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000
FXUS66 KLOX 181131
AFDLOX

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

.SYNOPSIS...18/327 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)...18/430 AM.

The latest water vapor shows a weak upper-level trough near 34N
and 134W, or about 900 miles west of Los Angeles, while a deeper
trough of low pressure sits upstream near 50N and 158W. Closer to
the surface, onshore pressure gradients are strengthening across
the area early this morning. The latest fog product imagery
indicates a low cloud pattern becoming well-entrenched across the
coastal waters and into all of the coastal areas early this
morning, but the exact extent of the low cloud field remains in
question due to the amount high level cloudiness moving over the
region. A thick middle and high cloud shield, making for some
difficulty seeing some of lower clouds on the infrared bands of
satellite, will likely push a decent amount of cloudiness across
the area today and this evening.

A partly to mostly cloudy day is shaping up across the region.
With the low clouds becoming more entrenched today and the weak
trough sitting off the coast, there is some concern that a gloomy
pattern could develop at the coast today. In fact, there is a
high (50 percent) chance that clouds could hug the beaches today,
struggling to clear from the land mass during the afternoon
hours. The latest GFS LAMP guidance for KSBA and KOXR hints at the
possibility of low clouds lingering into the afternoon hours today.
EPS ensemble cloud cover means moderately support this idea, but
some of cloudiness could be diluted with middle and high cloud
coverage as high-resolution multi-model ensemble members suggest.

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...18/0347Z.

At 00Z, the marine layer at KLAX was about 600 ft deep. The top
of the inversion was at 2200 ft with a temp of 19 deg C.

High confidence in desert and valley TAFs. Low confidence for
coastal TAFs with widespread low clouds are expected tonight, with
IFR to LIFR conds, and local VLIFR conds on the Central Coast.
High clouds may disrupt low clouds from forming.

KLAX...Moderate to low confidence in the 00Z TAF. There is a 20%
chance that cigs tonight will be in the MVFR category and a 20%
chance that cigs will not arrive until as late as 12Z. Any east
wind component should remain less than 6 knots.

KBUR...High confidence in the 00Z TAF with VFR conds thru the pd.
There is a 20% chance of LIFR cigs after 11Z Thu.

&&

.MARINE...17/947 PM.

A very shallow marine layer will continue to bring dense fog to
most waters south of Point Conception into this morning. There is
a chance that dense fog will also form off the Central Coast, but
low confidence. Improved but still reduced visibility is expected
during the day.

High confidence in generally light winds through Friday.
Southeast winds around 10 knots are expected each morning from the
Santa Barbara Channel to Orange County, with some local
enhancements through the Anacapa Passage and San Pedro Channel.

Northwest winds will be increasing over the weekend.

&&

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

&&

$$

PUBLIC...Hall
AVIATION...Phillips
MARINE...Kittell/Schoenfeld
SYNOPSIS...Hall

weather.gov/losangeles

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