424
FXUS66 KLOX 250442
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
942 PM PDT Wed Jul 24 2024

.SYNOPSIS...24/927 PM.

A significant heatwave will continue for locations away from the
coast through Thursday as a strong high pressure system remains in
place over the region. Temperatures will cool for the end of the
week and into the upcoming weekend as high pressure moves away to
the east. A shallow marine layer depth will continue to keep the
warming trend moderated along the coast and continue a threat of
dense fog each night through morning. A marginally moist air mass
will bring a low chance of showers and thunderstorms across the
higher mountains and interior portions of the area through
Thursday.

&&

.SHORT TERM (WED-SAT)...24/212 PM.

The western states upper anticyclone will be reaching its peak
intensity later today into Thursday, with maximum 500-mb heights
around 600 dam over the Lower Colorado River Valley. Correspondingly,
the areal coverage of the most significant HeatRisk for this
heatwave will be maximized through Thursday. Excessive Heat
Warnings continue through Thursday across most interior mountains
and valleys through the Santa Ynez Range -- where high
temperatures of 95 to 110 degrees and low temperatures of 72 to 82
degrees will be common, producing Major to Extreme HeatRisk. Heat
Advisories are in effect for many of the coastal valleys and
nearby foothills away from the coast -- where high temperatures
from the 90s to 105 degrees and low temperatures of 62 to 72
degrees will be common, producing Moderate to Major HeatRisk.
Cooler conditions with night and morning patchy dense fog are
expected over coastal areas, and there is a 30% chance for Dense
Fog Advisories to become necessary through Friday morning.

In addition, satellite data indicate a decrease in precipitable
water by around one-tenth to two-tenths inch over the region
today, in response to subtle drying behind a weak wave moving
through central California. However, with 1.1-1.3-inch
precipitable water values persisting from the San Gabriels and
Antelope Valley region through the northern Ventura County
mountains where temperatures have become very hot today, slight
chances for thunderstorms continue in these areas. Given the
modest decrease in tropospheric moisture, thunderstorm mention has
been restricted to areas where diurnally-enhanced orographic
ascent is optimized with the richest moisture and points
immediately downstream. This specifically includes areas from the
eastern San Gabriels through the eastern Antelope Valley to the
east of Lancaster and Palmdale, as well as the extreme northern
Ventura County mountains and nearby Lockwood Valley. The most-
likely timing for convective activity will be through 4PM PDT this
afternoon. The strongest storms will be capable of producing
frequent lightning, locally heavy rain and possible flooding if
convection were to back-build, as well as strong and erratic
outflow winds given significant dry air in the sub-cloud layer and
1500 J/kg of DCAPE.

Temperatures are forecast to begin a significant cooling trend
starting Friday and continuing into the weekend. High temperatures
are expected to fall by a few to several degrees from day to day
going into Friday and Saturday, with the most significant cooling
over areas away from the coast. These cooler conditions will come
with the eastward progression of a compact midlevel low north of
the region -- causing the aforementioned anticyclone and
accompanying ridge to break down and flatten. In response to
these developments, heights aloft will fall over the region,
ushering in the cooler air mass for Friday into the upcoming
weekend and bringing an end to the ongoing heatwave. The marine
layer is also expected to deepen with the lowering heights aloft
at the end of this week into the weekend, allowing night and
morning low clouds and fog to extend farther inland, perhaps
accompanied by drizzle at times.

With the onshore progression of the compact upper low across
central California, winds aloft will veer to more of a westerly
direction, advecting drier air into the region. As a result,
precipitation chances will be decreasing late this week. While an
isolated shower or thunderstorm cannot be ruled out over the
vicinity of the eastern San Gabriels through the eastern Antelope
Valley and the extreme northern Ventura County mountains on
Thursday, precipitation chances will only be around 10% --
decreasing further for Friday and Saturday. Dry conditions are
expected areawide by the upcoming weekend.

In addition, a building surface ridge across the eastern Pacific
waters for this weekend will cause offshore pressure gradients for
SBA-SMX to increase to around 3-3.5 mb. This will enhance the
potential for gusty Sundowner winds across the western Santa Ynez
Range and southwest Santa Barbara County coast on Friday and
Saturday. Without stronger upper support, the issuance of Wind
Advisories is currently unlikely -- less than 30 percent chance.

.LONG TERM (SUN-WED)...24/212 PM.

In the wake of the previously mentioned compact upper low, broad
upper troughing is expected to persist along the Pacific Coast
and over Southern California through Sunday. Sunday high
temperatures will range from the upper 60s at the beaches, to the
70s over coastal valleys, to the 80s over interior mountains and
valleys except the 90s across the Antelope Valley. These
temperatures will be a couple to several degrees below normal.

The trough will de-amplify for early next week while sub-tropical
ridging builds over the south-central states, causing heights
aloft to gradually rise across the forecast area. Correspondingly,
temperatures are expected to slowly rise, by a couple of degrees
from day to day, from early to middle parts of next week --
though significant heat impacts are unlikely. Onshore pressure
gradients will continue to maintain cooler conditions closer to
the coast, where night and morning low clouds and fog will
prevail.

Present indications are that dry conditions are expected to
persist into Sunday and next week. However, some model projections
indicate a convectively-enhanced midlevel cyclonic perturbation
advancing northward into the southwest states -- within the long
fetch of southerly flow along the western flank of the building
ridge. Those model solutions indicating the presence of this
impulse primarily depict its trajectory remaining east of the
region. However, there is typically significant uncertainty in the
evolution of such a mesoscale-driven midlevel perturbation, and
the predictability of its related impacts is often quite low
especially at this time range. As a result, there is a remote
chance (less than 10%) for a more prominent monsoonal moisture
influx to focus over the forecast area bringing precipitation and
perhaps cooler temperatures Monday through Wednesday. The chances
for such impacts occurring are too low for inclusion in the
forecast at this time.

&&

.AVIATION...25/0016Z.

At 23Z, the marine layer depth at KLAX was around 500 feet. The
top of the inversion was around 2500 ft with a temp of 31 C.

High confidence in TAFs for KPRB, KBUR, KVNY, KPMD, and KWJF.
There is a 10-15% chance of Thunderstorms near KPMD and KWJF this
evening through 06Z.

Low confidence for all coastal sites. Category change times may
be off +/- 5 hours from current forecast, and the minimum flight
category may be off by 1 category throughout the period,
especially after cigs arrive. MVFR-VFR conditions expected at
coastal sites by 15-19Z, and should remain VFR thru the afternoon.

KLAX...Low confidence in the TAF. 30% chance that cigs remain
above 005 after arrival. However, there is a 10% chance that cigs
could be lower than 002 and/or vsbys could be less than 1/2 SM
tonight between 10 and 16Z. High uncertainty of arrival time of
cigs/vsbys tonight, as they could arrive as early as 08Z and as
late as 13Z. Cigs should clear between 16 and 19Z tomorrow. No
significant east wind component is expected.

KBUR...High confidence in TAF.

&&

.MARINE...24/941 PM.

For the Outer Waters, moderate to high confidence in current
forecast. Tonight through Thursday mid-day, there is a 40% chance
of Small Craft Advisory (SCA) level winds across PZZ673/676 and a
30% chance across PZZ670. For Thursday afternoon through Friday
night, high confidence (70-90%) in SCA level winds across
PZZ673/676, therefore the existing SCA was extended through Friday
night. There is a 60-80% chance of SCA level winds lasting into Saturday
night for zones PZZ673/676. In Zone PPZ670, winds are expected to
remain below SCA levels tonight, but there is a 50-70% chance of
SCA level winds from Thursday afternoon through Saturday night.
There is potential for periods of SCA levels winds Sunday into
early next week.
For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Tonight through Thursday, high
confidence in winds and seas remaining below SCA levels. For
Friday through Saturday in the afternoon and evening hours, there
is a 40-50% chance of SCA level winds. For Sunday and Monday,
there is a 10-30% chance of SCA level winds in the afternoon and
evening hours.
For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. For the western half of the Santa
Barbara Channel, there is a 30-50% chance of SCA level winds
tonight through Thursday morning. Current SCA looks on track with
winds increasing Thursday afternoon into Thursday night. There is
a 50-70% chance of SCA level winds Friday through Monday night,
particularly in the afternoon to evening hours. Elsewhere across
the southern Inner Waters, there is a 20-30% chance of SCA level
winds across the Western Portions of PZZ655 Thursday through
Saturday in the afternoon and evening hours. High confidence in
winds and seas remaining below SCA levels Sunday and Monday.

Areas of dense fog will continue to impacts parts of the coastal
waters through Friday during the night and morning hours.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning remains in effect until 10 PM PDT
      Thursday for zones 38-344-345-351>353-376>383. (See LAXNPWLOX).
     Heat Advisory remains in effect until 10 PM PDT Thursday for
      zones 88-342-343-348-356>358-369>375-548. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 3 PM Thursday to 3 AM PDT
      Friday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 PM Thursday to 3 AM PDT
      Saturday for zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen
AVIATION...Lewis/Phillips
MARINE...Lewis/Phillips/RAT
SYNOPSIS...Hall

weather.gov/losangeles

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