078
FXUS66 KLOX 201009
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
309 AM PDT Wed Aug 20 2025

.SYNOPSIS...20/1244 AM.

A significant multi-day heat event is expected to start today and
last through Saturday with many areas likely to see near record
temperatures. There will be a slight chance of thunderstorms over
the Los Angeles and Ventura County mountains, and portions of the
Antelope Valley, during the afternoon hours Friday through Sunday.

&&

.SHORT TERM (TDY-FRI)...20/309 AM.

The heat is on...Today kicks off an extended heat wave that will
last into the weekend, peaking Thu and Fri. A 598 dam upper high
sits atop of the four corners area bringing 594 dam hgts to Srn CA
(Average 500mb hgts for Aug is 589 dam). At the sfc there is 1-2
mb of offshore flow from the north and only 1 to 2 mb of onshore
flow to the east. The weak flow has prevented any low clouds from
forming everywhere save for a small portion of the western SBA
county coast. The cooler marine air has be squished to about 600
ft and will only bring cooling to the cst. Most max temps away
from the csts will jump 3 to 6 degrees today with local 12 degrees
of warming in some mtn areas. Max temps only a few miles inland
from the coast will reach the upper 80s and lower 90s. The valleys
will warm into the 90s with 100-103 degree readings likely across
the western San Fernando Vly and the Calabasas area. While quite
warm these temps do not quite meet heat advisory criteria. Still
people outside should be aware of heat risk.

There will likely be no marine layer clouds again tonight. Min
temps will be well above normal esp in the vly/foothill region
between 1000 and 1500 ft elevations where the peak of the
inversion lies. Lows in this area will likely see lows in the mid
to upper 70s with 80s common for most of the night.

Continued high hgts, weak onshore flow and minimal marine
influence will bring another 3 to 6 degrees of warming to the
area on Thursday. This will boost max temps up to 8 to 12 degrees
over the already very warm August normals. Triple digit heat will
be widespread across the vlys, lower mtns elevations and the far
interior. The areas from the Vlys inland are all under an Extreme
Heat Warning starting at noon. The interior coastal sections are
covered by a heat advisory. Please see the product LAXNPWLOX for
all the details. The nearshore areas will also see above normal
temps and residents need to take heat precautions esp given the
general lack of air conditioning in these areas.

Not much changes at the upper levels on Friday, but there will be
onshore trends at the sfc. The far interior will warm up a few
more degrees while the csts and vlys see a little bit of cooling.
Whatever cooling does occur it will not be enough to mitigate the
extreme heat danger and there will be no change to the suite of
heat warnings and advisories. One additional twist to Friday`s
weather will be the addition of monsoon flow. While the moisture
will likely be high based and not have much effect on sfc level
humidities it will bring a slight chc or chc of TSTMs to the
VTA/LA mtns and then with drifting into the Antelope Vly.

Fire weather concerns will be in an heightened state as well
please see the fire weather discussion below for exact details.

.LONG TERM (SAT-TUE)...20/1224 AM.

All mdls showing 1 to 2 degrees of cooling on Saturday. This small
amount of cooling should be enough to eliminate the need for heat
advisories for the interior coastal sections. That said, the
latest guidance does not show the upper high weakening at all and
the new gradient forecasts show offshore trends compared to
Friday. If this new direction proves to be the correct direction
there will not be any cooling and, in fact, the csts will warm
some. The monsoon push still looks good enough for a slight chc of
showers/TSTMs over the mtns in the afternoon - possibly lingering
into the early afternoon.

Mdls are in good agreement that the high will weaken and shift to
the SW. Hgts will fall to 590 or 591 dam. There will be a slight
uptick in onshore flow as well. The increased sea breeze should
bring 3 to 6 degrees of cooling to most cst/vlys areas but only
about 2 degrees for the interior. This amount of cooling may not
be enough to eliminate the Heat Warning conditions for the mtns,
interiors and Antelope Vly. Even if Heat Warning conditions are
not met Heat Advisory Criteria will be. PWATs are still high
enough to warrant a slight chc of mtns TSTMs in the afternoon. If
they do develop they will likely be quite high based.

The upper high will pretty much fall apart on Monday and hgts will
fall to 589 dam. The onshore flow will increase as well. This
should bring 3 to 6 degrees of cooling to almost all of the area.
This should end the heat threat, although max temps will remain 3
to 6 locally 8 degrees above normal. The monsoon tap ends ending
the threat of afternoon convection.

Even more cooling is on tap Tuesday as the upper tries to reform
over Texas. Moderate onshore flow in the afternoon will return as
well. Most areas will cool 1 to 3 degrees.

Forecast mdls continue to forecast a pretty robust marine layer
cloud pattern across most coasts each morning. Looking at the
current cloud coverage and the lackluster onshore flow think that
this forecast is likely overdone (even way overdone) and it may
take until Tuesday before a substantial marine layer cloud
pattern redevelops.

&&

.AVIATION...20/0941Z.

Around 07Z, the marine layer depth was near 700 feet deep at KLAX.
The top of the inversion was near 1600 feet with a temperature
around 26 degrees Celsius. There was another inversion above up to
around 4100 feet.

High confidence in the current forecast, except for low confidence
in the current forecast for Central Coast and Los Angeles County
coastal terminals. There is a low-to-moderate chance of LIFR to
IFR conditions at Central Coast and Los Angeles County coastal
terminals through 16Z, and again after 10Z Thursday. Otherwise,
VFR conditions will persist.

KLAX...There is a 20 percent of IFR conditions through 16Z, and
after 13Z Thursday. Any easterly winds will remain less than 7
knots.

KBUR...VFR conditions are expected through the period. No wind
impacts are expected at this time.

&&

.MARINE...20/241 AM.

Generally, high confidence in the current forecast. Slightly
higher confidence in the forecast for seas relative to winds.

For waters southwest through northwest of the Channel Islands and
the nearshore waters along the Central Coast, there is a likely-
to-imminent (70-90 percent) chance of Small Craft Advisory (SCA)
conditions through early Friday, peaking during the afternoon and
evening hours. Local Gale Force gusts may occur this evening in
the windier spots, highest for the waters beyond 10 NM from the
Central Coast. Winds and seas for the nearshore waters with 10 NM
from the Central Coast will likely drop below SCA levels between
Friday and Saturday, but there is a moderate-to-high (30-40
percent) chance of SCA level winds over the weekend. There is a
high-to-likely (50-60 percent) chance of SCA level winds
returning to the waters beyond 10 NM offshore of the Central Coast
and south to near San Nicolas Island this coming weekend.

Inside the southern California bight, there is a moderate-to-high
(30-50 percent) chance of SCA level winds this afternoon and
evening, highest across the western portion of the Santa Barbara
Channel. Otherwise, winds and seas should drop below SCA levels
from Thursday through early Sunday. There is a moderate-to-high
(30-50 percent) chance Sunday afternoon and evening.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Extreme Heat Warning in effect from 11 AM Thursday to 9 PM
      PDT Saturday for zones
      38-88-342>345-348-353-369-371>383-548. (See LAXNPWLOX).
     Heat Advisory remains in effect from 11 AM Thursday to 9 PM
      PDT Friday for zones 341-347-356>358-367. (See LAXNPWLOX).
     Wind Advisory remains in effect until 5 AM PDT early this
      morning for zones 349-351. (See LAXNPWLOX).
     Extreme Heat Warning in effect from 11 AM Thursday to 9 PM
      PDT Friday for zones 351-352-368-370. (See LAXNPWLOX).
     Red Flag Warning in effect from 9 AM this morning to 9 PM PDT
      Saturday for zones 288-375>382. (See LAXRFWLOX).
     Red Flag Warning in effect from 9 AM Thursday to 9 PM PDT
      Saturday for zones 345-351>353. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect from 11 AM this morning to 1
      AM PDT Thursday for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 6 PM PDT this evening
      for zone 670. (See LAXMWWLOX).
     Gale Warning in effect from 6 PM this evening to 1 AM PDT
      Thursday for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Rorke
AVIATION...Hall
MARINE...Hall
SYNOPSIS...MW/Ciliberti

weather.gov/losangeles

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