Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
210 PM PDT Thursday Sep 3 2015
..marine discussion updated...
Moderate onshore flow will gradually weaken Labor Day weekend as an
upper level low departs to the east. As a result...a slow but steady
warming trend will begin...with above normal temperatures expected
by next Friday. Morning clouds extending into the valleys Friday
will retreat to some coastal locations by early next week.
Short term (tonight-sun)...overall...12z models in good synoptic
agreement through the period. At upper levels...low will move into
the Pacific northwest tonight/Friday...move south into northern
California on Saturday...then inland into the northern Great Basin
on Sunday. Near the surface...moderate onshore flow will prevail
through Saturday...then turn weakly offshore on Sunday.
Forecast-wise...nothing too exciting expected for the area through
the weekend. Cloud wise...there does not appear to be any noticeable
moisture moving around the upper low...so do not anticipate much of
any middle/high level clouds. However...the marine layer stratus will
remain a challenge. With continued onshore flow and lowering 500 mb
heights through Friday...marine inversion will remain rather deep
and stratus should push into the lower coastal slopes. For Friday
night and Saturday...500 mb heights increase a bit and onshore gradients
weaken slightly...so anticipate less widespread stratus coverage
across the area. By Saturday night/Sunday...weak offshore flow
should keep most areas stratus-free...with the only exception
potentially the la coast and San Gabriel valley where some
stratus/fog could develop.
As for winds...moderate onshore winds are expected through Saturday.
The winds will become gusty each afternoon/evening across the
mountains and deserts...but should remain below advisory levels. For
Saturday night/Sunday...the surface gradients turn weakly offshore
but upper level support is minimal. So...there will be some local
northeast winds in the usual places...but below advisory levels.
As for temperatures...Friday will be the coolest day in the short
term. On Saturday...temperatures will rebound a bit with less marine
influence and weaker onshore gradients. For Sunday...expect a very
noticeable temperature jump due to weak offshore flow. Most areas
away from the coast should be in the middle 80s to middle 90s on Sunday.
Long term (mon-thu)...overall...12z models continue to be in decent
agreement through the period. At upper levels...weak trough will
linger over the area Monday/Tuesday...then a ridge will try to move
in from the southeast Wednesday/Thursday. Near the surface...it
looks like weak diurnal flow will prevail...weakly offshore at night
and onshore during the day.
Forecast-wise...the excitement levels will continue to be minimal.
Monday/Tuesday should see a gradual return of the marine layer
stratus and a gradual cooling trend. For Wednesday/Thursday...the
marine inversion will likely shrink a bit and remain confined
generally to the coastal plain. With the less marine influence...the
temperatures on Wednesday/Thursday should exhibit a warming trend.
At 17z at klax... the inversion was about 3000 feet deep. The top
was near 6350 feet with a temperature of 17 degrees celsius.
Overall... moderate confidence in the 18z tafs along the coast and
in adjacent valleys and high confidence elsewhere. Ceilings continue at
a few locations but all taf sites will be VFR by 20z. Marine clouds
return tonight as early as 03z in some coastal locations and as late
as 14z at kprb but all coastal and adjacent valley locations will
have a return of LIFR/IFR ceilings at central coast sites and primarily
MVFR ceilings at the others with a similar clearing pattern tomorrow as
today. Otherwise and elsewhere VFR conditions will prevail.
Lax... moderate to high confidence in the 18z taf. Ceilings continue
at this time but should go VFR 18z-20z. Marine clouds return tonight around
05z with MVFR ceilings for klax. A similar clearing pattern tomorrow as
today. Otherwise VFR conditions will prevail.
Bur... moderate confidence in the 18z taf. Ceilings continue at this time but
should go VFR 18z-20z. Marine clouds return tonight around 06z with
MVFR ceilings for kbur. A similar clearing pattern tomorrow as today.
Otherwise VFR conditions will prevail.
Good confidence in Small Craft Advisory conditions developing this
afternoon and continuing through late tonight across the northern
two thirds of the outer waters. There is a twenty five percent
chance that a Small Craft Advisory will be needed for the inner
waters as well.
Small Craft Advisory conditions are expected again across the entire
outer waters Friday afternoon through Saturday night. There is a
thirty five percent chance that an advisory will be needed for the
northern inner waters during that period as well.
Meanwhile... storm force winds in the South Pacific generated a
swell that will affect the coastal waters of southwest California
over the weekend. Swell heights will be 2 to 4 feet with intervals
near 20 seconds. Mariners should be prepared for long period swells
that will likely have extra surging and currents. Shoaling is
likely in shallower water and in some Harbor entrances.
Beaches...a long period south swell will likely bring elevated surf
to south and southwest facing beaches over the Holiday weekend...
with a rather high risk of dangerous rip currents expected.
beach hazard statement (see laxcfwlox).
Small Craft Advisory (see laxmwwlox).