376
FXUS66 KLOX 171628
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
928 AM PDT Tue Sep 17 2024

.SYNOPSIS...17/854 AM.

Much cooler than normal temperatures will continue through Friday
before warmer conditions return for the weekend. There is a
chance of rain or drizzle Wednesday night and Thursday as a low
pressure system moves into the area.

&&

.SHORT TERM (TDY-THU)...17/915 AM.

***UPDATE***

Forecast remains on track with a little warming today, then little
change Wednesday and cooler on Thursday as the upper low moves
into the area with possible showers and thunderstorms. Models
continue to highlight the interior parts of SLO, SB, and Ventura
Counties as the most likely areas for thunderstorm development
Thursday afternoon. CAPEs are modest at around 500j/kg but
impressive for this time of year. In fact, looking at SPC`s
sounding climatology the model forecast of -19c at 500 mb would be
a record low temperature at that level since soundings have been
done in the VBG area in the late 40s. Given the instability and
rather slow steering speeds isolated heavy rain and flooding can`t
be ruled out, especially near burn areas. Outside those areas,
drizzle or light showers are possible just about anywhere late
Wednesday night into Thursday as the upper low approaches and
moves through, though based on the latest ensemble runs areas
north of Pt Conception are more likely to receive measurable rain.
If the upper low drops a little farther south and/or takes a
farther west track that would be a more favorable pattern for
showers in the south.

***From Previous Discussion***

A fairly unusual pattern for mid September as Srn CA is in between
a departing upper low and an approaching upper low. All of the
cold air form ydy`s low destroyed the marine inversion and low
clouds are confined to western SBA county. Rising hgts, plenty of
sunshine and weak onshore flow will bring 2 to 4 degrees of
warming to the csts/vlys and 4 to 8 degrees for the mtns and
interior. Despite this warming max temps will end up 10 to 15
degrees blo normals.

The next upper low will start its trek down the west coast early
Wednesday and by noon it will be just west of the Bay Area. The
marine inversion should reform and this along with stronger
onshore should allow for a little more coastal stratus to form
esp for the Central Coast and the LA cst. A slight chc of rain
will develop in the afternoon over SLO county as the low moves
further south. Max temps will warm a degree or two south of Pt
Conception and will cool a degree or two north.

The low with 563 dam central hgt will continue to move south
overnight and by dawn Thursday it will be centered just to the
north of SLO county. A chance of rain will overspread SLO And SBA
counties overnight. There is a slight chc of rain for VTA and LA
counties but more likely the upper low will gin up a deep marine
layer cloud deck and some drizzle (which could produce a few
 of an inch) Rainfall amounts overnight will not total any more
than .05 inches.

The upper low will skirt along the northern portion of the
forecast area on Thursday. It will bring cloudy skies and chc of
rain to the entire area. The system does not have much moisture to
work with and rainfall amounts will be under a tenth of an inch
with the only exception being the NW corner of SLO county which
could see a quarter inch. The cold core of the upper low will move
over the area in the afternoon and the instability created by the
cold temps aloft could be enough for TSTM development. At this
time there is only a 10 percent chc of a TSTM but the threat will
need monitoring.

Max temps on Thursday will plummet along with the quickly lowering
hgts and cloud cover. Look for 4 to 8 degrees of cooling across
the board with max temps coming in 10 to 20 degree blo normals.
Look for max temps in the 60s across the coasts and 70s in the
vlys.

Breezy northwest to onshore winds are likely Wednesday into
Thursday with the most wind prone areas potentially reaching
advisory levels.

.LONG TERM (FRI-MON)...17/312 AM.

All signs point to a robust warming trend starting on Friday. Both
the GFS and EC show a rapid rise in hgts to 588 or even 590 dam.
At the same time onshore flow weakens considerably with a
possibility of weak offshore flow during the morning hours. Marine
layer clouds will likely be confined to western SBA county.

6 to 12 degrees of warming is on tap for Friday with 5 to 10
additional degrees slated for Saturday. Sunday will be warmer
still but only be 1 or 2 degrees. Slight cooling is forecast
Monday as the hgts fall and onshore flow increases.

The weekend temps will be 4 to 8 degrees above normal with 70s at
beaches 80s a little further inland and 90s in the vlys, lower
elevation mtns and inland areas.

&&

.AVIATION...17/1544Z.

At 15Z at KLAX, the marine layer was 2600 feet deep. The top of
the weak marine inversion was near 7600 feet with a temperature
around 10 degrees Celsius.

Medium confidence in coastal TAFs, and medium to high confidence
in VFR TAFs elsewhere. Night through morning low clouds are
forecast to bring reduced cigs to coastal sites -- MVFR south of
Point Conception and IFR to LIFR north of Point Conception. There
is a 30% chance for cig categories to be off by one category, and
there is a 30% chance for timing of flight category fluctuations
to vary by +/- 2 hours from current forecasts.

KLAX...Moderate confidence in TAF. There is a 30% chance for cigs
to become IFR tonight and Wednesday morning. No significant east
wind component is expected.

KBUR...Moderate to high confidence in TAF. There is a 20% chance
for cigs to become MVFR tonight and Wednesday morning.

&&

.MARINE...17/844 AM.

For the Outer Waters, Small Craft Advisory (SCA) winds will
continue over the southern two zones (PZZ673 and PZZ676) through
Wednesday night, and extend into the northern zone (PZZ670) late
this afternoon into tonight. Steep and choppy seas will continue
through mid-week. For Thursday and Friday through the weekend, SCA
headlines will be unlikely, though a few spots of marginal SCA
winds cannot be ruled out (30% chance) from areas around San
Miguel and Santa Rosa Islands toward Point Conception.

For the Inner Waters north of Point Sal, SCA winds are expected
late this afternoon through tonight, with SCA conditions unlikely
thereafter.

For the Inner Waters south of Point Conception, SCA winds are
expected across portions of the Santa Barbara Channel --
especially the western two-thirds -- this afternoon and tonight
and again Wednesday afternoon and night, accompanied by steep and
choppy seas. For Thursday and Friday through the weekend, SCA
headlines will be unlikely, though a few spots of marginal SCA
winds cannot be ruled out (30% chance) over the Los Angeles and
Orange County waters within 20 NM from shore.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Wednesday for zones 645-650-670. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 PM Wednesday to 3 AM
      PDT Thursday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Thursday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...MW/Rorke
AVIATION...Cohen
MARINE...Cohen
SYNOPSIS...MW

weather.gov/losangeles

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