Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
800 PM PST Sat Dec 20 2014
Synopsis...high pressure will continue to build aloft to bring
warmer and drier conditions to Southern California through middle next
week. Gusty northwest to north winds will affect parts of the region
tonight through Sunday night...then gusty northeast winds
will affect Ventura and Los Angeles counties Monday through Tuesday.
Cooler conditions are expected for Christmas day into next weekend.
Short term (tonight-tue)...strong upper level winds riding over the top
of the building ridge in the eastern Pacific continue to send middle
and upper level clouds across the region tonight. The low level moisture
will gradually diminish as drier northerly flow begins to increase
across the area. The one exception will be the interior valleys and
interior mountains where considerable low level moisture will continue
to affect those areas. In fact...isolated showers will continue across
the north facing slopes of la/Ventura County mountains through
Sunday. Remaining areas will likely see less cloud coverage on
Sunday...with mostly sunny conditions in all areas by Monday. Gradual
warming trend next few days...with warmest locations expected to reach
80 degrees by Tuesday.
Other short term concern will be increasing northerly winds across the
region during the next 24-36 hours. 00z NAM continues to ramp up the
northerly upper level wind support overnight while tightening the
north-south pressure gradients at the surface. Wind advisories look
good for the sba County mountains and South Coast...as well as the
I-5 corridor in the la/Ventura County mountains. Not much drop off in
upper level wind support during the day Sunday...then slightly stronger
winds expected Sunday evening (especially over the sba County
mountains). In addition...the lax-Bakersfield and sba-Bakersfield gradients
are more offshore tomorrow night compared to tonight. As a
result...have extended Wind Advisory in the mountains through Sunday
night. Wind gusts around 40 miles per hour can be expected along the sba South
Coast...while the mountains will likely see gusts in the 45-55 miles per hour
range. Locally breezy conditions also expected across the Santa
Clarita and San Fernando valleys...Ventura County valleys...and
Santa Monica Mountains.
*** From previous discussion ***
Winds will become more northeasterly on Monday and continue into
Tuesday as the surface high pressure over the eastern Pacific builds
into the intermountain west through the day Monday and into Monday
night. The 12z NAM lax-dag pressure gradient is forecast to be -6.8
at 12z Tuesday...considered to be a moderate offshore gradient. Some
upper level support is evident up to between 850 and 700 mb as well.
Would expect that wind advisories will be needed for the typical
offshore wind prone areas for late Monday night into Tuesday.
Again...expect Tuesday to be the warmest day of the 7 day
forecast...well above normal...and under clear skies.
Long term (wed-sat)...the upper level High Breaks down on Wednesday
as another trough dives down toward the Pacific northwest. This will
cool wednesday's highs down several degrees at least...though still
reaching 5-10 degrees above average. Earlier model runs were
disagreeing a bit on the path of this trough...the European model (ecmwf) digging it
further west than the GFS. But both models are in better
disagreement on their 12z runs and the trough will ride across far
enough to our north to keep the area dry....but close enough to
increase the north and northeast winds as it does. Both the ec and
the GFS redevelop offshore flow on Friday and continue it into
Saturday as a result of another surface high that builds into The
Rockies behind the trough. This will be a colder high so don't
expect much of any warmup with these offshore winds..if any.
Marine layer at lax at 2340z is none.
Mostly higher level clouds moving through the district tonight.
As upper level northwest-north winds increase...looking for lower
level moisture to dissipate and become mostly confined to interior
valleys and interior slopes. Low level wind shear likely at ksba tonight. North-northeast
winds likely to surface Sunday morning at ksbp and ksmx.
Klax...mostly VFR conditions through period. There is a 30 percent
chance of MVFR ceilings this evening at klax.
Kbur...mostly VFR conditions through period. There is a 20 percent
chance of MVFR ceilings this evening at kbur.
Small craft for hazardous seas and wind conditions from the outer
waters north of San Clemente Island to Point Conception and over the
entire area north of Point Conception will likely persist through
Sunday. The swells today were generated from a distant storm earlier
in the week over the Gulf of Alaska from a 290-300 degree direction
relative to the coast. The long period swell peaked across the outer
waters earlier today...and is currently peaking in the inner water
tonight. Combined seas will remain greater than 10 feet through
late Sunday. Northwest to north winds will be on the increase across
the coastal waters this weekend...potentially reaching gale force
strength for zones 673/676 by Sunday afternoon.
Large surf today being generated by a distant storm in the Gulf of
Alaska. The swell direction was 290-300 degrees which focused the main
energy along the central coast. Due to the long period of the swell
(18 seconds)...some of this swell energy was able to refract into
the inner waters and impact the west facing beaches of Los Angeles
and Ventura counties. Cdip swell analysis charts showed the high
likelihood of some 15-20 foot breakers across favored locations
along the central coast. Morro Bay Harbor patrol did indicate maximum sets
to 16 feet earlier this afternoon. Meanwhile...maximum sets at Ventura
Harbor and Channel Islands Harbor were 11 feet and 10 feet
respectively. Surf will remain quite large on Sunday across the
central coast...but not as big as today. Meanwhile...surf across
west facing beaches of la/Ventura counties will remain at their peak
tonight...gradually subsiding on Sunday. Due to the unusually high
tides this weekend...there will also likely be some minor coastal
flooding of low lying areas along the central coast...near the time
of morning high tide. The large surf will also bring dangerous rip
currents to the region this weekend.
coastal Flood Advisory (see laxcfwlox).
Small Craft Advisory (see laxmwwlox).
High surf advisory (see laxcfwlox).
High rip current risk (see laxsrflox).
Marine weather statement (see laxmwslox).
Gale watch (see laxmwwlox).
Wind Advisory (see laxnpwlox).