405
FXUS66 KLOX 172127
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
227 PM PDT Sun Aug 17 2025

.SYNOPSIS...17/226 PM.

Temperatures will warm a little each day through Tuesday, but
remain below normal with low clouds over coasts and valleys. Gusty
Sundowner and I-5 winds will continue each night, strongest
Monday through Tuesday. High pressure will quickly build over the
region for the middle to end of the week, with several days of
hot conditions likely. Thursday and Friday look the hottest, with
many valleys and lower mountains approaching or exceeding 100
degrees. Monsoonal moisture surges are favorable Wednesday
through the end of the weekend, with a chance of afternoon
mountain and desert thunderstorms. Dangerous fire-weather
conditions are anticipated over the mountains and foothills of Los
Angeles and Ventura Counties from Wednesday through Saturday,
when a Fire Weather Watch is in effect.

&&

.SHORT TERM (TDY-WED)...17/226 PM.

***A SEVERAL DAY HEAT EVENT is coming Wednesday through next
 weekend, with a HIGH RISK FOR HEAT ILLNESS AND FIRE DANGER. Now
 is the time to adjust any plans for strenuous outdoor activities
 in the valleys and mountains to the cooler morning hours or for
 another week. If you live in a high fire danger area in the
 mountains or foothills, review your evacuation plans and route
 and stay tuned to your local emergency officials.***

The low pressure system just off the the Alaskan Panhandle that
has driven our recent 5+ day run of below normal temperatures will
move little through Tuesday. Couple that with stronger than usual
onshore flow and a deeper than normal marine layer, and the cooler
than normal run looks poised to last a couple more days. Highs in
the 70s will be common over coastal areas, 80s over the valleys,
and 90s over the deserts. Despite the seasonably deep marine
layer, dense fog will be an issue on the Central Coast.

Winds will remain gusty over southwest Santa Barbara County each
night, likely peaking Monday and Tuesday Night. A low-end Wind
Advisory continues for tonight, and will likely need additional
advisories after that. Some winds winds will expand to the east
and into the wind prone areas of the eastern Santa Ynez Range
starting Monday Night, but do not look strong enough to need a
Wind Advisory. Gusty southwest winds also expected each afternoon
over the Antelope Valley and nearby foothills, as well as
northwest winds through the I-5 Corridor each evening, but neither
should reach Wind Advisory levels.

Then, a more significant shift in the weather regime is expected
starting Wednesday, ushering in the onset of a significant
heatwave. High pressure aloft, currently parked over Arkansas,
will quickly shift to the west Monday and Tuesday, and settle into
the Four Corners region by Wednesday. This high is strong with
500 millibar heights of 597-599 decameters. This high looks to
move very little through at least Friday, and possibly through the
weekend. This is a major pattern shift with big weather changes
coming to our region. High temperatures on Wednesday are expected
to reach the 80s and 90s in many locations, with readings up to
100-105 degrees over some of the interior valleys and deserts.
Also, with the marine layer becoming increasingly shallow in
conjunction with the building upper ridge, a general transition
from marine-layer stratus to dense fog will be of increasing
concern from early to mid-week over the coasts and coastal
valleys, potentially warranting Dense Fog Advisories (30%
chance).

.LONG TERM (THU-SUN)...17/226 PM.

***A SEVERAL DAY HEAT EVENT continues into next weekend, with a
 HIGH RISK FOR HEAT ILLNESS AND FIRE DANGER. Now is the time to
 adjust any plans for strenuous outdoor activities in the valleys
 and mountains to the cooler morning hours or for another week. If
 you live in a high fire danger area in the mountains or
 foothills, review your evacuation plans and route and stay tuned
 to your local emergency officials.***

In terms of temperatures, they will climb quickly and projections
are trending towards a fairly long duration 5 or so day heat event.
Look for highs to increase sharply from 5-10 degrees below normal
on Monday to around 10 degrees above normal by Thursday and
Friday (likely the peak of this heat event). Confidence is high
for this sharp warming trend inland of the coasts, with fairly
remarkable consensus in the ensemble projections through Sunday
(Aug 24). Overnight temperatures will also warm, with many areas
seeing lows only in the mid 60s to mid 70s, and foothill areas
look poised to not drop below the 80s or even 90s. There is a
chance that the lows will be even warmer with the influx of
monsoonal moisture (more on that is a bit). Heat Advisories for
many valleys and mountains are looking favorable over all four
counties, with a 60 percent chance of localized warnings focused
on the hottest valleys and foothills. Despite max temperatures
only being 10 degrees above normal, the risk for heat illness is
high for this event because 1) This is the very hottest time of
the year climatologically, 2) This heat is coming quickly on the
heels of an extended cool period, 3) Overnight temperatures will
be above normal, and 4) Humidity levels may be elevated due to the
Monsoonal moisture push. Continued to push the official forecast
well above the NBM based on ensemble projections and historical
comparisons. Records are possible, but they are fairly high for
this stretch of time and may be just out of reach.

The coastal areas will be the trickiest. While onshore flow will
weaken, it stays onshore. So the marine layer will shrink in depth
and coverage, but may not go away. Many scenarios are in play,
ranging from dense fog remaining along many coasts all day with
mild coastal temperatures, to low clouds and fog completely
eroding away with highs well into the 90s and Heat Advisories
needed. The current forecast takes a middle ground approach, and
we will likely not know which scenario will win until a day or two
before the event starts.

The monsoonal potential is also rising for the middle to end of
the week thanks to the textbook several-day four corners position
of the high. Models continue to trend up in the moisture
transport, with the majority of ensemble solutions showing
precipitable water values between 1.25 and 1.50 inches. This is
plenty of moisture to kick off afternoon mountain and desert
thunderstorms. Introduced 20-30 PoPs for showers and thunderstorms
Wed-Sat as a result. Coastal and valley areas also have chances,
but that will be determined by any convective disturbances that
rotate up from Mexico, which is too far into the future for our
models to see at this point.

Additionally, a highly volatile fire-weather environment is
expected to develop in conjunction with the significant heatwave
across the mountains and foothills of Los Angeles and Ventura
Counties. Please reference the Fire Weather Watch and the Fire
Weather discussion section below for additional information.

&&

.AVIATION...17/2126Z.

At 20Z at KLAX, the marine layer was 2000 ft deep. The top of the
inversion was at 5500 ft with a temperature of 20 Celsius.

High confidence in CAVU TAFs for KPMD and KWJF with breezy
afternoon and evening SW winds.

Moderate confidence in TAFs for remaining sites. Low cigs/vsbys
are forecast to return tonight to coastal and coastal valley
sites -- onset within +/- 3 hours of forecast times. There is a
20% chance that low clouds do not re-develop at KSBP, KSMX, KSMO,
KLAX, and KLGB. Minimum cigs/vsbys may be off by one or two
categories.

KLAX...Moderate confidence in TAF. There is a 30% chance for
cigs/vsbys to become IFR tonight and Sunday morning. Any east wind
component should remain under 8 kts.

KBUR...Moderate confidence in TAF. There is a 30% chance for
cigs/vsbys to become IFR tonight and Sunday morning.

&&

.MARINE...17/226 PM.

High confidence in seasonably gusty NW winds across the waters
beyond 20 miles from shore between the Central Coast to San
Nicolas Island. Small Craft Advisory (SCA) conditions will be
common this week, except for some morning lulls. Localized Gale
Force gusts to 35 knots may occur in the evening hours tonight
from Point Conception south to San Nicolas Island in the late
afternoon through late night hours tonight and Monday, though
these conditions will be marginal and spotty, thus no Gale
Warnings are necessary.

The nearshore Central Coast waters will see SCA level winds each
afternoon and evening into at least mid-week.

The western Santa Barbara Channel will see periods of SCA
conditions during the late afternoon and evening hours from Sunday
afternoon through at least mid-week. The highest chances for
winds to spill into the eastern portion will exist in the
afternoon and evening Monday and Tuesday.

All areas will see choppy seas from these winds in the late
afternoon through late night hours.

&&

.FIRE WEATHER...17/226 PM.

***FIRE WEATHER WATCH IN EFFECT FROM WEDNESDAY MORNING THROUGH
 SATURDAY EVENING FOR AN UNSEASONABLY HOT AND UNSTABLE AIR MASS
 CAPABLE OF PRODUCING EXPLOSIVE 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***

The most significant heatwave of the summer season, so far, is
on the way middle to latter parts of this week and into next
weekend, especially Wednesday through Saturday. Expect
temperatures to soar well over 100 degrees over interior areas,
locally reaching near 110 degrees over some mountain valley and
desert locations. 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
explosive 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 Fire Weather Watch area covers portions of the mountains and
foothills of Los Angeles and Ventura Counties, which have been a
climatologically notorious area for large plume-dominated fires in
similar weather patterns to what is coming later this week. 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 -- southwest at 10-20 mph
gusting to 25-30 mph. 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 Saturday 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 explosive fire
behavior. And 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 Fire
Weather Watch 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, the
Bobcat Fire, and the Bridge Fire that burned significant portions
of the Angeles National Forest in 2009, 2020, and 2024,
respectively. 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 explosive fire
behavior and the growth of large fires. Interests in the Fire
Weather Watch area 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.

Fire-weather headlines may eventually need to be extended into
Sunday and into portions of interior Santa Barbara and San Luis
Obispo Counties.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory remains in effect from 6 PM this evening to 6
      AM PDT Monday for zones 349-351. (See LAXNPWLOX).
     Fire Weather Watch in effect from Wednesday morning through
      Saturday evening for zones 288-375>382. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
      for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until midnight PDT tonight for
      zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Kittell/Cohen
AVIATION...Cohen/Lewis
MARINE...Lewis/Cohen
FIRE...Cohen/Gomberg
SYNOPSIS...RK/Cohen

weather.gov/losangeles

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