Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
920 PM PST Friday Feb 27 2015
A cold low pressure system will bring scattered showers...possible
thunderstorms and mountain snow to the area this weekend. A weaker
system will be on the heels of the first arriving Monday into
Tuesday morning. Drier conditions and offshore breezes are expected
to bring above normal temperatures to the area by late next week.
Short term (tonight-mon)...plenty of stratocu covered the forecast
area this evening as a cool and moist airmass continued to filter
into the area from the north. There were isolated showers noted over
the eastern San Gabriel valley which produced a few hundredths of an
inch of precipitation. A slight chance to chance of showers will persist
across southwestern California through the night as an upper low continues to approach
the area from northern California. Good onshore flow produced gusty west to northwest winds
for most coastal areas this evening...and these winds may persist
for a while overnight...especially for vtu/l.A. Counties. There were
also gusty northwest to North Canyon winds affecting the sba S coast and mountains
this evening...with a Wind Advisory in effect there until 10 PM.
The upper low will continue to slide S on Sat to a position around
Monterey County in the afternoon...then to a position over sba
County Sat night and sun. This upper low will move slowly east Sun
night...then another upper level low will approach from the north on
Monday. These upper level low pressure systems will bring plenty of
showers and mountain snow showers to the forecast area Sat through sun
evening...with more showers into Monday. Cold...unstable air aloft (500
mb temperatures dropping to -26 to -28 degree c) will also bring a threat of
thunderstorms to the region Sat through sun evening. An isolated
thunderstorm cannot be ruled out Monday afternoon and evening as well.
Any thunderstorms that develop would be capable of producing brief
heavy rain...gusty winds and small hail.
This will be a showery weather system and precipitation amounts will be
quite variable. Overall...0.10 to 0.50 inch looks reasonable for the
coast and valleys...with 0.50 to 1.50 inches in the mountains. Isolated
precipitation totals could be up to an inch for the coast and valleys
and up to 2.00 inches or more for the mountains
Snow levels will fall to 5000 feet by Sat afternoon...to 4000 feet
Sat night...and to 3500 to 4000 feet on sun. Significant
accumulating snow over the weekend is expected for the mountains...with
6-12 inches possible in the mountains above 7000 feet...4-8 inches
from 5000 to 7000 feet...and 2 to 5 inches down to 5000 feet.
Accumulations less than an inch will be possible down to 4000 feet
mainly Sunday and Sunday night. Even so...there may be travel
problems over on the I-5 over the Grapevine late in the weekend. A
Winter Weather Advisory is in effect for the mountains of vtu/l.A.
Counties from late Sat afternoon to late Sun night.
Additional precipitation amounts later Sun night and Monday could total 0.10
to 0.25 inch for most areas...with an inch or two of snow
accumulation possible in the mountains down to 4500 feet.
Needless to say...with the cold upper level systems over southwestern
California...temperatures across the region will be several degrees below
normal Sat through Monday.
***From previous discussion***
Long term (tue-fri)...the upper low that zips in behind the weekend
low and moves through Monday night and early Tuesday is not as
strong or deep so expect precipitation amounts to be less. Height
rises move in on Tuesday to start a warming and drying trend for the
remainder of the week...and peaking Thursday...with some weak
offshore flow to keep skies clear.
At 2305z...the marine inversion at klax was at 4700 feet deep...the
top of the inversion was at 6300 feet with a temperature of 7 degree c.
Moderate confidence at best in the 00z tafs. Mostly VFR ceilings are
expected at the airfields through Sat...however there is a 40-50
percent chance of MVFR ceilings prevailing for most of the time during
the day on Sat for many airfields. Otherwise...MVFR ceilings may be
intermittent at times during the taf period. There will also be a
chance of showers mainly on Sat with some reduced visibilities at times. A
slight chance of thunderstorms can be expected as well on Sat
afternoon. Gusty winds will affect ksmx...ksbp and ksba this
evening...with low level wind shear possible at ksba from 02z-09z.
Gusty SW to west winds will affect kprb and kwjf through Sat.
Klax...moderate confidence at best in the 00z taf. Mostly VFR ceilings
are expected at the airfield through Sat evening...with some periods of
MVFR ceilings. However there is a 40-50 percent chance of MVFR ceilings
prevailing for most of the time during the day on Sat into Sat
evening. There will also be a chance of showers on Sat and Sat
evening with a 30-40 percent chance of MVFR visibilities at times. A slight
chance of thunderstorms can be expected as well Sat evening.
Kbur...moderate confidence at best in the 00z taf. Mostly VFR ceilings
are expected at the airfield through Sat...with some periods of MVFR
ceilings. However there is a 40-50 percent chance of MVFR ceilings
prevailing for most of the time during the day on Sat. There will
also be a chance of showers on Sat with a 30-40 percent chance of
MVFR visibilities at times.
For the outer waters...Small Craft Advisory conditions will continue through late
tonight for pzz670/673...and to 9 am Sat for pzz676. Winds will
diminish through the weekend then increase to gale or near-gale
Monday night into Tuesday...but this is lower confidence as model
agreement is not very good at this time.
For the inner waters...good confidence in Small Craft Advisory level winds across
pzz650/655 through middle morning Sat. The strongest winds will be
across the western sections from The Channel islands to Catalina
Island...where gusts to 30 knots will be possible. Short period
rough seas are expected.
Showers are expected across the waters this weekend as a couple of
frontal systems cross the area. From Saturday afternoon through
Sunday evening...there will be a slight chance of thunderstorms with
locally gusty winds...rough choppy seas...and possible waterspouts.
Winter Storm Watch (see laxwswlox).
Wind Advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).
High rip current risk (see laxsrflox).