000
FXUS66 KLOX 181617
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
917 AM PDT Thu Apr 18 2024

.SYNOPSIS...18/918 AM.

Strengthening onshore flow beneath southwest flow aloft will
bring a cooling trend through Friday with more extensive night
through morning low clouds and fog. A warming trend is expected
over the weekend as weak ridging aloft builds in. A significant
cooling trend with a deep marine layer depth is on tap for the
middle-to-late portion of next week.

&&

.SHORT TERM (TDY-SAT)...18/918 AM.

***UPDATE***

Marine layer stratus covers most of the coastal areas this morning
with areas of dense fog. Visibilities are improving as the stratus
lifts and break up, however there are still a fair amount high
clouds around. Gradients continue to trend onshore today as weak
troughing will remain over California through Friday, resulting in
cooling temperatures today. The marine layer is expected to deepen
tonight, so less dense fog at the coast but possibly some dense
fog farther inland in the valleys as low clouds push in there
later tonight.

***From Previous Discussion***

With more onshore flow and cloudiness, a cooling trend will
develop through Friday. The marine layer depth at KLAX, near 850
feet deep at KLAX a bit ago, will deepen through this morning to
around 1200 feet deep. As trough moves closer to the region
through tonight, the marine layer depth will likely deepen
additional. The latest 3-km WRF solutions suggest marine layer
depth approaching depths of 2500 feet deep by Friday morning, and
possibly as high as 3500 feet deep by Friday night. If this
materializes as modeled, this should push clouds well into the
coastal slopes of the mountains, especially Friday night. Patchy
morning drizzle cannot completely be ruled out on Friday morning
and overnight Friday night into Saturday morning as 850 mb mixing
ratios approach 9-10 g/kg and ample omega values exists through
the mixed layer.

Cyclonic flow turns anticyclonic into Saturday as ridging aloft
starts to nose into the California coast. A warming trend will
establish on Saturday and likely persist through the weekend as
the ridge axis slowly drifts over the region. GEFS and EPS
ensemble members agree well that a warming trend will take shape
starting Saturday.

.LONG TERM (SUN-WED)...18/432 AM.

The ridge axis will drift over the region sometime on Sunday and
the warm up should peak for most areas on Sunday. There is a
moderate chance that the warming trend could linger a little
longer into the interior areas. EPS ensemble members suggest a mix
of warmer and cooler solutions with means about the same at KPMD
and KWJF on Monday.

A weak boundary dropping south of Sunday, and falling apart north
of the area, could bring marginally gusty winds through the
Interstate 5 Corridor and across southern Santa Barbara County on
Sunday night and into Monday. EPS ensemble members indicate a tick
up of winds at KSDB and KSBA, but the strength seems to be in
question as there is a quite a bit of spread across the ensemble
members solutions. Fortunately, there is still sometime to resolve
the pattern setting up.

As we get into next week, a significant cool down is shaping up
through the week. All EPS, GEFS, and CMC ensemble members indicate
a cooling trend to be firmly in place for Tuesday and Wednesday
as trough of low pressure digs into the region. Almost all of the
ensemble member solutions have precipitation developing sometime
between Tuesday and Thursday, but there is quite a bit of spread
in regards to the timing. Amounts look light across the
perturbations, on the order of a tenth of an inch or less, but
there is a 10 percent chance that amounts could be more than that.
For the last several days, the forecast ensembles have
highlighted this period as a possibility for another opportunity
of rain, but it remains to be seen if it will amounts to drizzle
or more light to moderate rainfall across the region.

&&

.AVIATION...18/1247Z.

At 12Z, the marine layer at KLAX was about 1300 ft deep. The top
of the inversion was at 4000 ft with a temp of 16 deg C.

Widespread low clouds in coastal areas and in the Santa Ynez
Valley this morning, with generally LIFR to VLIFR conds. Some
VLIFR conds may push into the VTU County Valleys this morning,
and into the San Gabriel Valley for a few hours. Low clouds will
clear in most areas by late morning, but could linger into the
afternoon near the coast, especially across southern SBA County
and VTU County. Otherwise, expect lots of high clouds across the
region. Low clouds will return to coastal areas this evening,
then push into the valleys after midnight. Conds will be mostly
IFR to LIFR, then improve late at night in coastal areas.

KLAX...Moderate to low confidence in the 12Z TAF. LIFR to VLIFR
conds should improve to IFR by mid morning, but there is a 30%
chance that will not occur until at least 17Z. There is a 30%
chance that IFR to low MVFR cigs could linger thru 20Z. There is a
30% chance that cigs will arrive this evening as early as 01Z,
and start off in the IFR category. Any east wind component should
remain less than 6 knots.

KBUR...Moderate to high confidence in the 12Z TAF. There is a 20%
chance of IFR cigs from 13Z-17Z this morning, and there is a 20%
chance that cigs will not arrive tonight until at least 09Z.

&&

.MARINE...18/915 AM.

A very shallow marine layer will bring dense fog to a good
portions of the coastal waters this morning.

High confidence in conditions remaining below Small Craft
Advisory (SCA) levels thru Saturday morning. SE winds around 10
knots should occur each morning from the SBA Channel southward to
San Mateo Point. Locally stronger SE gusts are possible thru the
Anacapa Passage and San Pedro Channel.

NW winds will likely reach SCA levels in the outer waters Saturday
afternoon through Sunday, and across the inner waters north of
Point Sal and the Santa Barbara Channel during the
afternoon/evening hours over the weekend. There is a 30% chance
of SCA conditions in the outer waters Monday.

&&

.BEACHES...18/551 AM.

A long-period southerly swell cause elevated surf of 3 to 5 ft
with local sets to 6 or even 7 ft on south-facing beaches of L.A.
County, Catalina Island, and possibly VTU County. There will be a
high risk of rip currents on most beaches today.

&&

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

&&

$$

PUBLIC...MW/Hall
AVIATION...DB
MARINE...DB/RK
BEACHES...DB
SYNOPSIS...Hall

weather.gov/losangeles

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