694
FXUS66 KLOX 041645
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
945 AM PDT Tue Jun 4 2024

.SYNOPSIS...04/507 AM.

High pressure aloft over the region will continue a warming trend
through Wednesday, most pronounced away from the coast. Weakening
onshore flow will thin the marine layer depth some and keep clouds
confined to coastal and valley areas. A hot air mass will remain
in place across the interior valleys for much of this week, with
temperatures well above normal for this time of year. Closer to
the coast, the warming trend will be moderated due to persistent
night through morning low clouds and fog and a moderate to strong
onshore flow. A cooling trend will develop over the weekend with a
deepening marine layer depth.

&&

.SHORT TERM (TDY-THU)...04/944 AM.

***UPDATE***

Low clouds remained entrenched across most southern areas this
morning. Moderate offshore trends have supported breezy but sub-
advisory northeast winds above the marine layer. This combined
with rapidly increasing heights as our well advertised ridge
asserts itself today will support 5-10 degrees of warming with
highs in the 90s to near 100 for the warmest valleys to interior.
The marine layer will shelter most coastal areas from the initial
part of this heatwave. Tweaked cloud cover and fog through tonight
following closely to the latest HRRR model.

Forecast focus of today will be the heat with considerations for
expanding the heat advisory to some other interior areas
especially for Wednesday and exploring convection potential
focused across the interior late in the week.

***From Previous Discussion***

The latest water vapor imagery indicates a blocking ridge of high
pressure aloft in place over the West Coast early this morning.
High cloudiness continues to move over the region early this
morning but will clear out through the day. At the surface,
onshore flow remains in place at this surface, albeit much weaker
than the last several weeks. Through the breaks in the cirrus,
fog product imagery reveals a broken marine layer induced stratus
deck early this morning, but the low cloud field should fill in
across the coastal and valley areas south of Point Conception as a
moderate to strong onshore flow pattern is expected to stay
wedged in across the Southland. Low clouds and fog remain across
portions of the Central Coast and into the Santa Ynez Valley this
morning. Its time is likely limited as surface pressure gradients
are progged to turn offshore today through Wednesday. Offshore
flow should win out and be strong enough to push low clouds and
fog out of areas north of Point Conception. An early clearing is
likely across the Central Coast and Santa Ynez Valley as offshore
flow and drier air aloft will mix down as the day progresses.

With onshore flow weakening, the marine layer depth will thin over
the coming days. Onshore pressure gradients have weakened by a
millibar or two this morning. KLAX AMDAR soundings are already
showing a decrease of the marine layer depth by about 1000-1200
feet already, down to about 1500 feet deep this morning. This
seems to agree with the latest local 3-km WRF time height sections
for KLAX. The marine layer depth is expected to thin
additionally through Wednesday morning, then start to deepen
again for the latter half of the week as an upper-level trough of
the Baja California coast approaches the region.

A warming trend will continue to take shape through Wednesday,
turning significantly warmer away from the coast today. 950 mb
temperatures approach 35 degrees Celsius today across the interior
valleys and across some of the mountain areas then exceed the 35
degrees Celsius mark on Wednesday. With heat risk values climbing,
an excessive heat watch watch for interior San Luis Obispo County
valleys was converted to a heat advisory, while expanding it into
the southern Salinas Valley for Wednesday through Thursday
evening. The excessive heat watch was converted to an excessive
heat warning across the Antelope Valley and adjacent foothill
areas for Wednesday through Thursday evening. There is a chance
that a heat advisory could added for today interior San Luis
Obispo County starting this afternoon as temperatures get close to
criteria today. There is a little more uncertainty for Thursday
in interior San Luis Obispo County as some model solutions do have
southwest winds developing, cooling the area down in increased
onshore flow. In addition, a heat advisory for Wednesday cannot
be ruled out for the Cuyama Valley and across the interior
mountain areas as the daytime hours will approach criteria.

By far, the warmest temperatures will occur across the Antelope
Valley through Thursday. KPMD and KWJF typically record their
first 100 degree days around June 13th on average. The latest
forecast has these sites hitting 100 degrees this afternoon, about
9 days earlier than average, then warming to 105 degrees on
Wednesday. If any site fails to reach 100 degrees today, it is
virtually certain to happen on Wednesday as NBM solutions indicate
a greater than 95 percent chance of hitting 100 degrees on
Wednesday and a 90 percent chance or higher on Thursday.

The sky cover pattern will turn a little more complex between
Wednesday evening and Thurday. A developing southeast flow aloft
could bring an early start to the Desert Monsoon in southern
California on Thursday, while fighting the coastal stratus wedged
in. There are multitude of scenario that could develop. It is no
surprise that EPS solutions do offer up a little more spread for
Thursday as the increased cloud cover with any convection that
might develop could play tricks on the marine layer low cloud
field. In regard to the convection, WPC QPF estimates suggest
showers developing across portions of the mountains on Thursday
afternoon. EPS solutions indicate precipitable waters value
approaching 0.9 inch over KGXA and KPMD between Thursday and
Friday, or about 150-200 percent of normal for this time of year.
The forecast increases PoPs to mention isolated showers and
thunderstorms over the highest terrain at this time. While low
confidence exists in convection, it continues to be a non-zero
chance due to pattern recognition into the weekend. Forecast
ensemble solutions are at the mercy of convective
parameterization schemes working as advertised.

.LONG TERM (FRI-MON)...04/515 AM.

A southeast to south flow will likely linger into Friday and
Saturday as the upper-level trough off the Baja California lifts
out over the region. As is typical with troughs to the southwest
of the area, there is a lot of uncertainty with the forecast for
Friday and Saturday. There a non-zero chance of showers and
thunderstorms into the weekend, especially if the trough center
lifts out over the region. A cooling trend will likely begin with
a deepening marine layer depth as the trough approaches the
southern California coast, but drizzle, light showers, or
convection cannot be ruled out at times. The forecast goes with
NBM values for the current time to place the focus on the heat and
potential for convection for late week.

&&

.AVIATION...04/1637Z.

At 16Z at KLAX, the marine layer was 1900 ft deep. The top of the
inversion was at 5200 feet with a temperature of 20 Celsius.

Moderate confidence on better clearing today compared to
yesterday. High confidence in all valley airports becoming VFR by
19Z. Moderate confidence in VFR at all coastal sites by 19-22Z.
Chances of VFR today at KSBA (50%) at KOXR (50%) KCMA (70%) KSMO
(80%) KLAX (80%) KLGB (80%). Timing of cloud arrival and coverage
of low clouds tonight expected to be later and less than
yesterday. Ceilings likely to be lower as well.

KLAX...80% chance of ceiling clearing. 19Z is the most likely
time, but could be as early as 18Z or as late as 21Z. Clouds will
return as early as as 02Z and as late as 06Z. Ceilings should be
OVC008-012. High confidence in any east winds staying well below
08 knots.

KBUR...High confidence in VFR conditions today by 17-18Z. There is
a 50% chance of ceilings forming again tonight, as early as 09Z
and as late as 12Z. If ceilings form, expect 1-3SM BKN005-008.

&&

.MARINE...04/937 AM.

Gusty northwest winds will remain over the offshore waters off the
Central Coast, but will be less expansive. There is a 70 percent
chance of Gale Force winds this afternoon and evening beyond 30
miles from the coast. Otherwise, Small Craft Advisory (SCA) winds
and/or seas will impacts the offshore waters down to San Nicolas
Island and the nearshore Central Coast Waters. These winds will
generate choppy seas over most of the coastal waters.

SCA winds are likely through Thursday through the offshore waters
off the Central Coast. Otherwise, high confidence winds will stay
under SCA, although seas near 10 feet are possible offshore.
Southeast winds will be prevalent each morning south of Point
Conception, which locally could exceed 15 knots near islands and
Channels.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory in effect from 10 AM Wednesday to 8 PM PDT
      Thursday for zones 343-344. (See LAXNPWLOX).
     Excessive Heat Warning in effect from 10 AM Wednesday to 8 PM
      PDT Thursday for zones 381>383. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 9
      PM PDT this evening for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PDT Wednesday for
      zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Wednesday for
      zone 673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PDT this evening
      for zone 676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/RM
AVIATION...RK
MARINE...Kittell
SYNOPSIS...Hall

weather.gov/losangeles

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