883
FXUS66 KLOX 160406
AFDLOX

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

.SYNOPSIS...15/800 PM.

A warming trend will continue through Tuesday as high pressure
aloft over northern Mexico builds over the region. Areas of night
through morning low clouds and fog will continue near the coast
through Wednesday. Moisture from tropical storm Mario will move
into the area later Wednesday through Friday leading to a chance
of showers and thunderstorms at times, especially on Thursday. Dry
weather is expected over the weekend.

&&

.SHORT TERM (MON-THU)...15/905 PM.

***UPDATE***

The marine layer clouds didn`t stray too far from the coasts
today, prompting an early return to most coastal locations this
evening. As the marine layer is becoming more shallow, dense fog
poses a threat to all areas near the coast, though the highest
likelihood for dense fog will be for coastal locations north of
Point Conception.

Continued warming with heat peaking tomorrow is still on track,
though a little bit of uncertainty remains in the later portion
of the evening/overnight Tuesday. This uncertainty is due to model
disagreement in how fast (and where) the instability/lift moves in from
the low pressure system and tropical storm Mario (ahead of the
deeper moisture that moves in on Wednesday, that is). As a
result, there is a very small chance that some areas could see
drier thunderstorms move in Tuesday night, with the potential for
lightning induced fire starts. Otherwise, the forecast leans
toward a deeper marine layer Tuesday night, with clouds pushing
further past the beaches and possibly into some coastal valleys.

The forecast looks to be in shape, and no updates were needed this
evening.


***From Previous Discussion***

Significant warming occurred across inland areas today as onshore
flow weakened and high pressure was building aloft. These trends
did not make it to the coast where highs were mostly unchanged or
slightly cooler, especially along the Central Coast. The marine
layer is expected to continue to lower again tonight, keeping most
of the valleys clear but still pushing into most coastal areas.

After the typical morning marine layer Tuesday, 2-4 degrees of
additional warming is expected, including the coast areas, as the
upper high over the desert southwest continues to build into the
area and onshore flow continues to weaken. And it now looks like
Tuesday will be the warmest day of the week. Warmer valley areas
are expected to reach the lower 100s while inland coastal areas
will be in the 80s to lower 90s. Highs overall will be 4-8 degrees
above normal. While well above normal, heat risk values will
remain below advisory levels.

Tropical Storm Mario still remains around 1000 miles south of Los
Angeles. It is expected to weaken and become a remnant low over
the next 24-48 hours while drifting over cooler waters along the
northern Baja coast. High level moisture will begin to enter the
area Wednesday and the latest model projections suggest that rain
chances locally will hold off until at least later on Wednesday or
Wednesday night. The exception could be over the outer portion of
the coastal waters where some showers and thunderstorms could
develop earlier. PW`s will be increasing to between 1.5 and 1.75"
Wednesday leading to a very muggy day across the area and it now
appears that high temperatures in many areas will actually lower
slightly due to the added moisture aloft. However, while
temperatures will be slightly cooler, humidities will be
increasing and there is still a 20-40 percent chance that a heat
advisory will be needed across portions of LA County. Will re-
evaluate that on Tuesday.

Wednesday night into Thursday appear to be the most likely period
for showers and thunderstorms area-wide as the ensemble solutions
show Mario`s remnant upper low crossing into the US somewhere
near 32n/122w and moving north. PW`s will increase to almost 2"
and there will be enough instability to generate showers and
thunderstorms at times across the area. There is still a fair
amount of uncertainty with the timing and rainfall rates and
amounts as even a small deviation of the track could make a
significant change to the outcome. But a large percentage of the
ensemble solutions show at least some rain falling during this
period pretty much everywhere across the 4 county area. And there
are roughly 10-15% of the solutions showing an inch or more or
rain so there is certainly the potential for periods of very heavy
rain and flooding.

.LONG TERM (FRI-MON)...15/235 PM.

The threat of showers and thunderstorms is expected to continue
into at least the first half of Friday as the last of the
moisture moves through the area.

The weekend is now expected to be dry, though there are still a
small percentage of the ensemble solutions showing some additional
light showers associated with a quick moving upper low from the
northwest. Aside from that, the region will be slowly recovering
from the warm tropical pattern and it may take several days before
any marine layer reforms. Temperatures are expected to near
normal Saturday, then warming to slightly above normal Sun/Mon.

&&

.AVIATION...15/2357Z.

At 2259Z at KLAX, the marine layer was 1100 ft deep. The top of
the inversion was at 2400 ft with a temperature of 27 C.

High confidence in VFR TAFs for desert airfields (KWJF & KPMD).

Most uncertain in arrival times of CIGs (+/- 3 hours), & moderate
confidence in clearing times (+/-2 hours). Flight minimums may be
off one category.

Chances for CIGs tonight into Tuesday at KBUR (15%), KVNY (10%), &
KPRB (10%).

Chances for rain are increasing for Thursday, with a chance for
thunderstorms.

KLAX...Moderate confidence in 00Z TAF. Arrival and clearing times
likely to be accurate within +/- 2 hours from current forecast.
There is a 20% chance of lowering to 0VC004 from 10Z to 15Z Tue.
No significant east wind component expected through the period.

KBUR...High confidence in VFR through at least 06Z. 20% chance of
IFR conds 08Z-15Z Tue.

&&

.MARINE...15/823 PM.

Minimal risks of Small Craft Advisory (SCA) winds through
Wednesday Night, except for a 30% chance of low-end SCA winds off
the Central Coast Wednesday afternoon and evening.

Although, Tropical Storm Mario is expected to continue weakening
over the next couple of days - the remnants are expected to impact
southwest California Wednesday through Friday. A period of showers
is likely at some time, with Thursday having the highest chances,
but confidence is fairly low on timing and amounts. Thunderstorms
are also possible at any time in the time window, but again
confidence is low on the details. Any thunderstorm will bring a
risk of brief but strong winds and frequent lightning. In
addition, gusty Santa Ana- like winds are possible on Thursday due
to the proximity of the core of the weakened storm. Please
monitor the forecast if you have any plans on going out on the
water this week, as this will be a low- confidence and evolving
situation.

&&

.FIRE WEATHER...15/800 PM.

An upper level low pressure system developing off the coast will
likely bring periods of remnant subtropical moisture from tropical
system Mario (currently centered off the coast of southern Baja)
into the region this week. Subtropical moisture will likely stream
into the area starting as early as Tuesday afternoon/night when
there is a 5 to 15 percent chance of dry thunderstorms, with storm
activity mostly staying offshore. The chances of showers and
thunderstorms at times will increase late Wednesday into Friday,
with a decreasing threat into the weekend. The main thunderstorm
threats will be strong/erratic winds, frequent and possibly
pockets of dry lightning, and brief heavy rain inducing flooding.

While isolated dry lightning strikes may occur as early as
Tuesday afternoon into Tuesday night, the greatest threat for
drier thunderstorms and associated fire starts will be Wednesday
afternoon into Thursday morning (especially San Luis Obispo and
Santa Barbara counties) when there will be increased mid level
moisture, more favorable upper level dynamics, and relatively
drier conditions near the surface. With the potential for
strong/erratic outflow winds, fire starts will have the potential
for rapid fire spread. As a result, there is a 30 percent chance
of Fire Weather Watches being issued for portions of the forecast
area in this Wednesday afternoon to Thursday morning time frame.
As deeper moisture moves into the region on Thursday and Friday.
storms will generally become wetter with increasing flooding
concerns. There is a non-zero risk of localized significant flash
flooding/debris flow concerns. Most areas will likely receive
between 0.10 and 0.50 inches between Wednesday and Friday, with
pockets of one inch or greater possible. Rain rates are generally
expected to be less than 0.25 inches per hour, except 10 percent
chance of localized 0.50-1.00 inches per hour. Also of note, as
the remnants of the tropical wave passes through the region
Wednesday night into thursday, there will be the potential for
some gusty southeast to northeast winds, with gusts of 25 to 35
mph for lower elevations, and 40 to 50 mph across the mountains,
which will add to the fire weather concerns if thunderstorms
develop.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC...MW/KL
AVIATION...Black
MARINE...Kittell/Black
FIRE...Gomberg
SYNOPSIS...MW

weather.gov/losangeles

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