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000
FXUS66 KLOX 211217
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
517 AM PDT Sun Apr 21 2024

.SYNOPSIS...21/509 AM.

Onshore flow with a persistent marine layer will remain in place
through the week. Night through morning low clouds and fog will
continue across the coast and valleys through the period. A warm-
up will peak today, then significant cooling with a deep marine
layer will develop as a series of weak storm systems move over the
region. Drizzle or light rain is possible after Tuesday,
especially over the mountains.

&&

.SHORT TERM (TDY-TUE)...21/519 AM.

Weak ridging aloft over the region continues to the builds into
the state this morning. Onshore flow is weakening across the area
with onshore pressure gradients weakening between 2 and 3 mb
versus last night. To the north along the Central Coast, the flow
pattern is turning offshore this morning. The marine layer depth
is starting to thin some as subsidence aloft start to press down
on top of the marine layer.

Low cloudiness is much less extensive across the area versus the
last several nights and mornings, but stratus remains wedged in
across the southern California bight and into the South Coast of
California. Low clouds have been most extensive across the Santa
Barbara County South Coast and into the Ventura County coast.
Tendrils of low clouds made a good run up the Santa Clara River
and Ventura River valleys this morning with clouds spreading into
the Ojai Valley and just touching the western portion of the Santa
Clarita Valley. To the south in Los Angeles County, clouds have
been stubborn to form but appear to be regrouping for a morning
push. A patch of low clouds and dense fog does remain in the Santa
Ynez River valley this morning with the patch extending from near
Lompoc back toward Santa Ynez. This patch should mix out shortly
after sunrise as drier and warmer air aloft mixes down.

Subsidence and offshore flow trends will bring a warm-up for
today. The warming trend will be most pronounced away from the
coast across the interior portions of the area. The Antelope
Valley will likely see the warmest temperatures today with
Palmdale likely hitting its first 90 degree day of the calendar
year. NBM solutions give KPMD a 60 percent chance of hitting 90
degrees today. Many valley locales will warm into the mid 70s to
mid 80s today, or about 7-10 degrees above normal for this time of
year. With the ridge aloft building in, the South Coast beaches
will likely be stubborn to clear again. Low clouds will be hugging
the coastline and temperatures along the coast were trended
closer to persistence as a result.

The flow pattern will quickly reverse tonight through Tuesday as
strong onshore pressure gradients develop. Low clouds and fog
will likely become well-entrenched by Monday morning, then a
deepening marine layer ahead of a weak trough of low pressure will
push clouds well into the coastal slopes of the mountains on
Tuesday. This cutoff trough, located near 27N and 148W, or about
1600 miles west-southwest of Point Conception, will move towards
southern California through Wednesday. The marine layer depth,
near 800 feet deep at KLAX this morning, will deepen to near 2500
feet deep on Monday morning. Then, the marine layer depth will
deepen even further to around 4000 feet deep by Tuesday. The cloud
forecast follows suit of the EPS cloud cover means and local 3-km
WRF time height sections. With omega values present in time
height sections across the region on Tuesday morning, patchy
morning drizzle was added to the forecast.

.LONG TERM (WED-SAT)...21/519 AM.

The low pressure system will approach the southern California
coast between Tuesday night and Wednesday night. EPS, GEFS, CMC
ensemble members offer up a mix of solutions with light rain or
drizzle between Tuesday and Wednesday. Deterministic solutions
continue to suggest a cutoff trough moving through the southern
California bight between Wednesday and Wednesday night. A difluent
flow pattern setting up could bring the possibility of at least
mountain showers developing on Wednesday and Wednesday night, with
drizzle likely lingering into Wednesday night and Thursday across
the coastal and valleys. A deep marine layer with clouds well
into the coastal slopes is likely to persist and create a mostly
cloudy to cloudy pattern across the region between Wednesday and
Thursday. GFS solutions hint possible mountain convection
developing over the Ventura County mountains on Wednesday
afternoon and evening as lifted index values approach 0 degrees
Celsius. With 850-700 mb mixing ratios near 6 g/kg and EPS/GEFS
PWAT mean values approaching 0.70 inch, convection or showers
cannot ruled out over the mountains during the period. A handful
of ensemble solutions offer up a mix for precipitation, but this
should expected given the potential for a more convective pattern.

As we move later into the week and into next weekend, forecast
ensembles tend to favor an inside-slider type system dropping down
across the region between Friday and Sunday. Almost all of the
EPS, GEFS, and CMC ensemble favor light rain for the northern
areas including KLPC, KSDB, and KPRB. EPS solutions are wettest
relative to the GEFS and CMC, but the pattern seems to match up
well with deterministic solutions. If the trough moves about 100
miles farther to the west, it is possible more moisture could get
picked by the system. Given the uncertainty, PoPs break away from
NBM values for late week advertising a slight chance to chance of
showers. If the system plays out as advertised, it stands to
reason that southern Santa Barbara and Ventura Counties and
western Los Angeles County will be in rain shadow as downsloping
will wipe out any chance of rainfall.

Gusty winds will likely accompany this system also and an extended
period of at least wind advisory level winds could develop
through the Interstate 5 Corridor, across southern Santa Barbara
County, and down into the Antelope Valley west of Highway 14. EPS
solutions also develop a decent westerly push with a cold frontal
boundary passage on Friday evening at KOXR and KLAX.

&&

.AVIATION...21/1107Z.

At 1030Z, the marine inversion at KLAX was based around 900 feet.
The top of the inversion was 4200 feet with a temperature of
20 degrees Celsius.

Overall, moderate to high confidence in 12Z TAF package. This
morning, MVFR/IFR conditions are expected to be confined to
coastal sites south of Point Conception with moderate confidence
in timing of dissipation of current CIG/VSBY restrictions late
this morning/early this afternoon. For tonight, CIG/VSBY
restrictions should be more widespread, affecting the Central
Coast as well as the coastal valleys.

KLAX...Moderate confidence in 12Z TAF. Timing of dissipation of
current CIG/VSBY restrictions could be as early as 17Z or as late
as 22Z this afternoon. For tonight, high confidence in return of
CIG/VSBY restrictions, but moderate confidence in timing of return
(could be +/- 3 hours of current 03Z forecast). No significant
easterly wind component is expected.

KBUR...Overall, moderate confidence in 12Z TAF. There is a 30%
chance of MVFR VSBYs 13Z-17z. For tonight, high confidence in
return of CIG/VSBY restrictions, but only moderate confidence in
timing of return (could be +/- 3 hours of current 10Z forecast).

&&

.MARINE...21/407 AM.

For the Outer Waters, moderate to high confidence in current
forecast. Today through tonight, high confidence in Small Craft
Advisory (SCA) level northwest winds. For Monday and Tuesday, high
confidence in winds and seas remaining below SCA levels. On
Wednesday, there is a 40-60% chance of SCA level winds. On
Thursday, there is a 60-80% chance of SCA level winds.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Today through tonight, there is a
60-70% chance of SCA level winds, especially in the afternoon and
evening hours. For Monday through Wednesday, high confidence in
winds and seas remaining below SCA levels. On Thursday, there is a
40-50% chance of SCA level winds.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Today through Tuesday, high
confidence in winds and seas remaining below SCA levels. For
Tuesday night through Thursday, there is a 40-50% chance of SCA
level winds across the western half of the Santa Barbara Channel,
but high confidence in winds and seas remaining below SCA levels
elsewhere across the southern Inner Waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for zone
      645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 6 AM PDT Monday for
      zones 670-673. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Hall

weather.gov/losangeles

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