554
FXUS66 KLOX 041249
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
549 AM PDT Sat May 4 2024

.SYNOPSIS...04/528 AM.

An unseasonably cold storm system will move over the region
through Sunday. A cold front, dropping south today and tonight, will
bring rain and high-elevation snow to much of the area, along with
gusty winds. The storm will exit the region on Sunday night, then
a warming and drying trend will develop through at least
Wednesday.

&&

.SHORT TERM (TDY-MON)...04/528 AM.

The latest water vapor imagery shows a cold upper-level trough,
originating from the Gulf of Alaska, digging south along the
Oregon coast early this morning. A cold frontal boundary can been
over northern California and stretching out to the southwest.
After struggling to develop earlier in the night, the latest fog
product imagery indicates marine layer low clouds and fog
extending well into the coastal slopes south of Point Conception.
North of Point Conception, low clouds and fog are well entrenched
along the Central Coast and into the Santa Ynez Valley. Some
clouds are now starting to roll into the southern Salinas Valley
this morning. With the deepening marine layer depth likely
approaching 4000 feet deep this morning, patchy morning drizzle
or light rain cannot be ruled out.

A mostly cloudy day looks on tap with cooler temperatures across
much of the area. A thick high-level cloud shield is moving south
down the state currently, and this cloud deck should reinforce
the general premise of a mostly cloudy day. The cold frontal
boundary should push south down the state through the day. The
front should approach northwestern San Luis Obispo County late
this afternoon, then quickly move over the region overnight
tonight. The boundary should exit Los Angeles County by daybreak
Sunday. PoPs have been nudged closer to high-resolution multi-
model ensemble solutions, which advertise the Central Coast
getting about 3-4 hours of steady rain this afternoon. Confidence
lowers as the frontal boundary passes south of Point Conception
tonight. The forecast buys more heavily into high-resolution
multi-model ensemble solutions, but there is a moderate chance
that downsloping could occur and create a rain shadow effect
across Santa Barbara County, Ventura County, and western Los
Angeles County. One and six hour QPF probabilities in the multi-
model ensemble solutions introduce lesser chances as the frontal
boundary moves south of Point Conception, but it still produces a
high-to-likely chance of rainfall occurring.

QPF amounts in the forecast still follow WPC values at this time.
The latest forecast brings a bit more precipitation into the mix,
especially in the San Gabriel Mountains. Rainfall amounts should
range from 0.25 inch or less, except for 0.25 to 0.50 inch along
the Coast Ranges of Central California and in the San Gabriel
Mountains. A few sites such as Rocky Butte in northwest San Luis
Obispo County will likely see amounts up to 0.67 inch. Looking at
the less coarse EPS ensemble members and the spread across the
high-resolution multi-model ensemble solutions, there is slightly
less than a 50 percent chance that QPF values could be up to a
third wetter than forecast. Or put another way, about half of
these ensemble solutions suggest the values in the wettest areas
could range between between a third and two-thirds of an inch. If
this occurs, local amounts at the wettest sites like Rocky Butte
could approach 0.85 inch. Much will depend how much upslope flow
develops and for how long.

Snowfall amounts will be light. While snow levels will lower down
to 4000 to 4500 feet late tonight or early Sunday morning, most
of the moisture will likely vacate the area by that time. At
most, a dusting to an inch of snow are likely with local amounts
up to two inches.

Gusty winds will start to develop across the San Gabriel
Mountains, and the Antelope Valley and the adjacent foothills
this afternoon and this evening as the surface pattern tightens
ahead of the cold front. A wind advisory remains in effect for
this afternoon and tonight for these areas. After the front
passes over the region, a tight surface pressure gradient will
remain into Sunday and Sunday night. There is a good chance
that the advisory for the San Gabriel Mountains, and the Antelope
Valley and the adjacent foothills may need to be extended into
Sunday and Sunday night. Wind advisories were added for the entire
coastal areas and into the Santa Ynez Valley for gusty west winds
on Sunday afternoon and evening, and for the Transverse Ranges and
into southern Santa Barbara County for gusty northwest winds
developing late tonight through early Monday morning.

A cold air mass will spread into the area today and Sunday. There
is a moderate chance that low maximum temperatures records could
fall on Sunday out in the desert as the air mass will be cold for
May, more inline with February or March. Temperatures in the 60s
will be common across most areas the next several days, coldest on
Sunday for areas south of Point Conception.

.LONG TERM (TUE-FRI)...04/529 AM.

The forecast ensemble members continue to suggest a warming and
drying trend developing through Wednesday as high pressure aloft
reestablishes over the eastern Pacific Ocean and a low pressure
system dives through the Great Basin. There is a chance that a
cool down could arrive for late week as onshore flow increases.
EPS ensembles continues to keep cloud cover means free of the
marine influence, but confidence remains moderate at best due to
the time of the year.

&&

.AVIATION...04/1249Z.

At 12Z at KLAX, the marine layer was 3300 ft deep. The top of the
inversion was at 4200 ft with a temperature of 13C.

Deep marine layer with low clouds in all coastal/valley areas and
up to the coastal slopes. Conds were low MVFR in L.A. County.
except IFR-VLIFR in the foothills/mtns, and were mostly IFR-VLIFR
from VTU County northward. Skies will likely remain mostly cloudy
today, though cigs will gradually rise. Most areas should have MVFR
to possibly low VFR conds this afternoon, except IFR or worse in the
foothills/mtns. A frontal system will likely bring light rain to
SLO/SBA Counties this afternoon/evening, with a chance of showers
in L.A./VTU Counties this evening thru late tonight. Widespread MVFR
conds expected tonight with IFR-VLIFR cigs in the foothills/mtns.

KLAX...Moderate confidence in the 12Z TAF. There is a 30% chance
that MVFR conditions will linger thru the day, and not improve to
VFR from 20Z thru 04Z as predicted. There is also a chance that
MVFR conds tonight will linger thru 16Z Sun. No significant
easterly wind component is expected thru the pd.

KBUR...Moderate confidence in the 12Z TAF. There is a 30% chance
that MVFR conditions will linger thru the day, and not improve to
VFR from 20Z thru 03Z as predicted.

&&

.MARINE...04/510 AM.

In the Outer Waters, winds and seas may drop below Small Craft
Advisory (SCA) levels several hours late this morning/afternoon,
but are expected to increase again late today. Widespread SCA
level winds/seas are expected tonight thru Sun afternoon, except
will likely drop below SCA levels in the northern zone (PZZ670)
late tonight. Winds will increase Sun, with gales possible late
Sun into Sun night in the southern two zones (PZZ673/676) and SCA
conds in the northern zone. SCA conds are expected Mon thru Wed,
with a 40% chance of gales Mon afternoon/eve.

In the Inner Waters N of Pt. Sal, SCA level winds/seas are likely
in the afternoon/eve hours today thru Wed. There is a chance that
seas will remain at or above SCA levels even when winds weaken,
especially Sun night/Mon night.

In the Inner Waters south of Point Conception, SCA level W-NW
winds are expected to spread from west to east across the waters
this evening and early tonight, then continue thru Sun afternoon.
There is a 50% chance of gales Sun afternoon into late Sun night,
especially in western portions. SCA level seas may persist into
Mon. SCA winds are likely during the afternoon/eve. hours Mon thru
Wed in the SBA Channel, with a 30% chance of SCA conds extending
into the southern inner waters from Anacapa Island to Malibu.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 10 AM to 9 PM PDT Sunday for
      zones 87-340-341-346>348-354-355-362-366. (See LAXNPWLOX).
     Wind Advisory in effect from 11 PM this evening to 3 AM PDT
      Monday for zones 349>353-376>378. (See LAXNPWLOX).
     Wind Advisory remains in effect from noon today to 5 AM PDT
      Sunday for zones 379>383. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Sunday for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM this evening to 3 PM
      PDT Sunday for zones 650-655. (See LAXMWWLOX).
     Gale Watch in effect from Sunday afternoon through late
      Sunday night for zones 650-655-673-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Sunday for zone
      670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 PM PDT Sunday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

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