701
FXUS66 KLOX 271323
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
623 AM PDT Fri Sep 27 2024

.SYNOPSIS...27/449 AM.

High pressure aloft expanding into the area will continue a
warming trend away from the coast. Closer to the coast, the
temperature trend will be moderated as onshore flow will keep a
persistent marine layer and stronger sea breeze in place. Night
through morning low clouds and fog will struggle to clear away
from the beaches and immediate coast through the weekend. Some
cooling is forecast for Saturday, then a significant warming trend
is on tap for early next week as high pressure to the east builds
in again.

&&

.SHORT TERM (TDY-SUN)...27/448 AM.

The latest satellite imagery shows the area sandwiched between an
upper-level trough near 30N and 127W and an upper-level ridge of
high pressure over the Sonoran Desert. South-southwest flow aloft
will continue to bring an onshore flow into the region, but the
high to the east will expand into the region, warming the air
mass away from the coast. Clouds are well-entrenched along the
southern and central California coast early this morning. Some
intrusion of the low cloud field will likely push into the some
valley areas as the morning progresses.

With the high building in, the marine inversion will tighten and
remain fairly shallow. The latest AMDAR soundings from the KLAX
indicate a marine layer depth near 1150 feet deep, but what is
noted is the 10 degree Celsius change through the marine inversion
over the next 900 to 1000 feet above. The marine layer depth is
much shallower along the Central Coast as the deepening effect of
the coastal eddy in the southern California bight is lost. Low
clouds and dense fog are being observed this morning across the
broader Central Coast area and into the Santa Ynez Valley. A dense
fog advisory was added for this area through 9 am PST this
morning, but patchy dense fog may occur into the some of the
Southland valleys and foothills this morning. If you encounter
dense fog on your commute this morning, slow down, use low beam
headlights, and increase the space cushion around your vehicle.

The tightening marine inversion will also delay clearing this
afternoon. In a pattern similar to June, the marine inversion
will inhibit mixing and very likely keep the low clouds field
entrenched along the beaches later today. Clouds will very likely
sock in most beaches through the day and keep temperatures closer
to persistence at the beaches and immediate coastal locations.

The upper-level trough, sitting about 500 miles southwest of
Point Conception, will lift out and advance into the region
through Sunday. A cooling trend is expected for Saturday as the
marine layer will likely deepen slightly and onshore pressure
gradients will strengthen by about a millibar more onshore. The
trough will run into the blocking ridge pattern, then retrograde
once again for early next week. Some warming will start to take
shape on Sunday as the ridge reasserts itself to begin the week.

.LONG TERM (MON-THU)...27/446 AM.

High pressure aloft will expand back into the region for Monday
and Tuesday as the trough retreats again to the southwest. 500 mb
heights climb through the period while a low to middle level
southeast flow develop. High confidence exists in a warming trend
will develop with a hot air mass developing away from the coast,
but more questions remain for the coast and lower elevations of
coastal valley areas where a marine intrusion could remain. While
deterministic GFS solutions offer up weaker onshore flow,
deterministic ECMWF solutions turn the pattern weakly offshore.
EPS solutions suggest the deterministic ECMWF solution to be an
outlier, holding onto a decent onshore flow across the Southland
coastal areas. This would keep the air mass cooler for the South
Coast of California. Farther to the north along the Central Coast,
the air mass should be much warmer and there is a good chance of
the Central Coast could end up being 10 to 15 degrees above
normal. EPS solutions give a 30 percent chance of a 90+ degree
high on Tuesday at KSMX.

Still, the southeast lower level flow will be concerning if this
develops as progged in the deterministic solutions. The flow
pattern would be consistent with more significant warming for the
coastal areas. In addition, the flow pattern could be conducive
for monsoonal moisture to push into the region. PoPs were nudged
over NBM values for Tuesday and Wednesday to account for the
possibility of afternoon and evening showers and thunderstorms
over the mountains and desert. Nothing is mentioned in the
forecast yet, but it is a non-zero chance. The forecast ensembles
are trending higher with precipitable water values in the latest
runs, with more perturbations trending closer to 1 inch. A few
ensemble members pickup some low QPF number for Tuesday, but this
relies on convective processes to work properly as designed in the
models.

&&

.AVIATION...27/1322Z.

At 12Z at KLAX, the marine layer was 1500 ft deep. The top of the
inversion was 2400 ft with a temperature of 24 Celsius.

High confidence in the TAFs for KWJF, KPMD, and KPRB.

Otherwise, low to moderate confidence. Low clouds were widespread
in all coastal areas and the Santa Ynez and San Gabriel Valleys.
Clouds have pushed into the portions of the valleys of VTU
County and the southeastern San Fernando Valley as well. Conds
were mostly LIFR to VLIFR. Skies should clear by mid morning in
the valleys, and late morning on most of the coastal plain. Clouds
could linger into the afternoon near the beaches, especially
across southern SBA County and Ventura County.

Expect clouds to overspread the coastal plain this evening, and
push into most valley locations after midnight. The marine layer
may be a bit deeper, so conds should be IFR to LIFR, except
locally VLIFR on the Central Coast and in the valleys.

KLAX...Low to moderate confidence in the 12Z TAF. There is a 30%
chance that cigs will linger until as late as 20Z or 21Z. There is
a 20% chance that cigs will not clear at all. There is a 20%
chance that cigs will arrive as early as 01Z. Good confidence any
significant east wind component will remain under 6 kt.

KBUR...Moderate to high confidence in TAF. There is a 20% chance
that cigs will clear by 15Z. There is a 20% chance that skies
will remain clear tonight.

&&

.MARINE...27/550 AM.

In the outer waters, there is low to moderate confidence in the
forecast. Local Small Craft Advisory (SCA) level wind gusts are
quite likely this afternoon/eve, but for the most part, winds
should remain below SCA levels. However, there is a 30% chance
that SCA levels wind gusts will become widespread enough to
warrant an advisory. From late tonight thru Mon, winds are
expected to remain below advisory levels, but there is a 20%
chance of SCA level winds, mainly during the late afternoon/eve
hours, especially around Pt. Conception. There is a 30% chance of
SCA level seas Sat thru Mon, with the highest chance in northern
areas. SCA conds are not expected Mon night thru Tue night.

For the inner waters north of Pt Sal, the Santa Barbara Channel,
and inner waters from Pt Mugu southward, moderate to high
confidence that winds and seas will remain below SCA levels thru
Tue night. There is a 20% chance of SCA level winds this
afternoon/eve, mainly in western and southern portions of the
Santa Barbara Channel, and from near Anacapa Island to Point Dume.

Areas of dense fog with vsbys 1 NM or less will affect the waters
this morning, most widespread off the Central Coast.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Dense Fog Advisory in effect until 9 AM PDT this morning for
      zones 340-341-346>348. (See LAXNPWLOX).
PZ...NONE.

&&

$$

PUBLIC...Hall
AVIATION...DB
MARINE...DB
SYNOPSIS...Hall

weather.gov/losangeles

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