553
FXUS66 KLOX 210624
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1124 PM PDT Wed Aug 20 2025

.SYNOPSIS...20/117 PM.

A significant multi-day heat event is expected to start today and
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 Sunday. Cooler conditions are expected next week.

&&

.SHORT TERM (WED-SAT)...20/839 PM.

***UPDATE***

Highs warmed well today, with many areas about 4-8 degrees hotter
than Tuesday, except for the mountains of San Luis Obispo and
Santa Barbara Counties that saw 10-15 degrees of warming. Marine
layer clouds have been nearly non-existent, only making a brief
appearance over western portions of Santa Barbara County. We
likely won`t see much of it over the next few days as high
pressure and heat dominant the region.

Monsoonal moisture arrives Thursday, with precipitable water
values rising to 1.3" over southern areas, and hitting 1.5"
throughout the area by Friday morning. Current high resolution
model guidance shows potential for an isolated showers or storms
focused over interior LA County. However, guidance seems to
heavily favor Saturday over Friday for more widespread convection,
supported by 1000-1400 J/kg and precipitable water values up to
1.6" at times.

***From Previous Discussion***

The warming trend has begun with most areas up 3-6 degrees today
over yesterday. High pressure continues to build in from the
southeast with weakening onshore flow. Looking for at least
another 3-6 degrees of warming Thursday before temperatures level
off Friday then start a very slow cooling trend Saturday. During
the hottest days Thu/Fri highs will easily reach triple digits
across the valleys and deserts, possibly breaking some daily
records in the process. Closer to the coast temperatures will be
cooler but still 5-10 degrees above normal, including 90s for
Downtown LA and San Luis Obispo and high 70s and low 80s near the
beaches. Overnight lows will be quite warm as well, with desert
temperatures staying in the 80s for much of the night, and some
coastal valleys staying in the 70s. Heat advisories and Extreme
Heat warnings cover the entire four county area except for places
near the coast. 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.

As the high pushes in from the east the flow aloft will become
increasingly favorable for monsoon moisture to move into the area.
Ensemble models have been consistent indicating PW`s rising to
between 1.25 and 1.50" across LA/Ventura Counties by Saturday.
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, mainly during
the mid to late afternoon hours.

A slow cooling trend will begin Saturday but increasing humidities
will likely make it feel just as hot. May see some low stratus
returning to parts of the coast by Saturday as well.

.LONG TERM (SUN-WED)...20/208 PM.

High pressure will slowly weaken and retreat to the east as a
trough develops across the eastern Pacific. However, the cooling
trend will be very slow at least at first. The current suite of
heat hazards runs through Saturday but chances are very high that
at least some of the interior hazards will need to be extended
into Sunday. The one factor that could hasten the cooling trend is
if monsoon moisture produces more clouds and precip than currently
expected. However, most likely highs will still be 4-8 degrees
above normal in all areas except the immediate coast.

Additional cooling on the order of 2-4 degrees is expected per day
through Wednesday, leading to highs actually slightly below normal
in most areas by Tuesday and likely lasting through the rest of
next week. Night and morning low clouds and fog will return to
coastal areas and some coastal valleys.

&&

.AVIATION...21/0623Z.

At 0535Z at KLAX, the marine layer was 400 feet deep. The top of
the inversion was 1700 ft with a temperature of 30C.

High confidence in TAFs for KPRB, KSBA, KBUR, KVNY, KPMD and
KWJF.

Good confidence in TAFs for KSBP, KOXR, KCMA, KSMO, KLAX and
KLGB with a 20 percent chc of MVFR VIS 12Z-17Z and a 10 percent
chc of LIFR cig/vis 12Z-17Z.

Moderate confidence in TAF for KSMX with a 30 percent chc of LIFR
cig/vis 10Z-16Z.

KLAX...Good confidence in TAF. There is a 10 percent chance of 1SM
BR OVC002 conds between 10Z and 17Z. There is a 20 percent chc of
4SM HZ 12Z-17Z. No significant east wind component expected.

KBUR...High confidence in TAF.

&&

.MARINE...20/812 PM.

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, Small Craft
Advisory (SCA) conditions are likely through early Friday,
peaking during the afternoon and evening hours. Gale Force gusts
will continue through late tonight 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 (30-50 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, SCA level winds will likely
remain in the western half of the Santa Barbara Channel, away
from the coasts through Thursday evening. Local gusts to 21 kts
are possible near Point Dume and through the San Pedro Channel.
Then conditions are expected to remain generally below 20 kts
through Sunday morning. There is a moderate-to-high (30-50
percent) chance Sunday afternoon and evening.

&&

.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 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).
     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 until 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 until 1 AM PDT Thursday for
      zone 645. (See LAXMWWLOX).
     Gale Warning in effect until 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...Phillips/MW
AVIATION...Rorke
MARINE...Hall/Lewis
FIRE...Cohen/Gomberg/Lund
SYNOPSIS...MW

weather.gov/losangeles

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