Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 010013
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
513 PM PDT FRI SEP 30 2016
***UPDATED AVIATION SECTION***
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.
The marine layer at LAX at 2330Z was near 500 feet deep. The inversion
top was near 2200 feet, with a temperature of 25 degrees C.
Marine layer should deepen slightly tonight. Good confidence for
coastal TAF sites north of Point Conception, although timing of
the LIFR conditions could be off by a couple of hours. Also a good
chance for VLIFR conditions here, but timing and duration is less
certain. There is a 40-50% that fog will not reach KPRB Saturday
morning, but a brief period of VLIFR conditions will be possible.
South of Point Conception, sundowner winds should keep any stratus
out of KSBA, but LLWS will be possible this evening. Low
confidence in the stratus forecast for the coastal sites of
Ventura and LA counties. There is about a 30% chance of IFR/LIFR
conditions at KOXR from about 12-16z. There is a 40% chance for no
stratus issues across the LA basin late tonight and Saturday
KLAX...There is a 40% chance VFR conditions will prevail through
Saturday morning. Also a 30% chance for LIFR/VLIFR conditions from
KBUR...Good confidence that VFR conditions will prevail for the
next 24 hours. There is a 10% chance for a brief period of
IFR/LIFR/VLIFR conditions from 13-16z.
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.
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
For zone 645. (See LAXMWWLOX).
Small Craft Advisory in effect until 6 PM PDT Sunday For
zones 670-673-676. (See LAXMWWLOX).
Gale Watch in effect from Sunday evening through Monday
evening For zones 673-676. (See LAXMWWLOX).