547
FXUS66 KLOX 041312
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
612 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)...04/514 AM.

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...04/1312Z.

At 12Z at KLAX, the marine layer was 2200 ft deep. The top of the
inversion was at 6300 feet with a temperature of 19 deg C.

Widespread low clouds and fog in coastal and valley areas south
of Pt Conception, with the exception of the Santa Clarita Valley
and the interior valleys of Ventura County. Conds were mostly low
MVFR to high IFR, except LIFR to VLIFR in the higher valleys and
foothills. Expect skies to clear by mid morning in the valleys and
late morning on the coastal plain, except early afternoon near
the coast. N of Pt. Conception, areas of low clouds and fog were
affecting the southern Central Coast and Santa Ynez Valley with
mostly IFR to VLIFR cigs. Skies should clear by mid to late
morning. Expect widespread low clouds in coastal areas south of
Pt Conception tonight, with patchy low clouds on the Central Coast
and in the valleys. Conds should be mostly IFR to LIFR tonight.

KLAX...Moderate confidence in the 12Z TAF. There is a 20% chance
that cigs will linger until 21Z. There is a 20-30% chance that
cigs will not arrive until after 07Z tonight.

KBUR...Moderate confidence in the 12Z TAF. There is a 20% chance
that cigs could clear as early 16Z. There is a chance that cigs
will arrive as early as 09Z tonight.

&&

.MARINE...04/351 AM.

In the outer waters, Small Craft Advisory (SCA) level winds and
seas are expected to drop below SCA levels in the southern zone
(PZZ676) this evening, the central zone (PZZ673) late tonight, and
the northern zone (PZZ670) Wed evening. There is a 30% chance that
SCA conds will linger into Wed in the central zone (PZZ673). From
late Wed night thru Sat, SCA conds are not expected. However, in
the northern zones (PZZ670/673), there is a 20-30% chance of
advisory level seas Thu/Thu night, and SCA level winds Sat.

In the inner waters N of Pt Sal, SCA level NW winds are expected
(80% chance) this afternoon/eve. Then, SCA conds are not
expected late tonight thru Fri.

In the inner water S of Pt Sal, SCA conds are not expected thru
Sat. There will be SE winds late each night and morning thru Fri,
with isolated winds to 15 kt thru passages and channels, and
between islands.

Steep, choppy seas will affect the outer and northern waters thru
at least Wed night, pushing into western portions of the SBA
Channel at times.

&&

.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
AVIATION...DB
MARINE...DB
SYNOPSIS...Hall

weather.gov/losangeles

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