Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 302020
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
120 PM PDT FRI SEP 30 2016
A trough of low pressure dropping south from British Columbia
will bring cool conditions to the region Saturday through Monday
with night to morning low clouds spreading into the valleys. As
the trough departs to the east by the middle of next week, locally
gusty northwest winds are expected to transition offshore,
supporting a return to warm and dry conditions.
Overall a very similar day today as yesterday. West to east
gradients continue to trend onshore ahead of an upper level trough
moving into Oregon and nrn CA, though north to south gradients are
also increasing. Already seeing northwest winds increasing over
the coastal waters north of Pt Conception and a rapid decrease in
stratus there as well as drier air mixes down. Gusty sundowner
winds expected this evening in srn SB county as those northwest
winds filter down through the Santa Ynez range. Will likely be
some brief advisory level gusts in the favored areas west of
goleta, especially up in the foothills, but will hold off issuing
an advisory right now.
Cooler temps expected Saturday behind the trough. The increase in
northwest winds around Pt Conception is expected to spin up a
little eddy circulation which should help stratus formation from
Ventura county south by late tonight or early Saturday. Sub-
advisory level north to northwest winds expected late Saturday and
Sunday into Monday a second and colder trough will move through
northern and central California. Models seem to be leaning towards
the slightly farther north ECMWF solution, which means slightly
less cold air and upper level support, but still quite windy for
the usual northwest favored areas such as srn SB County again, the
I5 corridor, the Antelope Valley, and possibly the Santa Clarita
Valley. These winds will almost certainly be advisory level and
possibly some brief warning level gusts in the I5 corridor late
Sunday night into early Monday. Low level moisture is expected to
bank up against the north facing slopes near the Kern County line
Sunday night and there could be some light rain showers there
through early Monday in addition to the gusty winds. Amounts
should be less than a tenth of an inch. Marine lyr clouds should
mostly be wiped out with the trough passage except for possibly
some lingering clouds across southeastern LA County. Temps Monday
a few degrees below normal with breezy or windy conditions in many
Northwest flow will transition to northerly Tuesday then northeast
Wednesday as a weak upper level ridge builds from the west. This
offshore event looks quite a bit weaker than the last one as
there`s much less upper support and gradients aren`t as strong.
Temps will be warming back up to a few degrees above normal
Wednesday and warmer still Thursday, though offshore flow will be
weakening through the day as the ridge weakens and a trough
approaches from the west. Then slightly cooler Friday but still
slightly above normal.
A mid/upper level trough of low pressure upstream will approach. Upper
level moderate southwest winds will become strong southwest after
01/07z and mid level light southwest winds will prevail over the
area. Weak onshore pressure gradient through 01/02z and after
01/19z otherwise weak to moderate northerly gradient. Base of a
capping marine inversion was between .5-1kft this morning with an
overcast cloud field west and north of the Channel Islands and
isolated over the bight. The inversion base will differ by plus
.4kft with a scattered cloud field Saturday morning.
Marine layer at LAX at 1500Z is 451 feet deep and the inversion
top is at 2002 feet with a temp of 23.6 degrees C.
KLAX...Chance cigs 006 and vsby 4sm br between 01/11-01/17z.
KBUR...Very likely current sky/vsby conditions will persist.
virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less
Northwest wind will increase and small craft advisory conditions
will exist from Piedras Blancas to San Nicolas Island Friday
afternoon through next week Monday with occasional gale force
gusts each afternoon Saturday and Sunday. The gales force gusts
may become frequent south of the Northern Channel Islands Sunday
night through Monday and gale watch is in effect. Northwest winds
will likely fill in over portions of the inner basins each
afternoon Saturday through Monday.
Sundowners expected the next few nights for canyons and passes
below the Santa Ynez Range along the Santa Barbara County south
coast. Gusty northwest winds to about 35 mph will occur west of
Goleta tonight then will be more widely felt and peak near the
foothills from Montecito to Goleta by Sunday night when gusts
could be near 50 mph.
A cold low pressure system will move across northern California on
Sunday then eastward to the Great Basin by Monday. Much stronger
west-northwest winds will bring gusty conditions to all areas
Sunday afternoon with gusts to 35 mph for valleys and coasts, and
to 55 mph in the mountains and deserts. Relative humidities will
fall to single digits at ridgetop level on Sunday bringing several
hours of critical fire weather to higher elevation sites, but
temperatures will be below normal. No fire weather watch is
expected at this time. Similar dry conditions will occur on
Monday with winds turning northwest-north and remaining quite
gusty for the Antelope Valley and the San Gabriel Mountains.
High pressure will start to build on Tuesday and Wednesday with
north winds diminishing and temperatures rebounding back near
normal by mid-week.
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
For zone 645. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 PM PDT Saturday For
zones 670-673-676. (See LAXMWWLOX).
Gale Watch in effect from Sunday evening through Monday
evening For zones 673-676. (See LAXMWWLOX).