Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 230051

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
551 PM PDT Thu Oct 22 2020

Updated Aviation and Marine Sections

.SYNOPSIS...22/217 PM.

An approaching area of low pressure will bring an increase in
onshore flow, plenty of marine layer clouds and a cooling trend
through the weekend. Patchy drizzle will be possible through
Saturday as well. A chance of light rain is expected mainly south
of Point Conception Saturday night through Monday morning, with
mountain snow possible Monday morning. Clearing skies with gusty
Santa Ana winds are likely by Monday afternoon. A warming trend
can then be expected through the middle of next week.


.SHORT TERM (TDY-SUN)...22/210 PM.

Stratus persisted over much of the coast and some vlys early this
afternoon. There may be partial clearing toward the coast by
sunset, otherwise little change in the cloud cover can be
expected. The 21Z LAX-DAG gradient was +5.2 mb, with the NAM
forecasting +6.4 mb at 00Z this afternoon. This will contribute to
breezy to locally gusty S-W winds this afternoon, especially for
the foothills, mtns and deserts. Temps will be several degrees
below normal over the coast and adjacent vlys this afternoon,
while the interior vlys, mtns and deserts remain several degrees
above normal.

Weak upper level ridging this afternoon (H5 heights around 582
dm) will linger over the region tonight and Fri. Meanwhile, an
upper level low several hundred miles SW of the area today will
slowly move NE tonight through Friday night before moving to just
off the Central Coast on Sat with H5 heights lowering to 573-575
dm. The upper level low will slowly weaken while moving through
the forecast area Sat night into Sun morning then slide E of the
area Sun afternoon as a large upper level trof approaches the
area from the N.

A persistent marine layer can be expected tonight through Sat,
with the depth between 2500-3000 ft north to around 3500-4000 ft
south, then deepen further to 4000-5500 feet or so Sat night into
Sun morning in response to the passage of the upper level trof.
Low clouds will be extensive over the coast, vlys and coastal
slopes for much of the time tonight thru Sun, with only partial
afternoon clearing at best. The far interior vlys, higher mtns
and deserts will be partly cloudy for the most part thru Sat as
well, with a bit more cloudiness expected Sun. There will be
enough of an onshore push for patchy drizzle or even some isolated
light rain to develop night and morning hours especially for the
south-facing slopes and foothills tonight through Sat. Many areas
mainly S of Point Conception should then have a chance of light
rain from the deep moist layer Sat night and Sun, with up to 0.10
inch possible. Temperatures will be several degrees below normal
for many areas thru Sat, except remain a few degrees above normal
for far interior areas, then drop to to 5-9 deg degrees below
normal just about everywhere for Sun.

.LONG TERM (MON-THU)...22/213 PM.

The operational EC and GFS are in mostly decent agreement for the
extended period. They both track the deepening upper level trof
thru srn CA Sun night and Mon, altho the EC is deeper, wetter and
and slightly slower with the trof and forms an upper level low
over far Southeastern CA Mon afternoon. The upper trof/upper low
is forecast to move well E of the area Mon night and Tue while
upper level ridging builds in from the N. Upper level ridging
should then be over the area for the most part Wed thru Thu.

Leaned more toward the wetter EC solution thru Mon. There should
be more chances of rain over portions of L.A./VTU Counties Sun
night into Mon morning, with some snow possible above about 5000
feet, as the sharp upper level trof moves thru. Rainfall amounts
will be generally 0.10 inch or less except up to 0.25 inch or more
over the eastern San Gabriel mtns. Any snowfall should total 1 to
2 inches. The upper trof will also usher in a much cooler air
mass into the region along with increasing NE winds and rapid
drying by late Mon morning. There is the potential for strong and
gusty NE winds over VTU/L.A. Counties by early Mon afternoon and
persist thru Mon night, with gusts up to 50 mph or more possible
in the valleys and up to 65 mph in the mountains. The winds
combined with humidities dropping into the single digits and teens
will also bring the threat of critical fire weather conditions
Mon afternoon thru early Tue morning. Dry and warmer weather can
then be expected Tue thru Thu, with temps rising to several
degrees above normal by the middle of next week.



At 23Z KLAX, the marine layer was 3500 feet deep. The top of the
inversion was 4600 feet and 19 degrees Celsius.

Moderate confidence in similar timing over the next 24 hours as
the previous, with CIGS 005-010 higher and little to no VIS
restrictions. -DZ possible at some airports especially around 12Z.

KLAX...Low confidence through 06Z, with uncertainty around when
stratus returns. Otherwise moderately confident that MVFR/VFR CIGS
will be most common 06Z through at least 18Z. Low confidence
after 18Z Friday, with a 40 percent chance of VFR CIGS persisting,
and a 60 percent chance of rapid clearing. 90 percent chance of
any SE winds staying below 8 KT.

KBUR...Low confidence through 06Z, with uncertainty around when
stratus returns. Otherwise moderately confident that MVFR/VFR CIGS
will be most common 06Z through at least 18Z. Low confidence
after 18Z, but there is a 60 percent chance of VFR CIGS persisting
for most of Friday.


.MARINE...22/549 PM.

High confidence of winds and seas staying under Small Craft
Advisory (SCA) through at least Sunday morning, except for very
localized seas near 10 feet beyond 30 miles from the Central
Coast. Abnormally strong southeasterly winds are expected on
Friday, with localized gusts to near 20 kt between the Channel
Islands and around Point Arguello.

Gusty northerly winds are expected later Sunday, followed by a
potentially strong Santa Wind event, which could result in
dangerous conditions for the Harbors on Catalina and Santa Cruz





Gusty Santa Ana winds are likely for the start of the work week
across LA/Ventura Counties, possibly creating critical fire
weather conditions across the region. A dry air mass beneath
clear skies will bring near freezing overnight temperatures
across interior areas Tuesday and Wednesday.



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