Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 210440
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
940 PM PDT Wed Sep 20 2017

.SYNOPSIS...

An upper level trough will persist over the forecast area through
much of the weekend. A weak disturbance in the flow will move
across the region bringing a chance for some light rain or drizzle
across much of Los Angeles and Ventura Counties Thursday. High
temperatures will lower significantly Thursday with gusty winds
behind the the front. There will be a weak offshore flow
developing by Saturday and persisting into next week. Expect a
significant warming trend by late this weekend into next week.

&&

.SHORT TERM...(WED-SAT)

The latest infrared imagery indicates an upper-level trough of low
pressure along the Oregon coast. A cold front can be seen pushing
south across Northern California. This front will move over the
region Thursday and bring primarily a breezy to windy pattern with
some shower possibilities. A marginal wind advisory was added for
the Antelope Valley and for the Santa Barbara south coast and
mountains tonight as winds should reach criteria. Winds will
increase on Thursday and Thursday night, bringing high confidence
in advisory level winds to these areas. A wind advisory may also
need to be considered for the Central Coast and the Santa Ynez
and San Luis Obispo County Valleys for Thursday afternoon and
evening, if forecast guidance continues to fall inline with
advisory criteria being met. There is a chance that advisory level
wind criteria may also be met through the Interstate 5 Corridor
on Thursday night and into Friday morning.

Clouds will increase throughout tonight and into Thursday morning
as the front advances on the region. Thursday is shaping up to be
a mostly cloudy day for the Southland, clearing out as cold air
advection pushes into the South Coast Basin. Pops have been
nudged higher for the mountains and desert as jet dynamics and the
trough positioning should interact to bring the best rain chances
in these areas. A local mix of snow and rain cannot be ruled out
on the higher mountain elevations as the first `fall` system rolls
in. High temperatures on Thursday and overnight low temperatures
on Thursday night have been cooled. The first freezing
temperatures of the fall-winter season are forecast for the
mountains on Thursday night. Further cooling will likely need to
be introduced for Friday night, as well.

*** FROM PREVIOUS DISCUSSION ***

Over the weekend we`ll be transitioning, slowly, to offshore flow
and warmer temps. There`s still a weak eddy circulation in place
Saturday morning and with warmer air aloft the inversion should
strengthen enough to allow low clouds to reform at least across LA
County and possibly further north/west. Clouds may linger at the
beaches well into the afternoon and inland highs will still be
well below normal, but warmer than the previous couple days.

.LONG TERM...(SUN-WED)

Offshore gradients are expected to continue all of next week,
mostly on the light side, generally under 3mb, but enough to push
temps at least into the 80s for coastal areas and 90s for the
valleys each day. There is potential for higher temps and
stronger winds if gradients and upper support are stronger than
what the models are showing now but as it stands now neither
factor is really that impressive for most of the week. Tuesday
appears to have the best upper support as a second shortwave drops
into the mean trough over the Great Basin and brings a
reinforcing shot of cold air and northeast winds aloft across
southern CA. After that a ridge builds in from the west and the
air mass warms up quite a bit with thicknesses climbing each day
through Friday, up to 580dm and 950 temps near 30c. So we might
see an initial spike in high temps Tuesday west/south of the
mountains, then either little change or slight cooling Wednesday
before warming again at the end of the week as the ridge builds.

&&

.AVIATION...21/0300Z.

At 02Z, there was a double inversion at KLAX. The marine layer
depth was around 1600 feet deep at KLAX, with deeper moist layer
up to around 3800 feet. The top of the highest inversion was
around 6100 feet with a temperature of 16 degrees Celsius.

Low to moderate confidence in the current forecast. IFR to MVFR
conditions will become predominant through 13Z. There is a chance
of LIFR conditions at terminals north of Point Conception. There
is a chance that conditions could improve sooner than forecast at
terminals north of Point Conception. MVFR to VFR conditions at or
below 5000 feet will likely linger into the afternoon hours on
Thursday south of Point Conception.

KLAX and KBUR...MVFR conditions will become predominant through
13Z. MVFR to VFR conditions at or below 5000 feet will likely
linger into the afternoon hours on Thursday south of Point
Conception.

&&

.MARINE...20/900 PM.

Moderate to high confidence in the current forecast. Gale
Warnings were converted to Small Craft Advisory for all remaining
Outer Waters through late tonight before becoming warnings again
early Thursday morning. At least Small Craft Advisory conditions
will be prevalent by late Thursday afternoon. There is a 80
percent chance that gales for the nearshore waters north of Point
Conception and through Santa Barbara Channel, with a 40 percent
chance of gales for the Inner Waters south of Point Mugu to Dana
Point. Steep short-period seas are likely on Thursday afternoon
and evening across much of the waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until 4 AM PDT Friday for zones
      39-52. (See LAXNPWLOX).
     Wind Advisory in effect until 10 PM PDT Thursday for zone 59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 PM PDT Thursday for
      zones 645-650. (See LAXMWWLOX).
     Gale Watch in effect from Thursday afternoon through Thursday
      evening for zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM Thursday to 3 AM PDT
      Friday for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Thursday for
      zones 670-673-676. (See LAXMWWLOX).
     Gale Warning in effect from 3 AM Thursday to 3 AM PDT Friday
      for zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
No significant hazards expected.

&&

$$

PUBLIC...Hall/MW
AVIATION...Hall
MARINE...Hall
SYNOPSIS...Kaplan

weather.gov/losangeles


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