Area Forecast Discussion
Issued by NWS Los Angeles, CA

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404
FXUS66 KLOX 290022 AAA
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
522 PM PDT WED SEP 28 2016

...update for new aviation discussion...

.SYNOPSIS...

There is a slight chance of thunderstorms and showers for the Los
Angeles and Ventura County mountains and adjacent valleys through
early this evening. A broad upper level trough of low pressure will
arrive on Friday to cool temperatures to below normal and increase
the marine layer into early next week.

&&

.SHORT TERM...(TDY-SAT)

For today focusing on the potential for afternoon convection over
the LA/Ventura county interior. So far not seeing much vertical
development but still plenty of heating to go and what appears to
be some additional energy coming in from the northeast. The HRRR
model shows very little activity through 5 pm, then some showers
and possible storms in the early evening, mainly across the nrn
Ventura county mountains and the Antelope Valley. Precipitable
water not too high, generally around an inch, so the main concern
is for brief heavy rain, small hail, gusty winds, and lightning.
Probably not a big flood threat as northeast steering winds
should keep any storms moving.

Gradients will continue to trend onshore into the weekend as
a trough approaches from the northwest. There may be one more day
of some afternoon cu over the mtns but moisture and instability is
less than today so for now am going to keep storms out of the
forecast. Marine lyr coverage expected to be very similar as last
night, mainly just Central Coast. In fact, don`t see any solid
marine lyr south of Pt Conception until Saturday morning at the
earliest. Temps will continue their decline, though a little more
gradual now, mostly 2-4 degrees per day, until we are back to near
normal readings by around Friday and possibly even slightly below
normal Saturday.

The main impact from the upcoming trough besides the cooler temps
will be increasing northwest winds along the Central Coast, srn SB
County, the I5 corridor region, and Antelope Valley. Likely advisory
level winds at times, especially by Saturday afternoon and
evening.

.LONG TERM...(SUN-WED)

The trough that moves through Friday night into Saturday will be
the first of several that will mostly impact northern California.
There are a couple of weak cold fronts that will mostly wash out
before reaching SLO county, the first one late Sunday and then
another late Monday. Models aren`t fully in agreement but none of
them show any of these troughs holding together well enough to
generate any rain, even for the Central Coast. So will maintain a
dry forecast for now. Main impacts initially will be the cooler
temps and persistent northwest flow that will continue to generate
advisory level winds at times in the usual areas. As we get into
the middle and end of the week high pressure aloft will build into
California from the west while a surface high builds over the
Great Basin. While the air mass warm up gradients will be shifting
to offshore and we may be looking at another Santa Ana event by
Thu/Fri.

&&

.AVIATION...29/0010Z...

At 2310Z...the marine inversion at KLAX was near surface-based. The
top of the inversion was at 1300 ft with a temp of 26 deg C.

Generally hi confidence in the 00Z TAFs. There is a 60-70 percent
chance of low clouds and LIFR conditions at KSBP and KSMX from about
08Z-18Z tonight and Thu morning, altho the timing of the onset of
the low clouds may be off +/- an hour or so. Otherwise, VFR
conditions can be expected at all airfields thru Thu afternoon.

KLAX...Hi confidence in the 00Z TAF with VFR conditions expected
thru Thu evening.

KBUR...Hi confidence in the 00Z TAF with VFR conditions expected
thru Thu afternoon.

&&

.MARINE...28/200 PM...

Mostly light winds are expected across the coastal waters through
Thu with SCA conds not expected. Northwest winds are expected to
increase across the outer waters and northern inner waters
late Thu through Fri, with SCA conditions likely in those areas
Fri evening through at least Sun. There is a 50 percent chance
that SCA conds will spread into the SBA Channel and the southern
inner waters by Sat afternoon continuing through Sun.

There is a 20 percent chance of Gale Warning conditions sometime
this weekend mainly across the outer waters from Point Conception
to San Nicolas Island.

Extra surging and currents are possible near the coast due to a
long period south swell of 2 to possibly 4 feet by Thursday
morning, peaking late in the day into Friday before subsiding this
weekend.

Locally dense fog is possible across much of the coastal waters
tonight through Thursday morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC...MW
AVIATION...Sirard
MARINE...Sweet
SYNOPSIS...Seto/Sirard

weather.gov/losangeles



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