High Wind Watch 9, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
154 PM PST Wed Nov 25 2020

.SYNOPSIS...25/121 PM.

Patchy low clouds and fog will return tonight before gusty Santa
Ana winds develop Thursday, especially in Los Angeles and Ventura
Counties. The winds will peak in strength late Thursday night into
Friday morning, then weaken but still locally gusty into Saturday
through early next week. Afternoon temperatures warm on Thursday
and into the weekend. Another strong Santa Ana event is possible
next Tuesday and Wednesday.

&&

.SHORT TERM (TDY-SAT)...25/145 PM.

Low clouds have dissipated faster than models had shown today but
likely to redevelop this evening and overnight, mainly from
Ventura south. Clouds could spread as far inland as the San
Gabriel Valley before Santa Ana winds kick in Thursday morning.
As usual winds will start up first in the LA/Ventura mountains
then spread quickly into the valleys and eventually to the coast,
though how fast this happens will depend on the stubbornness of
the marine layer. If low clouds fail to reform or are just patchy
at best that's a good sign that winds will reach the coast much
sooner, but a solid low cloud deck would delay that arrival
potentially by several hours. Models certainly favoring the slower
arrival, at least for LA County coast but the clouds today
dissipated much faster than models had indicated so at this point
am more confident in faster arrival.

With this in mind went with wind advisories for most LA/Ventura
areas Thursday (minus the Antelope and San Gabriel valleys at
least initially), starting first in the mountains then valleys
and lastly the coast by late morning or early afternoon. Gusty
winds will also push out over the coastal waters but with this
first push they will be confined mainly to the waters off the
Ventura and Malibu coast.

Then the real crux of the event begins Thursday night with warning
level wind across the mountains (especially LA County, and
including the Santa Monicas) and possibly some of the valleys as
well. Typically even with these stronger events warning level
valley winds are confined to the elevated areas like the Santa
Susanna mountains (Wiley Ridge area) so am not super confident in
the watch there, at least for the more populated areas. Advisory
level winds also possible across the foothills of the San Gabriel
Valley but that's still more than 24 hours away. And gusty
northeast winds will push out of Orange County towards Catalina
Thursday night and bring gusty winds to the island into Friday
morning.

Winds expected to decrease Friday afternoon and all areas may drop
below advisory levels before sunset, but some pockets of advisory
level winds are possible again later Friday night into Saturday
morning which may require a small extension of the advisories in
certain areas, namely the LA mountains and the LA/Ventura valleys.

Temperatures will be on the mild side for this event as there is
some cold advection aloft. Daytime temps mostly 60s to lower 70s.
though some mid 70s by Fri/Sat as ridging aloft develops. In
areas without winds, like the Ojai Valley, Antelope Valley, and
interior SLO County, overnight temps will be quite cold, 20s and
30s.

.LONG TERM (SUN-WED)...25/153 PM.

Light offshore flow will continue through Monday while the ridge
aloft Sunday shifts east Sunday night and is replaced by a weak
trough Monday with essentially zero impact. Models coming into
better agreement for Tue/Wed showing another cold upper low
dropping into the Great Basin generating yet another moderate to
strong Santa Ana event. Most of the ECMWF ensembles show this with
potentially even stronger gradients and wind speeds than with the
event this week. Will see how models handle this going forward,
but very high confidence in this dry pattern likely extending well
into Dec with periods of offshore winds.

&&

.AVIATION...25/1804Z.

At 1730Z at KLAX...The inversion was 1600 ft deep. The top of the
inversion was around 2500 feet with a temperature of about 12
degrees Celsius.

Moderate to hi confidence overall in the 18Z TAFs. MVFR cigs
should linger at the start of the TAF period for KSMO, KLAX and
KLGB. Then there is a 60% chance of low clouds and IFR/MVFR cigs
at these airfields this evening to late tonight. Otherwise and
elsewhere, VFR conditions will prevail thru Thu morning. Gusty N
to N to NE winds are expected to develop Thu morning for KCMA,
KVNY and KPMD.

KLAX...Moderate to hi confidence in the 18Z TAF. IFR cigs are
expected 18Z-20Z, then there is a 60% chance of IFR cigs 01Z-04Z.
Cigs should improve to MVFR 04Z-14Z before offshore flow clears
the low clouds to VFR. The timing of the onset and dissipation of
any low clouds tonight may be off +/- an hour or two. Otherwise,
VFR conditions will prevail thru Thu afternoon. NE winds less than
8 kt are expected Thu morning.

KBUR...Hi confidence in the 18Z TAF with VFR conditions expected
thru Thu morning.

&&

.MARINE...25/151 PM.

Across the outer waters...Small Craft Advisory (SCA) level
conditions will prevail through late Thursday night. There is a
40% chance of Gale Force wind gusts tonight through early
Thursday morning. There is also a 50% chance of SCA level NE winds
for the southern waters on Friday. Conditions will be below SCA
level Friday night through Sunday, then there will be a 40%-50%
chance of SCA wind gusts at times on Monday.

Across the nearshore waters of the Central Coast... Winds will
increase to SCA level this afternoon and continue through early
Thursday morning. SCA-level swell will affect the area tonight
through Thu evening. Winds and seas should then be below SCA
levels late Thursday night through Monday.

Across the inner waters south of Point Conception... Gusty NE
winds to SCA levels will develop Thursday night and persist into
Fri morning, with a 50% chance of Gale Force wind gusts at times
S of Ventura out to near Santa Catalina Island for Thursday night.
Short period steep wind waves are expected to develop as well.
These hazardous conditions will affect Catalina Island, including
Avalon Harbor. Otherwise and elsewhere, conditions will be below
SCA levels through Monday.

&&

.BEACHES...25/151 PM.

A long-period northwest swell will bring high surf to the beaches
along the Central Coast through Thanksgiving Day. Surf will peak
tonight and Thursday morning at 12 to 15 feet with local sets to
18 feet along west to northwest facing beaches. Strong rip
currents are likely and beach erosion is possible. Minor coastal
flooding around the time of high tides will be possible.

Most of the swell energy will avoid the southern California
bight. Beaches south of Point Conception will have elevated surf
and a high risk of rip currents through Thanksgiving Day, but a
high surf advisory will not be needed for this area.

&&

.FIRE WEATHER...25/115 PM.

Forecast is on track. The Fire Weather Watches will be converted
to Red Flag Warnings. The areas under the warning are the same as
the watch, with the same 2pm Thursday start time, but the end time
is now 6pm Friday. This should match our San Diego office.

Moderate to locally strong Santa Ana winds will develop through
the day on Thursday, peak Thursday evening through Friday morning,
then gradually weaken into Saturday but stay locally breezy
through the weekend. Wind gusts between 40 and 65 mph are expected
over much of Los Angeles and Ventura Counties during the peak.
Meanwhile, humidities will increase tonight into Thursday morning,
then lower sharply Thursday afternoon and evening to 12 to 25
percent, lowering further to 5 to 15 percent for Friday through
Sunday. As a result, Red Flag conditions are expected to form
over much of Los Angeles and Ventura Counties by Thursday
afternoon or evening when the wind and drier air start to
coincide, then continue through Friday afternoon. There is a
50 percent chance for low-end Red Flag criteria to be met Friday
night through Saturday or Sunday over the mountains and windiest
valleys. There is a growing potential for moderate Santa Ana
winds and Red Flag conditions over Los Angeles and Ventura
Counties sometime Tuesday through Thursday.

We are urging the public to be extremely cautious with anything
that can start a fire. Some ignition sources are obvious
(campfires, welding equipment, cigarettes, fireworks) but others
are less so (lawn trimmers, dragging trailer chains, firearms).
Those living in wildland interfaces and areas vulnerable to
wildfire should have a plan and be ready to evacuate if a fire
starts.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 3 AM PST Friday for zones
      34-35. (See LAXCFWLOX).
     Wind Advisory in effect from 10 AM Thursday to 3 PM PST
      Friday for zones 40-41. (See LAXNPWLOX).
     Wind Advisory in effect from 7 AM to 10 PM PST Thursday for
      zones 44>46-88-547. (See LAXNPWLOX).
     High Wind Watch in effect from Thursday evening through
      Friday afternoon for zones 44>46-53-54-88-547. (See LAXNPWLOX).
     Wind Advisory in effect from 7 AM to 6 PM PST Thursday for
      zones 53-54. (See LAXNPWLOX).
     Red Flag Warning in effect from 2 PM Thursday to 6 PM PST
      Friday for zones 240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Thursday for
      zone 645. (See LAXMWWLOX).
     Gale Watch in effect from Thursday evening through late
      Thursday night for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until midnight PST Thursday
      night for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 PM PST Thursday for
      zone 673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Friday for zone
      676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SAT-WED).

An extended dry and warmer pattern is expected Saturday through
Monday with persistent elevated fire weather conditions. Another
strong Santa Ana wind event is possible next Tuesday and
Wednesday.

&&

PUBLIC...MW
AVIATION...Sirard
MARINE...Sirard
BEACHES...Sirard
FIRE...Kittell
SYNOPSIS...MW

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