-- Discussion containing changed information from previous version are highlighted. --
739
FXUS66 KLOX 271150
AFDLOX

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

.SYNOPSIS...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

.SHORT TERM (TDY-SUN)...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

.AVIATION...27/0117Z.

At 0004Z at KLAX, the marine layer was 1100 ft deep. The top of
the inversion was 2400 ft with a temperature of 25 Celsius.

High confidence in desert TAFs.

Moderate to high confidence in KPRB, KBUR, and KVNY. There is a
20% chance of VLIFR- LIFR conds at KPRB 07-17Z. There is a 10-20%
chance of IFR conds at KBUR and KVNY 11Z-15Z.

Moderate confidence in remainder of TAFs through 03Z, then low
confidence. Timing of flight cat changes could be off by up to
+/- 3 hours and off by 1 flight cat. Better chance for earlier
arrivals north of Point Conception. For KSBP and KSMX, there is a
30% chance of VLIFR conds 05-16Z. For KSBA, there is a 10% chance
of VLIFR conds overnight.

For KOXR and coastal TAF sites south there is a 30% chance of
cigs as early as 03Z. LIFR conditions are possible for KLAX and
KLGB from 10Z-15Z.

KLAX...Moderate confidence through 03Z, then low confidence. Cigs
could arrive as early as 03Z or as late as 08Z. There is a 20-30%
chance of LIFR conds between 10Z-14Z. Good confidence any
significant east wind component will remain under 6 kt.

KBUR...Moderate to high confidence in TAF. There is a 10-20%
chance of brief LIFR to IFR cigs 12Z-15Z.

&&

.MARINE...26/904 PM.

Across the outer waters, there is low to moderate confidence in
the forecast. Localized Small Craft Advisory (SCA) winds are
occuring around Point Conception this evening, but at this time
are not expected to become widespread enough to warrant an
advisory. Friday afternoon there is a 40% chance of local SCA
gusts around Point Conception again, followed by a 20% chance
Friday through Monday. Elsewhere, there is a 30% chance of SCA
level seas late Sat thru Mon, with the highest chance across the
northern areas. Otherwise for the rest of the outer waters, SCA
conditions are not expected.

Across 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
Mon night. There is a 20-30% 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.

Patchy dense fog with visibilities of 1 NM or less may affect the
coastal waters tonight through Friday morning, with the highest
chances off the Central Coast.

&&

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

&&

$$

PUBLIC...Hall
AVIATION...Phillips
MARINE...Lund/Schoenfeld
SYNOPSIS...Hall

weather.gov/losangeles

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