541
FXUS66 KLOX 191835
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1135 AM PDT Thu Sep 19 2024

.SYNOPSIS...19/914 AM.

An unsettled weather pattern will develop across the region
through Friday as an early season low pressure system moves over
the region. A cool air mass will remain in place through Friday,
but turn more showery through tonight. While not every location
will see showers and thunderstorms, any shower activity will be
capable of producing heavy rainfall, especially across interior
portions of Ventura, Santa Barbara, and San Luis Obispo Counties
through this evening. Some showers or thunderstorms could linger
into Friday before the low pressure area and the associated air
mass exits the region. A significant warming and drying trend
will develop over the weekend and into early next week.

&&

.SHORT TERM (TDY-SAT)...19/932 AM.

***UPDATE***

Not a lot of change from earlier forecasts and discussions. The
upper low has drifted south and is now just west of the Central
Coast. Some showers and thunderstorms developed near the
SLO/Monterey border and those are moving towards the northwest
into Monterey County and over the adjacent waters. Based on the
latest reflectivity forecasts from the HREF, probably not a lot of
activity in our area until later this afternoon as the low drifts
farther south and better instability develops with cooling aloft
and daytime heating. Still looking at the southern portion of interior
SLO County through interior SBA County as being the likely focus
area of showers and storms later today into this evening. Hi res
ensembles still indicating at least a 50% chance of rain rates of
a quarter inch per hour in that area and a 30% chance of a half
inch per hour. So the potential is definitely there for some very
heavy rain later today into this evening in and around the
interior SLO/SBA border area.

The slow moving upper low will shift the focus to areas south of
Pt Conception later tonight into Friday, though as we get into the
backside of the upper low steering flow increases quite a bit from
the north which, if true, would limit the amount of rain falling
on any one area. Rates could still be heavy, but durations would
be much less. Hi res models are not quite as confident on specific
areas of focus as the low moves south so for Friday the forecast
is taking a more broadbrushed approach with chances for showers
and thunderstorms in all areas south of Pt Conception.

***From Previous Discussion***

The latest satellite imagery shows a vigorous early season trough
off the central California coast early this morning. This trough,
located near 35N and 122W, or about 80 miles west of Point
Conception will wobble around the Point Conception area through
this afternoon while digging slowly southeast. Shower activity is
already developing across interior San Luis Obispo County this
morning, while a weaker band is currently located just offshore
closer to the trough. Isolated thunderstorms cannot be ruled out
for northern San Luis Obispo County this morning as the most
favorable lifted index values will push along the San Luis
Obispo-Monterey County border this morning. A few lightning
strikes have been observed to the north in Monterey County.
Shower and thunderstorm activity should become more expansive as
the day progresses and daytime heating steepen lapses rates and
destabilizes the atmosphere. PoPs mostly remain untouched through
this afternoon with only a few minor tweaks, but PoPs were
increased and a slight chance of thunderstorms were added to the
forecast tonight through Friday morning as the 500 mb cold pocket
will move across the southern California bight tonight through
Friday morning. High-resolution multi-model ensemble members keep
the highest probability of rain across the interior portions
across San Luis Obispo, Santa Barbara, and Ventura Counties this
afternoon and tonight. A flood watch remains in effect for these
areas where the highest probability of the heaviest rainfall
exists and includes Hurricane and Apache burn scars.

To south, low clouds and fog are filling in across the Southland
and into southern California bight and Santa Barbara Channel, but
low confidence exists in the cloud forecast as middle and high
level clouds streaming over the region are disrupting the marine
layer stratus formation. Where there are clouds, the instability
with the trough will likely produce patchy drizzle this morning as
the instability interacts with the deep marine layer depth in
place. The latest AMDAR soundings from KLAX indicate a 3200 foot
deep marine layer depth in place. Clouds should continue to fill
in across the Southland as the trough position should promote
low-level moisture to increase.

A cooler air mass and unsettled air mass will remain in place for
the next couple of days as the trough center slowly drifts
southeast across the area, coming ashore in Orange County on
Friday morning. Shower chances linger in the forecast into Friday.
It should be noted that the trough position is favorable for
wrap-around shower activity across the interior areas into Friday.
PoPs have been extended into Friday for this reason. A breezy
afternoon looks on tap for Friday as the surface pattern tightens
some. It appears to be marginal for any wind advisories, but a
wind advisory cannot ruled out across southern Santa Barbara
County on Friday evening.

A drying and warming trend should begin on Saturday as high
pressure aloft starts to build into the region. Some low clouds
and fog could remain across the region, but less low cloud
coverage should be expected into the weekend.

.LONG TERM (SUN-WED)...19/456 AM.

High pressure aloft will build into the region for early next
week and bring a warming trend. The latest temperature forecast
breaks away from NBM values for Sunday and Monday when high
pressure and onshore flow weaken. Potentially, weak offshore flow
could develop across the region and make temperatures several
degrees warmer than indicated. EPS cloud cover means indicate
minimal clouds for early next week during the night through
morning period, thus marine layer induced low clouds and fog
coverage could be less in coverage for Sunday and Monday than what
the current forecast alludes to.

Some cooling should develop for the latter half of next week as
another trough will attempt to dig south. This trough is not
expected to be as strong as the current one and will only increase
onshore flow and make the low cloud field a bit more expansive.
There is quite a spread in the forecast ensembles for late week as
there are plenty of solutions offering warmer solutions. The
forecast trends the way of the majority of the cooler solutions
with more low clouds and fog.

&&

.AVIATION...19/1834Z.

At 1653Z at KLAX, there was a 4500 ft deep moist layer. The top
of the weak marine inversion was at 5500 feet with a temperature
near 10 degrees Celsius.

High confidence in TAFs for KPMD and KWJF.

Low confidence in remaining TAFs. Cig heights will vary
frequently due to presence of upper level low moving over the
region. Timing of flight cat and VCSH/VCTS changes could be off by
+/- 5 hours.

For KPRB, TSTM (30% chc) and rain shower chances diminish after
06Z.

For KSMX, KSBP, and KSBA, low confidence in cig height and timing.
Thru 12Z, there is a 20-40% chance of -DZ, with highest chances
thru 06Z. There is also a 20-40% chance of isolated thunderstorms
thru 12Z, with best chances at KSMX and KSBP before 06Z. LIFR
conds possible (30% chance) at KSBP and VLIFR conds possible (30%
chance) at KSMX from 10Z thru 17Z. There is a 20% chance of brief
IFR cigs at KSBA between 08Z and 16Z.

For Ventura and Los Angeles County TAFs, there is a 20-30% chance
of -DZ is possible at all sites from 00Z thru the remainder of the
period tomorrow. During this period, there is also up to a 20%
chance of isolated thunderstorms. For KOXR and KCMA, there is a
40% chance that conds remain MVFR or VFR thru the period.

Thunderstorms may produce brief +RA and gusty, erratic winds.

KLAX...Low confidence in TAF. There is a 40% chance that cigs
below BKN035 will arrive +/- 3 hours from current forecast. Cigs
may vary between BKN015-035 frequently from 04Z thru at least 18Z.
There is a 30% chance of -DZ from 04Z through 18Z, and a 20%
chance of isolated thunderstorms during this period. No
significant east wind component is expected.

KBUR...Low confidence in TAF. There is a 40% chance that cigs
below BKN035 will arrive +/- 3 hours from current forecast. Cigs
may vary between BKN015-035 frequently from 04Z thru at least 18Z.
There is a 10% chance of brief BKN008-010 cigs between 04Z to
17Z. There is a 30% chance of thunderstorms from 04Z to 18Z.

&&

.MARINE...19/925 AM.

For the Outer Waters (Central coast south to San Nicolas Islands),
moderate to high confidence in the forecast. High confidence in
winds and seas remaining below Small Craft Advisory (SCA) level
thru early next week in the northern zone (off the Central Coast),
and thru this afternoon for the southern zones (Pt Sal to
San Nicolas Island). This evening there is a 20-30% chance of low
end SCA level winds in the two southern zones, mostly from Pt
Conception to San Nicolas Island. Then, there is a higher chance
(50%) for more widespread SCA winds tomorrow afternoon thru late
night in these zones. From Sat thru early next week, moderate to
high confidence in winds and seas remaining below SCA level in the
southern outer waters.

For the inner waters along the Central coast, moderate to high
confidence in the forecast. There is a 20% chance for brief
isolated low end SCA level winds late tonight. Otherwise, high
confidence in winds remaining sub advisory level thru early next
week.

For the Santa Barbara Channel, moderate confidence in the
forecast. There is a 20-30% chance of low end SCA level wind gusts,
isolated the far western portion of the channel this afternoon
thru evening. Higher chances in the same areas tomorrow afternoon
thru late night. Then, moderate confidence in conditions remaining
below SCA level Sat thru Tues.

For the inner waters off the coast from LA and Orange County, high
confidence in conditions remaining sub advisory level thru early
next week. Low end SCA level gusts are possible in the afternoon
thru evening today and tomorrow, isolated to the San Pedro
Channel.

Isolated to scattered thunderstorms will be possible across all
of the coastal waters this afternoon through tomorrow morning, as
an upper level low pressure system drops from Northern California
to near Pt. Conception by the afternoon. The trough will move
southeastward across the southern California bight tonight and
onshore over southern California through Friday. The northern
waters have a greater chance to be impacted thru late tonight,
while the southern zones have a better chance from tonight thru
tomorrow morning. A Marine Weather Statement is in effect to
communicate the hazards from thunderstorms to the marine
community.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch in effect until 11 PM PDT this evening for zones
      38-344-345-377. (See LAXFFALOX).
PZ...NONE.

&&

$$

PUBLIC...MW/Hall
AVIATION...Lewis
MARINE...Lewis/Lund
SYNOPSIS...MW/Hall

weather.gov/losangeles

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