pattern will develop through Monday ahead of an
approaching cold front. Cooler conditions will linger into
Monday...but the interior valleys will warm somewhat as downslope
effects develop on Monday with the sea breeze circulation.
Model solutions are struggling to handle the trough centered over
the Pacific northwest and the cold air mass breaking out with it.
Earlier model solutions indicated more snow than wind for the area
for the storm system...but the latest model solutions are backing
off the snow and advertising stronger winds. High confidence
exists in colder temperatures moving in for middle-to-late this
week...but confidence is less certain for snow and winds. With
the amount of cold air mass spreading south...it is would seem
more apparent for strong winds developing across the area...but
model solutions downplay it. With much time until the event...the
next shift will be briefed accordingly.
Short term...a very cold storm dropping out Canada today will
continue to move south and reach Southern California late Tuesday
night. Due to its terrestrial trajectory it's pretty starved of
moisture but not entirely. It will pick up a little moisture as
the westerly flow around the south end of the low moves over water
and this will bring the southeastern portion of the forecast area,
mainly southern la County its best chance of precipitation either late
Tuesday afternoon or evening. Other areas will have a shot at some
isolated showers but very light amounts. As the low continues to
drop south and east Tuesday night the low level flow will quickly
shift to northeast. As this happens concern will shift to the
la/Ventura mountains and the Antelope Valley as snow levels drop
to 2000 feet or lower and northeast winds generate orographic lift.
Models indicate the best lift will be across the eastern half of
the av and san gab mtns, basically from Highway 14 east. Although
amounts may struggle to reach true warning level criteria due to
the limited moisture, given the impacts on Holiday travel over
Interstate 5 and through the Mojave Desert it was felt that a
Winter Storm Watch was justified.
Coast and valley shower chances will diminish Wednesday morning with
the downslope flow while precipitation across the interior is expected to
taper off Wednesday afternoon as the low moves farther east and
south of the area. Rainfall amounts to the Lee of the mountains expected
to be under a quarter inch. For the mountains and av a quarter to half
inch liquid equivalent, translating to around 3" of snow for the av
floor and the Grapevine and as much as a foot for the north slopes
of the eastern san gab mountains
Long term...a cold start to 2015 expected with lows in the
valleys, including the Pasadena area in the lower 30s and possibly
as low as the middle to upper 20s in the colder wind protected areas
such as Ojai and the western sfv. Otherwise dry and mostly clear
skies through next weekend with a very slow warming trend. By
Sunday highs should be back to near normal levels.
At 0600z...there was no marine inversion at klax.
VFR conditions are expected throughout the period...except for some
MVFR visibilities possible in the Salinas valley overnight.
Klax...high confidence in the 06z taf with VFR conds expected
through Monday evening. There is a 20 percent chance that MVFR ceilings may
arrive as early as 03z Tuesday...and may be IFR instead.
Kbur...very high confidence in the ceiling and visibility unlimited 06z taf.
Marine...28/900 PM PST.
Winds will increase across the area through Tuesday. At
least...Small Craft Advisory level winds will be prevalent after
Monday evening across the outer waters. There is a 30 percent
chance of gale force winds across the coastal waters between
Tuesday night and Wednesday night as a cold front drop south into
the region. Winds may be enhanced near shore below passes and
canyons. Steep short period seas may also develop Tuesday night
and Wednesday morning.
Winter Storm Watch (see laxwswlox).
Frost advisory (see laxnpwlox).
Freeze warning (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).