Acton Weather

000
FXUS66 KLOX 220023
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
523 PM PDT Mon Sep 21 2020

.SYNOPSIS...21/1248 PM.

Little change in temperature is expected through Tuesday with
night and morning low clouds and fog across the coasts and lower
valleys. A slight warming trend will occur Wednesday and Thursday
followed by additional warm and dry conditions this weekend.

&&

.SHORT TERM (TDY-THU)...21/131 PM.

Still warm and dry across interior sections today, which has
sparked additional plume development over the Bobcat Fire in the
San Gabriel Mountains and Antelope Valley foothills. Temperatures
were in the 90s with many humidities between 5-15 percent. A few
southwest wind gusts of 25 to 35 mph in this region will result in
brief periods of critical fire weather until sunset this evening.
Elsewhere, rather benign weather concerns with a marine layer
that jumped up to 1700 feet south of Pt Conception and less than
800 feet to the north. Low clouds have retreated off the coast for
most areas except from Gaviota westward and along the central
coast this afternoon. Expect a return of stratus and locally dense
fog in these areas and mainly clouds for the Ventura and LA
Coasts including the LA Valleys late tonight through Tuesday
morning.

Onshore pressure gradients will be slightly weaker the next few
days as high pressure slowly builds into the region, so not quite
as gusty across the mountains and deserts in the afternoons. Very
minimal changes are expected with temperatures and humidities
however, just a couple of degrees lower Tuesday then higher on
Wednesday. This will continue to create some elevated fire
weather concerns each afternoon when humidities drop into the
lower teens. Wind direction begins to veer to the west and
northwest late Wednesday which could be the first evening
Sundowners returning to western hills of the south Santa Barbara
County coast. Building heights will help to lower the marine
inversion through Wednesday, so less inland push of low clouds
but more likely to see locally dense fog for coastal areas.

On Thursday, there is a weak trough that slides across northern CA
that strengthens the northwesterly wind flow heading into Friday.
NW to N offshore pressure gradients of 2-3 mb develop over south
Santa Barbara County Thursday evening for a potential wind
advisory Sundowner by then. Not much change in temperatures yet
through the short term.

.LONG TERM (FRI-MON)...21/129 PM.

A storm system will pass through the Pacific Northwest on Friday
and into the northern Rockies by Saturday. This helps high
pressure build off the West Coast during the period with
northerly flow increasing over southern CA. Warming should take
place for the majority of the area with the marine layer becoming
more restricted to the LA Basin by Saturday morning. Sundowner
winds and I-5 corridor winds will likely add downslope warming to
many areas with heights building to 592dm or higher by Saturday.

Looking beyond Saturday, the GEFS and ECMWF ensembles both hint at
a significant warm up late this weekend and early next week as
high pressure encompasses the Great Basin and West Coast. Heights
build to 594dm or higher by Monday, which is outside of model
climatology (less frequent than 1-day every 10-years) for this
time of the year. The bottom-line is that we could be flirting
with record temperatures once again with highs 95-105 degrees in
the coastal and interior valleys, especially if we get any
significant offshore wind regime (ECM deterministic shows -3mb
LAX-DAG and -9mb LAX-TPH). The hot and dry scenario will likely
become a headline not only for the potential heat stress and
record highs, but for fire weather concerns for new and existing
fires. Large plume growth would likely contribute to rapid fire
growth and fire behavior.

&&

.AVIATION...22/0006Z.

At 2308Z at KLAX, the marine layer was 1100 feet deep. The top of
the inversion was around 3000 feet with a temperature of 26 C.

High confidence in desert TAFs and KPRB. Moderate confidence in
coastal and valley TAFs.

Will continue with a persistent forecast overall over the next
24-hour period, except there is a 40% chance that stratus could
clear out an hour earlier across coastal areas, especially N of
Point Conception.

There is a 30% chance that VLIFR Conds will not occur for coastal
TAF sites N of Point Conception overnight into Tue morning. There
is a 20% chance that IFR Conds will continue across coastal TAF
sites S of Point Conception and not increase to MVFR CIGs.

KLAX...Moderate confidence in 00z TAF. There is a 30 percent
chance of marine clouds arriving +/- an hour from forecasted
time. There is a 20% chance that an easterly compononet wind will
exceed 8 knots between 13-18z.

KBUR...Moderate confidence in 00z TAF. There is 30 percent chance
of marine clouds arriving +/- an hour from from forecasted time.

&&

.MARINE...21/120 PM.

A Small Craft Advisory has been issued for zone 670 as wind gusts
will just reach 25 knots this afternoon into tonight. Elsewhere,
winds are expected to remain below SCA criteria. By Tuesday
afternoon, there is a 60% chance of SCA level winds in zone 670
again with a 40 percent chance in other sections of the outer
waters. Gusty NW winds will spread southward by Wednesday and
there is a 70% chance of widespread SCA level winds across the
outer waters through San Nicolas Island Wednesday into Thursday.
Good confidence that SCA level winds will continue through Friday.
There is a 30-40% chance that SCA level winds may bleed into the
western portion of the Santa Barbara Channel Wednesday through
Friday.

Patchy fog, locally dense with visibility of one nautical mile or
less, will likely affect the coastal waters through this morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zone 670. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

Strong high pressure is expected to build over the West Coast
Sunday and Monday. Heat stress and elevated fire weather
conditions will be possible, especially on Monday.

&&

$$

PUBLIC...EB/Rorke
AVIATION...Kaplan
MARINE...Sweet
SYNOPSIS...EB

weather.gov/losangeles