778
FXUS66 KLOX 230314
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
814 PM PDT Sun Sep 22 2024

.SYNOPSIS...22/153 PM.

Warm and dry conditions can be expected through Monday as high
pressure aloft builds over the region. The warmer air mass will
likely remain in place through much of the week away from the coast,
with the hottest daytime temperatures forecast for Monday and
Tuesday. It should turn cooler late this week into next weekend.
Mostly clear skies can be expected through the period except for
night and morning coastal low clouds and fog.

&&

.SHORT TERM (SUN-WED)...22/813 PM.

***UPDATE***

Temperatures today were able to warm up to the high 80s to mid 90s
across the interior areas, including the San Fernando Valley.
Coastal valleys reached 80s to 90s, while the beaches were
largely in the 60s thanks to the the persistent marine layer
clouds. A combination of very weak offshore flow setting up and
slight increase of upper level highs overnight will lead to the
potential of patchy dense fog, and less expansive marine layer
clouds overnight into tomorrow morning. Low temperatures tonight
will generally be a few degrees higher than last nights` due to
very weak offshore flow setting up overnight, though areas at the
coast will remain pretty similar.

As previously mentioned, very weak offshore flow will set up due
to a broad trough over the Great Basin and a low pressure system
developing a few hundred miles off of the Central Coast. The weak
offshore flow and slight rise in 500 mb heights will allow for
better afternoon clearing of the marine layer tomorrow, and in
turn result in up to 10 degrees of warming compared to today. The
interior areas (including the interior valleys such as SCV and
SFV) will be in the mid to high 90s tomorrow, the remaining
valleys will be in the 80s to low 90s, and as you get closer to
the beaches, temperatures will be in the high 60s to high 70s.
Temps at the beaches will be fairly normal for this time of year,
however most areas will be a few degrees to up to 10 degrees above
normal.

Forecast is in good shape, and no updates were needed at this
time.

***From Previous Discussion***

The upper level low will edge slowly N Mon night thru Wed, and by
Wed will weaken and become a broad upper level trof. H5 heights
over SW CA will be in the 585-586 dam range thru Tue then fall to
581-584 dam by Wed with the upper level flow turning to the S and
SW.

The marine layer will shrink through tonight thanks to
significant warming between about 900 mb and below. By early Mon
morning, the inversion should be around 800 ft or so S of Point
Conception and possibly near surface-based on the Central Coast.
Little change in the inversion depth is expected thru Tue, then it
should increase only slightly for Tue night into Wed and be about
1000 ft or less.

A marine layer pattern with night and morning low clouds and fog
mainly along the coast followed by clearing back to or off the
coast during the late morning and afternoon hours can be expected
tonight through Wed. Due to the shallow marine inversion
developing, patchy dense fog is possible with the low clouds
especially later at night into the morning hours thru Tue.
Elsewhere and otherwise across the forecast area, mostly clear
skies are expected thru Wed, with breezy onshore winds each
afternoon.

Pressure gradients to the N and E are forecast to trend offshore
tonight into Mon morning which will allow temps to warm
considerably. Highs on Mon are forecast to be 5-10 deg above
normal except along the immediate coast. Weak pressure gradients
will support continue significantly above normal temps into Tue,
especially away from the immediate coast. Also of note, overnight
lows will be quite a bit above normal for many areas, with temps
likely remaining in the 70s for the foothills and lower mtns. By
Tue night and Wed, temps should start to trend cooler as onshore
flow increases. Highs on Wed are expected to be a few degrees
below normal to near normal for the coast and vlys, and 3-7 deg
above normal for interior areas including the mtns and deserts.

.LONG TERM (THU-SUN)...22/152 PM.

The GFS and EC ensembles and deterministic are in generally good
agreement thru the extended period. An upper level low is forecast
to develop about 500 miles SW of the fcst area by Thu then
meander well off the coast thru next Sun. Upper level ridging will
linger generally over the southern Great Basin into SE CA thru
the period. H5 heights over the forecast area should be in the
586-588 dam range thru the extended period altho the EC
deterministic does increase the H5 heights to 590-592 dam next
weekend.

The marine inversion should remain relatively shallow Thu and Fri
then increase some to about 1000-1500 ft or so Sat and Sun. Night
thru morning low clouds and fog mainly for the coast can be
expected Thu thru Sat, then expand some inland on Sun. Otherwise
and elsewhere, mostly clear skies can be expected thru the period.

Temps Thu and Fri should be from a few degrees below normal for
the coast and vlys to a few degrees above normal for interior
areas, then cool several degrees over the weekend as onshore flow
increases. By Sun, temps should be several degrees below seasonal
norms for most areas, but remain a few degrees above normal in the
far interior vlys and deserts.

&&

.AVIATION...23/0135Z.

At 2358Z at KLAX, the marine layer was 1100 feet deep. The top of
the inversion was 2400 feet with a temperature of 26 degrees C.

High confidence in TAFs for KPRB, KPMD, and KWJF. There is a 10%
chance for VLIFR-LIFR cigs/vsbys at KPRB 10Z-15Z.

Low confidence in KBUR and KVNY. There is a 30% chance of VLIFR-
LIFR conds 10Z-15Z.

Low confidence in L.A. coastal TAFs. Cigs could linger until 22Z
Mon. There is a 10% chance for VLIFR conds at KLAX/KSMO, and a
20% chance for LIFR conds at KLGB 04Z-15Z.

Low confidence in remaining coastal TAFs north of L.A. county.
Cigs could linger until 22Z Mon. There is a 40% chance of VLIFR
conds at KSMX and a 30% chance at KSBP. There is a 30% chance that
cigs remain IFR at KOXR, KSBA, and KCMA through 16Z.

KLAX...Low confidence in TAF. There is a 30% chance of cigs
lingering until 22Z Mon. There is also a 10% chance for cigs
OVC001-OVC002 04Z-15Z, and a 10% chance of OVC010-15. Good
confidence in no significant east wind component through the
period.

KBUR...Low confidence in TAF. There is a 30% chance of
OVC001-OVC002 cigs and/or vsbys 1/2SM or lower.

&&

.MARINE...22/635 PM.

High confidence in the current forecast through Wednesday, then
moderate confidence thereafter. Winds and seas will remain below
Small Craft Advisory (SCA) levels through at least Wednesday,
except for 30-40% chance of local low end SCA level wind gusts
near Pt Conception, around Malibu, and the San Pedro Channel thru
this evening. There is also a 20-30% chance for SCA level winds
in the inner waters off the LA and OC Coasts in the afternoon thru
Wednesday evening.

Then, moderate to high confidence in winds and seas increasing to
SCA level Thursday thru Friday, with highest confidence in the
Outer Waters (Central Coast south to San Nicolas Island) Friday.
There is also a 20-30% chance for Gale Force winds Friday
afternoon thru late night in the Outer Waters, with highest
confidence from Pt Conception north.

&&

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

&&

$$

PUBLIC...Sirard/Lund
AVIATION...Lund
MARINE...Lewis/Schoenfeld
SYNOPSIS...Sirard

weather.gov/losangeles

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