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000
FXUS66 KLOX 201612
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
912 AM PDT Sat Apr 20 2024

.SYNOPSIS...20/914 AM.

Low clouds and fog will be present through at least the middle of
next week, least expansive on Sunday. Sunday will be the warmest
with temperatures 5 to 15 degrees above normal, with steady
cooling to follow. Drizzle is expected Tuesday through Thursday
with a chance for light rain around Friday.

&&

.SHORT TERM (TDY-MON)...20/914 AM.

***UPDATE***

The marine layer is up to 3000 feet at LAX and 1800 feet at
Vandenberg. Low clouds have pushed across all coastal and valley
areas this morning. There are some indicators for better clearing
compared to yesterday, with onshore pressure gradients trending
weaker and a northwest clearing trend pushing down the Central
Coast. With that said, this is a potent marine layer with a
healthy capping inversion, so still expect a lot of coastal areas
to be slow to clear this afternoon, if at all. The Central Coast
would be the one exception where that clearing trend will likely
bring much more afternoon sun than yesterday. The current forecast
is in good shape with no significant changes needed.

The forecast challenge for today is to figure out just how much
warming we will see tomorrow, and how significant the chances of
light rain are for the end of next week. The other interesting
wrinkle is that there is some instability over the mountains this
afternoon. While the moisture is limited, 850mb dewpoints are in
the 8-10 C range which traditionally is the starting point to
start thinking about convection. The threat for thunderstorms is
very small but not zero. The most likely outcome is for a few
afternoon cumulus clouds to form over the mountains that do not
amount to much.

***From Previous Discussion***

This looks to be last day of a deep marine layer as ridging aloft
can be seen building into the West Coast this morning. Subsidence
aloft will start to assert its influence on the marine intrusion and
low cloudiness will likely clear a little better for areas north
of Point Conception. A weak cold air advection pattern setting up
just north of the area will scour out the low clouds field through
this afternoon and flip the flow pattern to weak offshore across
the Central Coast late tonight. South of Point Conception, the flow
will turn more northerly by tonight, but there will not be enough
of an offshore push to knock out the stratus field and its
associated marine intrusion. Some semblance of marine layer depth
will likely remain in place for the weekend and keep night through
morning low clouds and fog in the forecast. EPS cloud cover means
and 3-km WRF time height sections back up this idea, with less
cloud cover and moisture in the boundary layer for the northern
terminals, while keeping cloud cover means higher and moisture in
the boundary layer for KLAX. With the ridging aloft building in,
there is some concern Sunday could be equally gloomy at the
beaches as warming aloft with the subsiding air could tighten the
marine inversion. EPS cloud cover means suggest this to be a
10-20 percent chance of occurring. For now, the forecast keeps a
persistent marine layer depth in place and night through morning
low clouds and fog continuing across the Southland.

With high pressure aloft building in, a brief warm up,
establishing today, will peak on Sunday, especially away from the
coast and outside the marine influence. The warmest locations will
likely be the interior valleys. Still, most valley locations will
warm into the upper 70s and 80s on Sunday. The hottest locations
with the brief warm-up will be out in the Antelope Valley, where
Palmdale and Lancaster could record their first 90 degree day of
2024. in the calendar year. The forecast goes with 89 degrees for
KPMD and KWJF, but the latest NBM solutions indicate there to be
a 5 percent chance of KWJF hitting the 90 degrees mark and a 35
percent chance of KPMD touching 90 degrees on Sunday.

Southwest flow aloft redevelops on Sunday night and into Monday.
EPS cloud cover means respond in advertising a broader night
through morning low cloud field across the coastal and valley
terminals. Local 3-km WRF solutions deepen the marine layer depth
at KLAX from around 1500 feet deep on Sunday afternoon to near
2500 feet deep by Monday morning. To the north, local 3-km WRF
solutions bring back the boundary layer moisture aggressively for
KSMX, forming a 1200 foot marine layer depth on Sunday evening
and deepening it to 2500 feet deep by Monday morning. With
southwest flow aloft developing and strengthening through the
period, the seems to make sense and the forecast follows with this
idea. The forecast introduces a cooling trend for Monday and add
a bit more low cloud coverage into Monday morning.

.LONG TERM (TUE-FRI)...20/406 AM.

The cooling trend will turn much more significant into Tuesday and
Wednesday as a trough over the Central Pacific Ocean near 160W
moves into region around midweek. Due to its cutoff nature, the
forecast models and ensembles are struggling to get a handle on
this trough. EPS, GEFS, and CMC forecast ensemble members have
backed off some on the precipitable water means and the number of
solutions with precipitation. Still, pattern recognition of this
trough in the deterministic model solutions suggest a favorable
pattern developing for showers toward mid-to-late next week. GFS
and ECMWF solutions have some timing differences, but offer up a
trough with a decent southerly fetch. The difluent flow pattern
that sets up would be conducive for convection to develop,
especially over the mountains. Slight chance PoPs remain in the
forecast, most prevalent across the higher terrain of southwest
California and breaking away from NBM values.

Forecast ensembles hint a better chance of rain with a
reinforcing inside slider trough for late next week. The ECMWF
deterministic solutions shows a piece of a low pressure system in
Siberia ejecting out across the Pacific and phasing with the semi-
permanent Aleutian Low. This wave overruns the ridge and drops
quickly down into eastern Pacific Ocean by Thursday or Friday. A
trough carves over southern California and this trough should
pipes much colder air into the region. EPS solutions buy into a
wet pattern for the mountains of southern California as ensemble
means favor precipitation for KSDB, KL35, and KTSP. The forecast
keeps a slight chance to chance across the higher terrain of
southwest California, and continues to break away from NBM values.

Much caution should be placed in the forecast for late next week
as the cutoff flow pattern could bring much change to the forecast
period and showers could linger longer than what is indicated. It
should be noted that cluster analysis favor height falls over the
region with a trough axis over the state. Stay tuned as this
portion of the forecast could differ greatly.

&&

.AVIATION...20/1052Z.

At 1030Z, the marine inversion at KLAX was based at 2500 feet.
The top of the inversion was 4800 feet with a temperature of
14 degrees Celsius.

Overall for 12Z TAF package, high confidence in desert TAFs and
moderate in coastal/valley TAFs. Current MVFR/IFR conditions are
expected to dissipate by late this morning/early this afternoon,
but timing could be +/-2 hours of current forecasts. For tonight,
moderate confidence in CIG/VSBY restrictions remaining confined
to coastal sites south of Point Conception.

KLAX...Moderate confidence in 12Z TAF. Timing of dissipation of
MVFR CIGs could be +/-2 hours of current 21Z forecast. For
tonight, high confidence in return of MVFR conditions, but low
confidence in timing (could be +/- 4 hours of current 05Z
forecast). No significant easterly wind component is expected.

KBUR...Moderate confidence in 12Z TAF. Timing of dissipation of
MVFR CIGs could be +/- 2 hours of current 18Z forecast. For
tonight, there is a 20-30% chance that IFR/MVFR conditions could
return after 06Z.

&&

.MARINE...20/830 AM.

For the Outer Waters, high confidence in current forecast. Today
through Sunday night, Small Craft Advisory (SCA) level winds and
choppy seas are expected, generally north of San Nicolas Island
with the most widespread winds across PZZ670/673. For Monday
through Wednesday, winds and seas will remain below SCA levels.

For the Inner Waters north of Point Sal, high confidence in
current forecast. SCA level winds are expected in the afternoon
and evening hours both today and Sunday. For Monday through
Wednesday, winds and seas will remain below SCA levels.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. For most of the southern waters,
high confidence in winds and seas remaining below SCA levels today
through Wednesday. The only exception will be the western half of
the Santa Barbara Channel where there is a 50-60% chance of SCA
level winds late Monday afternoon through Monday night.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect from 2 PM this afternoon to 9
      PM PDT this evening for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect from 2 PM this afternoon to 6
      AM PDT Monday for zones 670-673. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/Kittell
AVIATION...RAT
MARINE...RAT/Sirard
SYNOPSIS...Kittell

weather.gov/losangeles

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