Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
150 PM PST Friday Dec 13 2013
..added marine discussion...
Surface high pressure will build into the Great Basin tonight...
resulting in warmer and very dry conditions...along with gusty
offshore winds for parts of the area through the weekend. A cooling
trend will likely begin by the middle of next week as low pressure
approaches the West Coast.
Short term (tonight-mon)...main issue is the developing moderate to
locally strong Santa Ana wind event tonight that lasts through the
weekend. Conditions this afternoon were relatively mild with
temperatures about 2-6 degrees cooler overall compared to
yesterday...with the exception of warmer conditions across the
Antelope Valley. There were some isolated north wind gusts to 30-35
miles per hour and about 20 reporting sites in the mountains with humidity from
10-20 percent this afternoon.
Winds near 6000 feet will start to shift to northeast early this
evening and strengthen overnight across Ventura and Los Angeles
counties. We should start to see 25-35 miles per hour winds with gusts to 50-60
miles per hour in the mountains by midnight but it will take additional time
for these winds to mix downward into the valleys and then to the
coasts around sunrise. Wind gusts of 40-50 miles per hour will be possible for
the lower elevations. Typical Santa Ana winds decrease some in the
afternoon and early evening hours...however this one keeps its
intensity through these hours with a peak 7 mb offshore pressure
gradient. There will be a slight decrease in wind speeds Saturday
night into Sunday morning...then further weakening Sunday afternoon.
Humidities below 15 percent and critically dry fuels in combination
with the winds and above normal temperatures expected both Saturday
and Sunday has resulted in a red flag warning and extreme fire
danger for many areas of Ventura and Los Angeles counties. See
discussion below and the latest laxrfwlox statement.
The Santa Ana will bring warm temperatures to many areas this time
around compared to the cold event we had earlier in the week.
Readings will push well into the 70s and even some lower 80s on
Sunday and Monday.
Long term (tue-fri)...
continued warm and dry weather on Tuesday as we remain in weak
offshore flow...although coastal areas will likely start to cool in
the afternoon as winds start to push back onshore. An upper low
currently over 1000 miles west of Baja California will lift northeast to the
central California coast on Wednesday. Additional cooling should occur
across southwest California and the possibility of low clouds for beaches
Wednesday morning as the onshore breezes increase.
For Thursday and Friday...will continue with a dry but cool
forecast. There has been much model inconsistency with the strength
of the eastern Pacific high pressure ridge across the Gulf of Alaska
and whether a storm system moving down the West Coast will have
over-water trajectory or stay mostly inland. Model trends for the
last few days and the latest GFS/European model (ecmwf) runs favor the inland track.
Will leave the forecast dry but we could eventually need a few light
showers for the mountains on Thursday when the trough passes by. The
higher probability event will be another Santa Ana threat by the end
of the week and possible warming next weekend.
Moderate to locally strong Santa Ana winds will quickly develop this
evening across the mountains...with offshore winds descending into
coastal and valley areas overnight. Offshore winds are expected to
peak on Saturday across Los Angeles and Ventura counties...when gusts
between 45 and 55 miles per hour are expected across the mountains...except
isolated gusts over 60 miles per hour in favored locations. Meanwhile...coastal
and valley areas will generally see gusts in the 40 to 50 miles per hour range.
Slight weakening of Santa Ana winds are expected on Sunday...however
gusts between 30 and 45 miles per hour will still be common. Peak lax-Daggett
pressure gradients on Saturday and Sunday are projected to range
between -6 and -7 mb.
Humidities are expected to fall to around 15 percent or less across
the mountains early Saturday morning...with the drier air filtering
into coastal and valley areas in the afternoon. Further drying is
expected Saturday night into Sunday...when there will be the
potential for widespread humidities between 8 and 15 percent. A red
flag warning has been issued for much of Los Angeles and Ventura
counties for Saturday into Sunday due to the combination of moderate
to locally strong offshore winds...low humidities...warm
temperatures...and dry fuels. Offshore winds are not expected to be
as strong across San Luis obsipo and Santa Barbara
counties...however wind gusts between 25 and 35 miles per hour with warm and
dry conditions will bring elevated fire danger to these areas.
Offshore breezes will continue to bring warm and dry conditions
on Monday and Tuesday...maintaining elevated fire danger across
much of Southern California.
Marine...a Santa Ana wind event will bring gusty NE winds to the
near shore waters late tonight through Sunday morning. The strongest
winds will be over the waters from Point Mugu to Santa Monica where
gusts of 25 to 30 knots are expected on Saturday. Gusty winds will
also affect the waters from Ventura to Point Mugu...and the San
Pedro Channel with gusts of 20 to 25 knots. Hazardous short period
waves are also expected. Easterly wind chop will build to 3 to 4
feet Saturday and Sunday...especially beyond 5 nm from shore. The
short period seas will affect east facing harbors such as Avalon
Harbor...and any other harbors or landings with east exposure.
A long period west swell will begin to move into the waters Sunday
afternoon...building to 4 to 6 feet at 18 seconds in the near shore
waters north of pt Conception by late afternoon...and to 6 to 8 feet
at 17 seconds Sunday night. The swell will begin to move into the
waters south of pt Conception Sunday night...building to 3 to 4 feet
at 17 seconds near shore early Monday. Building surf will likely
approach the high surf advisory threshold late Sunday into Monday.
Overall...moderate to high confidence in 18z taf package. High
confidence in VFR conds for all sites as offshore develops across
the area...except 10% chance of fog/low ceilings forming over ksmx and
ksbp as winds subside late tonight. There will likely be some
low level wind shear/turbulence issues through/below passes and canyons late tonight
into Saturday morning...and at kbur.
Klax...high confidence in 18z taf...with VFR conds through tonight.
There is a 20% chance of low level wind shear developing after 12z Sat due to NE
Kbur...moderate to high confidence in 18z taf. Expecting
low level wind shear/turbulence after 10z Sat as offshore flow develops.
red flag warning (see laxrfwlox).
Wind Advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).