Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 200632
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1132 PM PDT Tue Sep 19 2017

.SYNOPSIS...
An upper level trough will continue to persist over the region
allowing for partly to mostly cloudy conditions through Wednesay.
As the trough deepens, a cold front will move through the area
Thursday bringing a slight chance for a few showers and drizzle.
Cooler air behind the front will bring much cooler and breezy
conditions, especially for inland areas. Weak offshore winds are
expected to build late this weekend into early next week with
significant warming expected, especially for coast and coastal
valleys.

&&

.SHORT TERM...(TUE-FRI)

Another trough of low pressure off of Victoria Island will pivot
the broader troughing pattern over the West and dig south into
Southern California through Friday. A weak marine inversion in
place this evening is keeping a muddled cloud pattern over the
South Coast Basin this evening. Clouds should fill in overnight
and into Wednesday, but lower confidence is placed in the sky
cover forecast for tonight. Highest confidence exists for Los
Angeles County areas, with moderate to high confidence for the
Santa Barbara and Ventura Counties.

NAM-WRF solutions are increasing winds through the Interstate 5
Corridor tonight and into Wednesday morning. NAM-WRF boundary
layer winds increase to 25-30 knots across the Interstate 5
Corridor in Northern Los Angeles and Ventura Counties and 850 mb
winds increase to 40 knots across the same area. A marginal Wind
Advisory was added to the forecast for tonight and into Wednesday
morning.

With the troughing in place, a cooler than normal air mass should
remain across the region. As the trough`s vort max drops south, a
showery weather pattern could develop late Wednesday night into
Thursday morning as the trough interacts with the deep marine
layer in place. NAM BUFR time heights sections suggest Los
Angeles County areas picking up a few hundredths of
precipitation.

*** FROM PREVIOUS DISCUSSION ***

Then Thursday with the trough deepening look for the marine layer
depth to increase again south of Pt Conception with possibly some
morning drizzle or light rain, especially up along the south
facing slopes. Highs Thu should be 5-10 degrees cooler than Wed
and 10-20 degrees below normal for inland areas. Clearing will be
slow and possibly not at all in some areas south of Pt Conception.
Cool and breezy along the Central Coast with some morning marine
layer but not as extensive as further south.

By Thursday night and early Friday with the trough axis moving
through the area it`s possible the inversion will become so weak
that marine layer clouds will have a very difficult time
reforming. In any case, another day of well below normal temps
Friday with coast/valleys mostly in the 70s. If marine lyr
struggles to reform Thu night at the very least some stratocu
should form during the day Friday as the low levels remain quite
moist.

.LONG TERM...(SAT-TUE)

Remainder of the forecast for the weekend into early next week
remains more or less unchanged. We`ll be shifting from a deep
marine/strong onshore flow pattern to an offshore/Santa Ana
pattern but it will take a couple days for this shift to really
take effect. So still some lingering marine layer Saturday morning
but early clearing and warmer temps. Then turning weakly offshore
Sunday and slightly stronger Monday and Tuesday which should be
the warmest days of this 7 day period. Still not seeing a lot of
upper support with this offshore event but enough to boost temps
several degrees Sunday and then again Mon/Tue. Will have to see as
we get closer if we`ll see much in the way of winds with this but
at this point the upper support doesn`t quite look strong enough
to get anything more than some locally breezy conditions through
passes and canyons. We should start seeing some 90s in the valleys
by Sunday and possibly some near 90 temps for inland coastal areas
early next week. The rest of next week looks quite warm with weak
offshore flow. Not enough for advisory level winds but certainly
enough for above normal temps, especially for coastal areas which
should stay in the 80s and lower 90s all week, perhaps mid to
upper 90s around downtown LA by Wed/Thu.

&&

.AVIATION...20/0630Z.

At 05Z, the marine layer was around 3900 ft deep at KLAX. The top
of the inversion was around 6500 ft with a temperature of 18C.

Low confidence in the current forecast as a weak marine inversion
is in place across the area. Expect stratus to develop across most
coastal and valley areas south of Pt Conception, with generally
high MVFR cigs or possibly VFR cigs expected. Clearing is expected
by mid to late morning in the valleys and by early afternoon
across the coastal plain.

KLAX and KBUR...Low confidence in the 06Z TAF. There is a 30-40%
chance that conds will remain VFR overnight, either due to no
cigs, or cigs in the MVFR category. If MVFR cigs develop as
expected, there is a 20-30% chance they will linger thru 22Z.

&&

.MARINE...19/900 PM.

Winds struggling to reach gale warning levels this evening. A Gale
Warning for the Southern Outer Waters have been replaced with a
Small Craft Advisory through 3 AM Thursday morning. A Gale Watch
was also added for the Southern Outer Waters from Thursday morning
through Thursday afternoon. This watch will likely need to be
extended into Thursday evening, as well. All other headlines for
the Outer and Northern Waters have been extended until Thursday
afternoon for now, but they likely will need to extended until
early Friday morning.

Western portions of the Santa Barbara Channel will have SCA level
winds through late tonight and possibly into each afternoon and
evening through Thursday night.

Combined seas will increase through Wednesday as building short-
period waves combine with the first northwest swell of the season.
Seas will build to 7 to 10 feet at 14 seconds on Wednesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until 10 AM PDT Wednesday for zones
      53-54. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 PM PDT Thursday for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Wednesday for
      zone 650. (See LAXMWWLOX).
     Gale Warning in effect until 3 PM PDT Thursday for zones
      670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Thursday for
      zone 676. (See LAXMWWLOX).
     Gale Watch in effect from late Wednesday night through
      Thursday afternoon for zone 676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(FRI-TUE)
No significant hazards expected.

&&

$$

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

weather.gov/losangeles



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