820
FXUS66 KLOX 211003
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
303 AM PDT Thu Aug 21 2025

.SYNOPSIS...21/203 AM.

A significant multi-day heat event will continue today and will
last through the weekend 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
Monday. Cooler conditions are expected next week.

&&

.SHORT TERM (TDY-SAT)...21/227 AM.

Three days of dangerously hot weather is on tap for the Southland.
A 598 dam upper high will sit over the four corners region of the
county. 594 dam hgts today will lower to 592 dam for Friday and
Saturday. There will only be weak onshore flow with the
possibility of brief offshore flow each morning from the north.
The marine layer has been smooshed to under 500 ft and with no
eddy and not much onshore flow there will be no marine layer
stratus.

Since this is not a Santa Ana event there will be temperature
gradient from the csts to the interior. The beaches will mostly
escape the heat with highs in the mid 70s to lower 80s but moving
a few mile inland the interior coastal sections will see temps in
the mid 90s. The vlys, lower mtn elevations, far interior and the
Antelope Vly will all have max temps ranging from 100 to 109
degrees.

Heat advisories cover the interior sections of the coast and heat
warnings cover the rest of the interior. Only the beach areas will
not have dangerous heat. Please see the product LAXNPWLOX for all
of the details about this long durations and dangerous heat wave.

Get outdoor activities done as early in the morning as possible
and don`t leave people or pets in closed vehicles for any amount
of time.

Forecast mdls persist in cooling Saturday`s temps a few degrees,
which probably a result of slightly increased humidities and some
mid and high level clouds. Still it will be a dangerously hot day.

The four corners location of the upper high is very favorable for
monsoon moisture transport and indeed PWATs will rise to as high
as 1.5 inches. The best chances for showers and storms will be
across the eastern San Gabriels Friday and then spreading west
into the Ventura mtns Sat/Sun. Steering flow from the south may
push the activity into the Antelope Valley as well. Brief moderate
to heavy downpours will be possible in these areas through the
weekend, during the mid afternoon to early evening hours.

.LONG TERM (SUN-WED)...21/257 AM.

The upper high will slowly weaken each day before disappearing on
Wednesday. Hgts do not fall all that much falling from about 590
dam Sunday to 588 dam on Wednesday.

The more important factor will be the increase in onshore flow
both to the east and north. Marine layer stratus will also likely
reform, although it will likely only affect the near shore area.

The csts and vlys will see 3 to 6 degrees of cooling Sunday. but
the interior will not see too much change and heat products may
need to be xtnd into Sunday for some areas there. On Monday all
areas will experience 3 to 6 locally 8 degrees of cooling and
this will end the heat wave. 1 to 3 degrees of cooling is forecast
for Tue and Wed and this will bring max temps on Wed to 2 to 5
degrees blo normals.

Ensemble based convection forecasts keep a slight chc of afternoon
TSTMs across the LA/VTA mtns and Antelope Vly all the way through
Tuesday.

&&

.AVIATION...21/0853Z.

Around 07Z, the marine layer depth was around 500 feet deep at
KLAX. The top of the marine inversion was around 1600 feet with a
temperature near 28 degrees Celsius. There was another inversion
up to around 2100 feet.

Moderate to high confidence in the current forecast. There is a
low-to-moderate chance of LIFR to IFR conditions at coastal
terminals through 16Z, and again after 08Z Friday.

KLAX...There is a 10 percent chance of LIFR to IFR conditions
through 16Z, and again after 08Z Friday. Otherwise, VFR conditions
are expected. Any easterly winds should remain less than 7 knots.

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

&&

.MARINE...21/203 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, then there is a high-to-likely
(50-70 percent) chance of SCA level winds for Friday afternoon and
evening. Local Gale Force gusts will continue through late
tonight in the windier spots, highest for the waters beyond 10 NM
from the Central Coast during the afternoon and evening hours.
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 (30-50 percent) chance
of SCA level winds early next week. Beyond 10 NM offshore of the
Central and south to near San Nicolas Island, there is a high-
to-likely (50-70 percent) chance of SCA level winds over the
weekend and into early next week, highest during the afternoon and
evening hours.

Inside the southern California bight, winds and seas should remain
below SCA levels through Sunday. Local SCA gusts are possible each
afternoon and evening across the western portion of the Santa
Barbara Channel, near Point Dume, and into the San Pedro Channel
through Sunday. Winds will start to increase across the bight on
Sunday and Monday with a moderate-to-high (30-50 percent) chance
of SCA levels winds, highest across the western portion of the
Santa Barbara Channel.

&&

.FIRE WEATHER...20/402 PM.

***RED FLAG WARNING IN EFFECT UNTIL 9 PM PDT SATURDAY FOR AN
 UNSEASONABLY HOT AND UNSTABLE AIR MASS CAPABLE OF PRODUCING
 EXTREME FIRE BEHAVIOR FROM VERTICAL PLUME GROWTH...LOW RELATIVE
 HUMIDITY...AND LOCALLY BREEZY WINDS FOR PORTIONS OF THE MOUNTAINS
 AND FOOTHILLS OF LOS ANGELES AND VENTURA COUNTIES***

***RED FLAG WARNING IN EFFECT FROM 9 AM PDT THURSDAY TO 9 PM PDT
 SATURDAY FOR AN UNSEASONABLY HOT AND UNSTABLE AIR MASS CAPABLE OF
 PRODUCING EXTREME FIRE BEHAVIOR FROM VERTICAL PLUME GROWTH...LOW
 RELATIVE HUMIDITY...AND LOCALLY BREEZY WINDS FOR PORTIONS OF THE
 MOUNTAINS AND FOOTHILLS OF SANTA BARBARA AND SAN LUIS OBISPO
 COUNTIES***

The most significant heatwave of the summer season, so far, has
begun and will continue through at least Saturday, and likely into
Sunday. Expect temperatures to soar into the 95-105 degree range
over interior areas, locally reaching near 110 degrees over some
mountain valley and desert locations, peaking Thursday and Friday.
The intense surface heating will bring strong vertical mixing to
depths extending over 15-17 thousand feet -- highest Thursday and
Friday. Resultant unseasonably strong instability will create a
fire environment capable of producing extreme fire behavior
through significant vertical plume growth, as minimum relative
humidity ranges from 8 to 20 percent in the dry air mass.
Overnight relative humidity recovery will be poor to moderate,
while the shallow marine layer generally remains displaced closer
to the beaches.

The Red Flag Warning areas cover portions of the mountains and
foothills of Los Angeles, Ventura, Santa Barbara, and San Luis
Obispo Counties, which have been a climatologically notorious area
for large plume-dominated fires in similar weather patterns to
what is coming this week into the weekend. While background
surface winds are not expected to be strong, or even close to
reaching nominal Red Flag Warning criteria, they will be locally
breezy in the afternoon and evening over the western Antelope
Valley Foothills and vicinity and in other wind-favored areas such
as through passes and canyons -- sustained winds 10-20 mph
gusting to 25-30 mph following diurnal- wind patterns. These winds
will exacerbate the spread of any plume-dominated fires, and add
to the potentially dangerous fire- weather environment.

Moreover, the western edge of a southwest-states monsoonal
moisture influx will overlie Los Angeles and Ventura Counties,
which will contribute to the development of atmospheric buoyancy
by Friday and this weekend without significantly moistening the
air mass -- i.e., displaced away from the core of the moisture
source. While lightning-induced fire ignitions are a distinctive
possibility on the peripheries of rain cores, the increase in
buoyancy will further aid in vertical plume growth accompanied by
potential pyrocumulus development and related extreme fire
behavior. Combined with significant fuel loading of exceptionally
dry fuels, the fire environment in the mountains and foothills of
Los Angeles and Ventura Counties will become especially volatile
for Wednesday through Saturday, warranting the issuance of a Red
Flag Warning upon collaboration with local area National Weather
Service Core Partners.

Analogs to the forthcoming, highly volatile fire-weather
environment in Los Angeles County include the Station Fire in
2009, the Bobcat Fire in 2020, the Lake Fire in 2024, and the
Bridge Fire in 2024, which burned significant portions of
forested areas. In each case, winds were generally below Red Flag
Warning criteria, yet the extreme heat combined with buoyancy
greatly offset the sub-marginal wind to create extreme fire
behavior and the growth of large fires. Interests in the Red Flag
Warning areas should be prepared for similar activity if fires
were to start. Also, be aware of passing outflow boundaries that
could bring sudden wind-shifts and increased fire-spread rates,
even from distant thunderstorms.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Extreme Heat Warning in effect from 11 AM this morning 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 this morning to 9
      PM PDT Friday for zones 341-347-356>358-367. (See LAXNPWLOX).
     Extreme Heat Warning in effect from 11 AM this morning to 9
      PM PDT Friday for zones 351-352-368-370. (See LAXNPWLOX).
     Red Flag Warning in effect until 9 PM PDT Saturday for zones
      288-375>382. (See LAXRFWLOX).
     Red Flag Warning in effect from 9 AM this morning to 9 PM PDT
      Saturday for zones 345-351>353. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect from 11 AM this morning to 9
      PM PDT this evening for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Saturday for
      zones 670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for zone
      676. (See LAXMWWLOX).

&&

$$

PUBLIC...Rorke
AVIATION...Hall
MARINE...Hall
FIRE...Cohen/Gomberg/Lund
SYNOPSIS...MW

weather.gov/losangeles

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