417
FXUS66 KLOX 151128
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
428 AM PDT Mon Sep 15 2025

.SYNOPSIS...15/134 AM.

A warming trend will continue through at least Tuesday as high
pressure aloft over northern Mexico builds over the region.
Onshore flow remaining in place across the area will keep areas
of night through morning low clouds and fog in the forecast for
many coast and coastal valley areas through Tuesday. A monsoonal
flow pattern will develop between Tuesday and Wednesday and bring
the possibility of showers and thunderstorms for the mid-to-late
week. A very warm and humid air mass is likely to settle into the
region during this period.

&&

.SHORT TERM (TDY-WED)...15/309 AM.

A deep marine layer remains in place over the region this morning
as onshore flow remains intact. The latest fog product imagery
shows clouds becoming well entrenched along the southern and
central California coast this morning as an eddy circulation
continues to spin near the Channel Islands, and a fresh push of
the stratus field arrives along the Central Coast. Low clouds and
fog extend into the valley areas but are becoming a little more
problematic for the Southland valleys as the eddy circulation
drifts a little farther offshore this morning. The low cloud field
will be monitored closely over the next several hours for
potential updates.

The low cloud field will likely become more problematic over the
coming days as high pressure aloft builds into the region and a
monsoonal flow pattern starts to shape up. The latest satellite
imagery shows an upper-level ridge over northern Mexico early this
morning. This ridge will build over the region into Tuesday night
and expand into the Great Basin for Wednesday. A warming trend
will continue over the area through at least Tuesday as 500 mb
heights climb through Tuesday night. Onshore flow will weaken and
subsiding air aloft will press down on top of the marine intrusion
and fit it into a smaller area. As a result, less low cloud
coverage is expected over the coming days, and temperatures will
rise even further. The latest heat risk parameters suggest the
situation to be marginal for heat advisory headlines, but this
will need to be closely watched. Higher confidence exists in the
temperature forecast through Tuesday, but less confidence exists
in the warming trend continuing for Wednesday due to increase in
monsoonal moisture.

Increasing monsoonal moisture will start to spread into the region
between Tuesday and Wednesday as southeast flow aloft establishes.
Moisture from redeveloping Tropical Storm Mario will become
embedded with the monsoonal flow and get transported into southern
California between Tuesday and Wednesday. As is frequent with
these patterns, there is a great amount of uncertainty how much
precipitation and cloud coverage will occur. A dense cloud shield
with the increase in moisture could cut into temperatures on
Wednesday and Thursday. Low confidence should be exercised in the
forecast for precipitation as convective activity be could more
or less widespread than indicated. The forecast remains consistent
for now with a blanket slight chance of showers and thunderstorms
between after Wednesday, but there is a moderate chance that the
moisture could arrive as soon as Tuesday afternoon or Tuesday night.
NBM values trend wetter for PoPs, and that could certainly
develop, but given how the NBM has struggled with the last several
events, the forecast remains consistent for now. As the moisture
arrives on Tuesday night or Wednesday, there is a moderate chance
that a period of dry thunderstorms could develop, or thunderstorms
that contain little to no rainfall due to the dry air mass
beneath the moisture.

Moisture will very likely continue to spread into the region
through Wednesday night and Thursday. EPS and GEFS precipitable
water value means approach 1.75 inches by late Wednesday night, or
values above the 97th percentile. This is when the air mass could
turn wetter and add the potential for flooding. Brief heavy
downpours will certainly be possible as soon as Wednesday
afternoon or evening with any showers or thunderstorms.

.LONG TERM (THU-SUN)...15/318 AM.

The pattern could turn more active between early Thursday and
Friday night. The latest deterministic solutions are struggling
with modeling of the remnant circulation of Mario, GFS solutions
push it north into along the Central Coast, while NAM-WRF
solutions bring the center of the circulation into the Los Angeles
Basin. While there is uncertainty with the actual movement, there
is good agreement across the forecast ensemble for the moisture
to be in place. A vast majority of the EPS and GEFS solutions are
suggesting the most likely period for rainfall from showers and
thunderstorms to occur between early Thursday and Friday night.
The forecast continues chance PoPs across the region, but future
shifts may need to consider upping PoPs into the likely category.

All forecast ensemble members suggest the precipitable water
value means reaching maximum on Thursday morning, then moisture
receding thereafter. Chances of showers and thunderstorms will
could linger into Friday or Saturday, then become confined to the
mountains and desert over the weekend.

While temperatures will cool with the increase in moisture and
the potential for shower activity, temperatures will likely remain
warm and humid into late week.

&&

.AVIATION...15/1127Z.

At 1025Z at KLAX, the marine layer was 1700 ft deep. The top of
the inversion was at 3400 ft with a temperature of 24 C.

High confidence in KPMD and KWJF TAFs.

Low confidence in TAFs for KSMX and KSBP. There is a 40% chance
of no low clouds at KSMX and a 40% chance of brief cigs at KSBP
through 17Z.

Moderate confidence in remaining TAFs. Timing of flight cat
changes may be off by 2 hours and flight cat may be off by one at
times. Cigs may scatter out and reform at times through 12Z. 30%
chance for LIFR to IFR cigs at KPRB, KBUR and KVNY after 08Z
Tues.

KLAX...Moderate confidence in TAF. Clearing times may be off by 2
hours. There is a 15% chance that there is no VFR transition
through the period or is very brief. No significant east wind
component expected through the period.

KBUR...Moderate confidence in TAF. Clearing times may be off by 2
hours. 30% chance for LIFR to IFR cigs at KBUR after 08Z Tues.

&&

.MARINE...15/131 AM.

Sub-tropical moisture will move northward into the coastal waters
this week, bringing chances of showers and thunderstorms to the
area as early as Tuesday, with better chances Wednesday through at
least Friday. Any thunderstorm that forms may create localized
dangerous ocean conditions with choppy seas, gusty erratic winds,
and/or frequent dangerous cloud to surface lightning. Boaters
should closely monitor weather conditions and avoid traveling near
showers and thunderstorms.

Small Craft Advisory (SCA) level NW winds (20-30 kts) will impact
the outer waters through this morning, with seas peaking at 6-8
feet. Conditions are expected to weaken and remain below advisory
levels through Wednesday morning. Low to moderate confidence in
winds increasing close to SCA levels for portions of the waters,
especially south of Point Conception Wednesday afternoon through
late Friday.

For the inner waters along the Central Coast and south of Point
Conception, conditions are generally expected to remain sub-
advisory through the week, except for low to moderate chances for
WNW winds 20-25 kts across portions of the Southern California
Bight in the afternoon through evening hours through Thursday, but
winds may be too isolated to warrant an SCA issuance.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from 11 AM PDT this morning
      through late Tuesday night for zones 354-362-366. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 9 AM PDT this morning
      for zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall
AVIATION...Lewis
MARINE...Lewis
SYNOPSIS...Hall

weather.gov/losangeles

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