Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 240330
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
830 PM PDT Sun Sep 23 2018

.SYNOPSIS...23/830 PM.

Afternoon temperatures will cool a bit early in the week, warm
through the end of the week, then cool again during the weekend.
Overnight and morning low clouds and fog will develop along the
coast and at times will push into the adjacent valleys.

&&

.SHORT TERM (SUN-WED)...23/828 PM.

The latest fog product imagery shows the marine layer stratus
more entrenched this evening over the Southern California Bight,
while clinging the Central Coast. Stronger onshore flow has led to
a more organized marine layer stratus deck as previously
premised. A deep marine layer near 1000 feet currently at KLAX
will deepen some into Monday morning to around 1600 feet. Stratus
intrusion into the valleys looks a bit better later tonight and
into Monday morning. Latest GFS LAMP and NAM MOS guidance bring
some clouds and reduced visibilities into KBUR around daybreak
Monday.

Onshore pressure gradients continue to trend slightly stronger
onshore tonight as an upper-level trough of low pressure over the
Washington State brushes Northern California. All signs point to
a cooling trend continuing into Monday. Locally gusty onshore
winds could impact the higher valleys such as the Santa Clarita
Valley, the mountains, and Antelope Valley on Monday afternoon
and evening as onshore pressure gradients should be a tad
stronger.

The forecast was updated earlier to increase the marine layer
stratus coverage into Monday morning.

***From Previous Discussion***

For Monday night into Tuesday, there will not be too much change
in the low clouds and fog coverage as the even the LA valleys will
again see some patchy low clouds and fog. However, the heights
start to rise and the onshore pressure gradients will weaken
enough to allow some warming across the forecast area, especially
in the mountains and deserts.

By Tuesday night into Wednesday, a rex block forms in the eastern
Pacific, with a high latitude upper high and a cutoff low just to
its south. The cutoff low will be well off to the west and its
only effect will be to amplify a downstream ridge over southwest
California. Temperatures will rise across the entire district due
to higher heights and weaker onshore flow. At this point, the
marine clouds should become shallow enough to be restricted to
only the coastal sections.

.LONG TERM (THU-SUN)...23/126 PM.

The warming trend will continue into Thursday, which should be the
warmest day of the next 7 days. High temperatures on Thursday will
soar close to triple digit territory in the warmest locations,
like the interior valleys and the deserts. Patchy low clouds and
fog should remain along the immediate coastal sections.

On Friday, the upper high off the Pac NW coast breaks down and
allows trof energy to approach the Washington and Oregon coasts,
which in turn, will kick the cutoff low eastward and toward
Northern California. Accordingly, heights will drop, the onshore
flow will again increase, and the temperatures will drop across
the region.

For next weekend, the extended models are in decent agreement that the
cutoff low will push into far Northern California on Saturday.
Little change is expected for Sunday, before another deeper trof
pushes into Northern California early next week.

&&

.AVIATION...24/0017Z.

At 21Z, the marine layer depth was around 1400 feet deep at KLAX.
The top of the marine inversion was around 1950 feet with a
temperature near 21 degrees Celsius. There was another inversion
up to around 2400 feet.

Moderate confidence in the current forecast. Timing of could be
up to one hour earlier than forecast or up to two to hours later.

North of Point Conception, LIFR to IFR conditions will spread
into coastal and coastal valley terminals through 08Z. Conditions
should become predominantly LIFR between 05Z and 08Z. There is a
chance of VLIFR conditions between 08Z and 14Z. VFR conditions
should redevelop between 16Z and 17Z.

South of Point Conception, IFR to MVFR conditions will spread
into coastal terminals through 10Z. Higher confidence in MVFR
conditions south of KOXR. There is a chance of IFR to MVFR
conditions for valley terminals between 10Z and 16Z. VFR
conditions should redevelop between 16Z and 18Z.

KLAX...MVFR conditions will spread into KLAX as early as 03Z or
as late as 09Z. VFR conditions should develop between 16Z and
17Z, but there is a 30 percent chance that MVFR conditions delay
until as late as 20Z. Any east winds that could develop between
11Z and 15Z should remain less than 5 knots.

KBUR...There is a 40 percent chance of IFR to MVFR conditions
between 10Z and 16Z. No wind impacts are expected at this time.

&&

.MARINE...23/814 PM.

A marginal Small Craft Advisory northwest of the Channel Islands
will continue through late tonight. There is a 30 percent chance
that a Small Craft Advisory near Point Sal and Point Conception
south to the Channel Islands could be issued for Monday afternoon
and evening. Otherwise, conditions will remain below advisory
levels through Thursday.

Dense fog with visibility one nautical mile or less will affect
portions of the coastal waters through Monday morning. Highest
confidence for dense fog exists for the coastal waters north of
Point Conception, but the Santa Barbara Channel may also be
affected late tonight and early Monday morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for
      zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

No significant hazards expected.

&&

$$

PUBLIC...Hall/Sweet
AVIATION...Hall
MARINE...Hall
SYNOPSIS...Kj

weather.gov/losangeles


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