-- Discussion containing changed information from previous version are highlighted. --
031
FXUS66 KLOX 180347
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
847 PM PDT Tue Sep 17 2024

.SYNOPSIS...17/143 PM.

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

&&

.SHORT TERM (TUE-FRI)...
-- Changed Discussion --
17/841 PM.

***UPDATE***

A super cool day with most max temps across the csts and vlys only
in the upper 60s to mid 70s. These temps were about 5 degrees blo
normal for the beaches, 5 to 10 degrees blo normals for the rest
of the coastal area and a remarkable 10 to 15 degrees cooler than
normal for the vlys. Skies were clear save for some pesky morning
low clouds across the LA coast and the Central Coast.

Currently skies are pretty much clear everywhere and it looks like
the low clouds will fill in late (after midnight) over the the
LA cst while the Central Coast will slowly fill up with low clouds
over the next few hours.

The rain/thunder fcst still looks on track for late Wed and Thu.

Small update will be issued to reduce this evening`s clouds.

***From Previous Discussion***

Low impact weather expected through most of Wednesday.
Temperatures will remain much cooler than normal both at night and
during the day as a second low pressure system approaches from the
northwest. A weak marine inversion will remain in place through
tomorrow so clouds will be spotty and very difficult to predict,
but otherwise not a significant issue.

The main weather impact will be the arrival of the upper low late
Wednesday into Thursday. Models have been very consistent with the
track and depth of the low, however there still is a fair amount
of uncertainty what the exact impacts will be locally. The upper
low is unusually cold for this time of year. A look at SPC`s
sounding climatology shows that the forecasted -19c at 500mb at
VBG would be the coldest on record since soundings started there
in the 1940s. With that context in mind, it`s reasonable to
assume that some very usual weather will take place during the
peak of the instability on Thursday. If the track holds, the most
likely area for the biggest impacts (ie, thunderstorms with
pockets of heavy rain) would be across the interior parts of SLO,
SB, and Ventura counties, mainly east and north of the coastal
mountain ranges. Ensemble based guidance indicates up to a quarter
to half inch of rain in those areas, but with maybe a 10% chance
of as much as an inch. If those heavier showers occur over a
recent burn scar that could cause some debris flows. Probabilities
are too low for any sort of flash flood watch but not completely
out of the question.

For most coastal and valley areas, the impacts from this low will
be just some areas of light rain or drizzle, mostly under a tenth
of an inch. If the upper low ends up deviating a little farther
to the south, LA and Ventura Counties could be in a favorable
position for slightly higher rain amounts up to around a quarter
to half inch.

The upper low is expected to exit Thursday night with dry and
warmer conditions expected Friday.

.LONG TERM (SAT-TUE)...17/211 PM.

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.
-- End Changed Discussion --

&&

.AVIATION...17/1926Z.

At 20Z at KLAX, the marine layer was 2500 feet deep. The top of
the weak marine inversion was near 7200 feet with a temperature
around 10 degrees Celsius.

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

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...17/1227 PM.

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) later
this afternoon into tonight. Areas of steep and choppy seas will
continue through mid-week. For Thursday and Friday through this
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 Pt. Conception.

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

For the Inner Waters south of Pt. 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 this 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 until 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