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000
FXUS66 KLOX 221627
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
927 AM PDT Mon Apr 22 2024

.SYNOPSIS...22/501 AM.

A cool and cloudy weather pattern will setup for much of this
week as strong onshore flow and a deeper marine layer will bring
low clouds and fog to the coast and coastal valleys. High
temperatures will cool significantly to below-normal for this time
of year by Tuesday as a series of low pressure systems will move
over the area this week. There will be a chance for drizzle or
very light rain this week with the passage of these systems.

&&

.SHORT TERM (TDY-WED)...22/931 AM.

***UPDATE***

The forecast is well on-track, and no changes have been made to
the ongoing forecast.

***From Previous Discussion***

Low clouds and fog are well entrenched this morning as strong
onshore flow continues to develop in southwest flow aloft. Clouds
have pushed well into the valleys this morning with low clouds and
fog reaching the coastal slopes of the mountains north of Point
Conception and into the Santa Clarita Valley south of Point
Conception. AMDAR soundings from around 09Z at KLAX indicate a
marine layer depth around 1900 feet deep. Drizzle has developed
across the southern Santa Barbara and Ventura Counties this
morning and into western Los Angeles County with several rain
gauges picking up measurable drizzle. Pavement surfaces are wet
here at the office in Oxnard from morning drizzle. High-
resolution multi-model ensemble solutions have handled PoPs
decently for this morning and favor was shown to the solutions in
advertising measurable drizzle over the next couple of days and
nights. Multi-model ensemble members keep the focus on southern
Santa Barbara County through the day today, then the threat for
measurable or light rain will spread to most coastal and valley
areas tonight and into Tuesday morning.

Low clouds will be quite extensive over the coming days as the
marine layer depth is progged to deepen to between 3500 and 4000
feet in local 3-km WRF time height sections. Much cooler
temperatures were added into the forecast, undercutting the NBM
solutions on Tuesday and Wednesday as strong onshore pressure
gradients develop. The land mass will likely struggle to clear the
next several days ahead of a weak trough near 38N and 142W. Water
vapor imagery shows several weak shortwaves kicking out ahead of
the low pressure system. These feature will serve to deepen the
marine layer depth and provide some instability to lift the marine
intrusion and squeeze out some drizzle or light rain. The coolest
day of the short-term looks to be Wednesday when upper 50s and
60s will be common across the coastal and valley areas. Closer to
the coast, a persistence forecast will very likely be the best bet
over the coming days as clouds will struggle to clear, and at
best, hug the shoreline.

With the stronger onshore push, gusty southwest winds will slowly
increase each day through Wednesday. Wind advisories will likely
be needed for interior valleys and the interior slopes of the
mountains on Tuesday and Wednesday as the cooler marine air mass
will be create a pressure difference across the mountains.
Antelope Valley sites like Lake Palmdale will turn quite gusty as
we get closer to midweek.

The upper-level trough will move over the region between Wednesday
and Wednesday night. GFS deterministic solutions hint at mountain
showers developing on Wednesday afternoon and evening. Pattern
recognition would suggest this to be the case, but instability
will need to be monitored closely as GFS solutions suggest lifted
index values less than zero degrees Celsius over the Ventura
County mountains. Isolated thunderstorms cannot be ruled out with
daytime heating on Wednesday afternoon and evening. For now, the
forecast keep PoPs higher than NBM values for the period, but this
could change with the trough`s position.

.LONG TERM (THU-SUN)...22/501 AM.

The trough passage should mix out the marine layer depth and
weaken the marine inversion between Wednesday night and Thursday,
but a cooler than normal weather pattern is likely to persist and
keep temperatures in the 60s across much of the coastal and valley
areas. A break in the action looks to occur for Thursday, then
another much colder, but inside-slider type trough will dig south
into the region between Friday and Saturday. A wave will eject
out of the semi-permanent Aleutian Low Pressure System on
Wednesday, overrun the ridge in supergeostrophic flow, then drops
down into California on Friday. Due to its trajectory, PoPs favor
the northern areas in the latest forecast, while still planning
for a possible shift back to the west. PoPs are likely too dry for
areas north of Point Conception and into the northern slopes of
the mountains. EPS ensemble members solutions continue to suggest
precipitation for KSDB. All solutions of the EPS for KSDB have
measurable precipitation with run-to-run consistency going back
the last several days. Confidence is growing for the late week
precipitation to primarily the northern areas.

Winds may end up being the main story with this low pressure
system. The GFS surface pressure pattern is impressive, agreeing
well with EPS ensemble wind gusts. Gales will be possible across
the coastal waters with gusty west to northwest winds on Friday.
Advisory level wind gusts are likely for the Central Coast, the
Santa Ynez Valley and into the coastal sections south of Point
Conception. EPS wind gust means are consistent in advertising
gusty winds at KSMX, KLPC, and KIZA north of Point Conception, and
KSBA, KOXR, and KLAX into the coastal areas south of Point
Conception. Gusty winds, potentially reaching damaging levels in
the Antelope Valley foothills, are likely at KSDB, KWJF, and KPMD
especially between Friday and Saturday.

A tight northerly pressure gradients is looking to linger into
early next week and keep the potential for additional wind
headlines in the forecast.

&&

.AVIATION...22/1110Z.

At 1030Z, the marine inversion at KLAX was based at 2000 feet.
The top of the inversion was 3600 feet with a temperature of
21 degrees Celsius.

Overall for the 12Z TAF package, moderate confidence in coastal
and valley TAFs and high confidence in desert TAFs. Lower
confidence for coastal/valley sites due to uncertainties in timing
of flight category changes. For tonight, there is a a decent
chance of drizzle which will likely bring IFR CIGs.

KLAX...Moderate confidence in 12Z TAF. Timing of flight category
changes between IFR and MVFR conditions could be +/- 3 hours of
current forecasts. There is a 40% chance of VFR conditions
21Z-03Z. No significant easterly wind component is expected.

KBUR...Moderate confidence in 12Z TAF. Development of VFR
conditions could be as late as 20Z. Timing of return of IFR/MVFR
conditions tonight could be +/- 3 hours of current forecasts.

&&

.MARINE...22/818 AM.

For the Outer Waters, moderate to high confidence in current
forecast. Today through Wednesday, high confidence in winds and
seas remaining below Small Craft Advisory (SCA) levels. For
Thursday and Friday, high confidence in winds increasing to SCA
levels with a 40% chance of Gale force winds across PZZ673 and
PZZ676 Thursday and increasing to 70% on Friday.

For the Inner Waters north of Point Sal, high confidence in
current forecast. Today through Wednesday, winds and seas will
remain below SCA levels. For Thursday and Friday, winds will
increase to SCA levels, mainly in the afternoon and evening hours.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Today through Wednesday, high
confidence in winds and seas remaining below SCA levels. For
Thursday and Friday, there is a 50-70% chance of winds increasing
to SCA levels, especially across western sections.

&&

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

&&

$$

PUBLIC...Hall/Cohen
AVIATION...RAT
MARINE...RAT/jld
SYNOPSIS...Hall

weather.gov/losangeles

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