861
FXUS66 KLOX 160106
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
606 PM PDT Sun Sep 15 2024

.SYNOPSIS...15/200 PM.

Much cooler than normal temperatures will continue through the
next several days, before warmer conditions return for next
weekend. Gusty winds are expected at times, especially tonight
through Monday night. Periodic drizzle and light rain showers
will affect the region, and the highest chances for this activity
will be tonight into Monday morning, and again Wednesday night
into Thursday. Areas of low clouds will be common over the coast
and coastal valleys, with occasional clearing each afternoon.

&&

.SHORT TERM (SUN-WED)...15/200 PM.

High-amplitude upper troughing will remain anchored over the
western states going into this week. Satellite loops depict
significant energy aloft digging through the west side of a deep
upper trough along the Pacific Coast, causing the upper vort
center to quickly deepen off the Oregon coast toward northern
California. By Monday, this vort center will begin an eastward
trajectory across central California, as a shortwave trough
quickly advances through the base of its surrounding large area
of midlevel cyclonic flow.

The progression of this impulse will provide the upper support
necessary for winds to continue increasing through tonight,
especially for areas south of Point Conception, in proximity to
the midlevel speed maximum enhanced around the shortwave trough.
The gusty winds will continue through Monday night, before upper
support translates eastward thereafter. Wind Advisories remain in
effect through Monday evening for the Antelope Valley and nearby
western foothills, where southwest winds 20-30 mph gusting to 45
mph are expected. In addition, enhanced winds will extend farther
upstream through the interior slopes of the San Gabriels tonight
through Monday evening in response to the progression of stronger
momentum aloft. The Wind Advisory has been expanded to include the
San Gabriels, as well as the eastern Antelope Valley foothills,
where gusts up to 50 mph will be of concern. In addition, the
strongest low-level mass response to the progressive shortwave
trough will bring terrain-channeled flow through the Santa
Barbara Channel eastward to the Ventura County coast and nearby
coastal valleys, warranting Wind Advisories in these areas for
gusts up to 45 mph. There will be gusty winds over the Santa Ynez
Range and the Santa Barbara County South Coast tonight through
Monday night, though the overall westerly-flow orientation and
lack of a stronger pressure gradient provide limited confidence in
Advisory level wind gusts occuring. However, there is a 30%
chance for the Wind Advisory to be expanded to include these areas
generally south of Point Conception in later forecasts. Winds
should be decreasing by Tuesday morning as stronger flow aloft
departs the region, with areas of gusty winds returning by
Wednesday in association with the next reinforcing vort max.

With the cold core aloft being closest to the area on Monday,
Monday high temperatures will be 10-20 degrees below normal in
most areas, generally in the 60s and 70s. Highs on Tuesday are
forecast to increase a few degrees as heights aloft rebound behind
the departing shortwave trough and first vort max, and then fall
again by a degree or two for Wednesday as height falls overspread
the region with the next reinforcing vort max.

The combination of the deepening moist layer and differential
cyclonic vorticity advection ahead of the first vort max will
support drizzle and light rain showers spreading across the region
tonight into Monday morning. The greatest chance for measurable
rain will be along the coastal slopes of the Transverse Ranges
owing to upslope-flow enhancements, where up to a quarter inch of
rain may fall. Elsewhere, rainfall amounts should be generally a
tenth of an inch or less. Rain will end by later in the day
Monday, and then return as early as Wednesday in advance of the
next reinforcing vort max.

Clouds will remain abundant for the rest of today, with slow and
partial clearing ongoing. The moist-layer depth has already
increased to 4500 ft at KLAX, and additional deepening is expected
over the next day in response to ascent ahead of the approaching
vort max. This will continue to reinforce marine stratus over the
coasts and well inland across the coastal valleys to the nearby
coastal slopes, with only slow and partial clearing later this
afternoon. Clouds will be reinforced tonight, before cooling at
the top of the moist layer on Monday may completely mix out clouds
causing early-day clearing. There is substantial uncertainty
regarding cloud coverage and clearing trends on Tuesday and
Wednesday, though night through morning low clouds should be
prevalent especially near the coast -- as moist-layer depths are
boosted by the reduced midlevel heights that get further reduced
for Wednesday in advance of the next vort max.

.LONG TERM (THU-SUN)...15/200 PM.

Deep upper troughing will persist over the region late this week,
as a reinforcing vort max tracks southward over coastal
California amid broad cyclonic flow across the western states.
Another round of very light showers and gusty winds will be
possible continuing into Thursday from Wednesday, with well-
below normal temperatures across the area on Thursday. Thursday
high temperatures will only be in the 70s in most places, with
readings in the 60s near the beaches. Temperatures will be warming
by next weekend, though there is some uncertainty regarding the
onset timing of more significant warming owing to phasing
differences between model solutions. However, by next weekend,
highs in the 80s to lower 90s should be common areawide, as an
amplified upper ridge moves into the region from the east Pacific.

The sharpness of height gradients aloft -- surrounding the
reinforcing vort max -- do not appear as significant as those
associated with the earlier-week disturbance, and wind headlines
do not appear likely at this time. However, coasts and coastal
valleys of Ventura County vicinity northeastward through the high
terrain across Los Angeles County may experience gusty winds
Wednesday into Thursday, and there is a 30% chance for Wind
Advisories to become necessary -- especially if the upper low were
to trend deeper. Otherwise, marine stratus will be prevalent
across much of the region from the coasts to the coastal slopes
ahead of the reinforcing vort max through Wednesday night, before
potentially scouring out later in the week and becoming confined
to areas near the immediate coast.

&&

.AVIATION...16/0106Z.

Around 00Z, a weak marine layer was around 4000 feet deep at
KLAX. The top of the marine inversion was near 5000 feet with a
temperature around 16 degrees Celsius.

Low confidence 00Z TAFs for coasts and coastal valleys. Timing of
flight changes may be off by 3 hours and more than one flight
category at times. There is a 20-50 percent chance of -DZ/-SHRA
focused with vsbys between 2-6SM between 06Z-18Z except locally
earlier especially for KSBP and KSMX.

Low confidence in KPRB with a 40 percent chance that VFR conds
prevail through the period.

There is a 20-30 percent chance of MVFR cigs between 10-18Z for
KPMD with otherwise high confidence for the desert TAFs.

KLAX...Low confidence in 00Z TAF. There is a 20% chance of vsbys
between 2SM and 4SM due to drizzle between 06Z and 15Z. Cigs may
become >030 by 14Z. No significant east wind component expected.

KBUR...Low confidence in 00Z TAF. There is a 20% chance of brief
cigs 008-009 and/or vsbys 2-4SM due to drizzle between 07Z and
16Z.

&&

.MARINE...15/139 PM.

For the Outer Waters (Central Coast to San Nicolas Island),
moderate to high confidence in the forecast. For the northern
zone (Pt Piedras Blancas to Pt Sal), winds should remain near or
above Small Craft Advisory (SCA) level throughout the day until
late tonight. Then, winds are likely to remain below SCA level
until Tues afternoon, where there is a 50-60% chance of SCA level
winds thru early Wed morning. From Wed afternoon thru late Sat,
moderate confidence in winds remaining below SCA level, with the
highest chances in the afternoon thru evening hours Fri and Sat.
For the southern zones (Pt Sal to San Nicolas Island) moderate to
high confidence in winds persisting thru late Tues or early Wed.
Moderate to high confidence in winds then picking back up to SCA
level Wed afternoon thru late, especially from Pt Conception to
San Nicolas Island. From Thurs thru Sat, moderate confidence in
winds remaining under SCA thresholds, with a 20-30% chance of SCA
winds Sat afternoon.

For the Inner Waters off the Central Coast. High confidence in
SCA level winds lasting thru late tonight, with local gusts to 35
kt possible in the far northern portion of the zone. High
confidence winds will remain sub advisory level thru Tues midday,
then there is a 30-50% chance in SCA winds in the afternoon thru
evening hours Tues thru Wed, with best chances Tues. Moderate to
high confidence in winds being sub advisory Thurs thru Sat.

For the Santa Barbara Channel, high confidence in winds reaching
SCA level, but moderate confidence in timing. Winds may increase
as early as this evening, but higher confidence in late tonight
or early tomorrow. High confidence in GALES across the channel,
specifically from Santa Cruz Island east to the coast beginning
tomorrow mid day and lasting thru at least late Tues. An upper
level trough passing overhead will lead to an abrupt enhancement
of the surface level winds, leading to very choppy seas and
strong rip currents. Winds will decrease into the early morning
hours Tues, with moderate confidence in them remaining above SCA
level thru late Wed, especially in the afternoon and evening.
Then, there is a 20-40% chance of SCA level winds Thurs thru Sat,
with best chances Fri and Sat.

For the nearshore waters off the Los Angeles and Orange County
coasts, SCA level winds will develop in the morning throughout
much of the zone. In the northwest portion of the zone from
Malibu west to the Channel Islands, and as far south as Santa
Barbara Island, winds will likely (70-80% chance) rapidly
increase to GALE FORCE by midday. Seas will be choppy in this
area, and exposed beaches may see strong rip currents. Winds will
drop off to SCA level late Mon/early Tues. Moderate to high
confidence in winds remaining below SCA level Tues afternoon thru
Sat.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from noon Monday to midnight PDT
      Monday night for zones 354>357. (See LAXNPWLOX).
     Wind Advisory in effect from midnight tonight to midnight PDT
      Monday night for zones 379-380-382. (See LAXNPWLOX).
     Wind Advisory remains in effect until 9 PM PDT Monday for
      zones 381-383. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for
      zones 645-670. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 AM to noon PDT Monday
      for zone 650. (See LAXMWWLOX).
     Gale Warning in effect from noon Monday to 3 AM PDT Tuesday
      for zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM to noon PDT Monday
      for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT Tuesday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen
AVIATION...Munroe/Lewis
MARINE...Lewis
SYNOPSIS...Cohen

weather.gov/losangeles

Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox