Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 250005

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
505 PM PDT Sat Oct 24 2020

.SYNOPSIS...24/133 PM.

Temperatures will remain cool through the weekend with mostly cloudy
skies and patchy drizzle through tonight. There is a chance of
drizzle or light rain across Los Angeles and Ventura Counties
tonight and Sunday then strong and gusty Santa Ana winds will
develop Sunday night and Monday and then weaker but still gusty
into Tuesday. Below normal temperatures expected through at least
Monday then warming up through the end of next week. Freezing
temperatures during the early morning hours possible across the
Antelope Valley beginning Monday.


.SHORT TERM (TDY-TUE)...24/137 PM.

Most of the area still mired in a deep marine layer this
afternoon and below normal temperatures. Clouds have cleared
across inland areas but most of the coast and coastal valleys are
still cloudy. Models continue to advertise a weak upper low
passing just west and south of the area tonight into Sunday
morning which will maintain or even deepen the already deep marine
layer and possibly help squeeze out some drizzle or light rain
across parts of LA/Ventura Counties. Amounts were never that
significant with this event but with the low now tracking a little
farther south amounts now are almost negligible and many areas
will likely see little to no precip. Foothills and south facing
mountain slopes have the best chances but even there amounts
likely not exceeding a tenth to a quarter inch.

As the inversion deepens it will weaken and this should help to
dissipate the clouds more than the last couple days so there`s a
decent chance for at least some sunshine most areas Sunday
afternoon, though temps still on the cool side.

After that we rapidly transition to a Santa Ana wind event that
will focus over LA/Ventura Counties. 12z models today showed
little change from earlier solutions which suggested a moderate to
strong wind event Monday with critical fire weather conditions.
Still somewhat lower confidence in the timing at lower elevations
as models typically are a few hours too fast at pushing the winds
down to the coast. Otherwise, by mid-morning Monday a large part of
LA/Ventura counties will be feeling gusty northeast winds and
rapidly dropping humidities. And because there is good cold
advection with this event winds will push all the way out over the
coastal waters, including the Channel Islands and Catalina. Even
parts of the San Gabriel Valley will likely get some gusty winds.

This will be a much colder event than earlier Santa Ana events so
daytime temps will mostly be in the 60s and 70s for the initial
couple days. Also, with that dry, colder air coming in places that
are protected from the winds during the overnight hours such as
the Antelope Valley will experience their first freeze of the
season. So a freeze watch will be issued for that area. Other
areas like interior SLO county and the Ojai Valley may need frost
advisories early next week.

Winds still look weaker Tuesday though the NAM still has a -5.8
LAX-DAG gradient Tuesday morning with 50kt winds aloft. The
decrease in gradients and support aloft is much more significant
with the GFS/EC solutions and the ensembles are suggesting at
least a 40% reduction in wind speeds from Monday. So it`s more
likely that we`ll see winds more in the advisory category Tuesday
but still gusty and still extremely dry, in fact likely drier than
Monday. So very critical fire weather conditions are likely again
Tuesday despite the lighter winds. Daytime temperatures also
expected to be a few degrees warmer as some of the cold air exits
but still topping out mostly in the 70s.

.LONG TERM (WED-SAT)...24/130 PM.

Light offshore flow expected to continue right on through the
remainder of next week. This should keep the marine layer away
from the coast but minimal winds expected and a warming trend in
most areas except for the immediate coast which will level off as
gradients trend onshore. Still quite cool at night with the dry
air and freeze warnings/frost advisories may be needed for some
interior areas for a few more mornings but warming up to the 70s
and 80s in the afternoon.



At 2312Z at KLAX, the marine layer was 4500 feet deep. The top of
the inversion was 5600 feet with a temp of 14 degrees Celsius.

Moderate confidence in TAFs. VFR cig to MVFR cig transitions may
be off by as much as two hours. There is a 30 percent chc that
sites with MVFR cigs forceast will remain VFR and a 20 percent
chc that sites will only VFR cigs forecast will have MVFR cigs
08Z- 15Z. There is a 20 percent chc of SCT conds at any TAF site
05Z- 16Z.

KLAX...Moderate confidence in TAF. MVFR cigs may arrive as late as
09Z. There is a 30 percent chc that CIGs will remain between 035
and 040. There is a 20 percent chc of SCT conds 01Z-16Z. High
confidence that there will be no east wind component greater than
3 KT.

KBUR...Moderate confidence in TAF. There is a 20 percent chc of
MVFR cigs 08Z-15Z. There is a 20 percent chc of SCT conds 05Z-


.MARINE...24/157 PM.

High confidence that winds and seas will stay below Small Craft
Advisory (SCA) levels over all coastal waters thru at least Sun.

There is high confidence that a moderate to strong Santa Ana wind
event will affect the inner coastal waters and possibly the
southern outer coastal waters late Sun night thru Mon. This is a
cool Santa Ana wind event, which tends to affect more of the
waters than a typical event,

There is a 50%-60% chance of Gale Force NE winds developing late
Sun night and continuing thru Mon from Ventura to Santa Monica and
out to Anacapa and Santa Cruz Island, thru the San Pedro Channel
and out to Catalina Island, and across portions of the southern
outer waters zone (PZZ676). A Gale Watch has been issued for
these areas for late Sun night thru Mon afternoon. There will be
a 20%-30% chance SCA level wind gusts may continue in some of
these areas Mon night into Tue morning.

The strong and gusty winds and short period and steep NE wind
waves will likely make for dangerous conditions for the harbors
on Catalina and Santa Cruz Islands, especially Mon. Take
precautions now. Northeast wind gusts of 25 to 35 knots with gusts
to 40 knots are possible, with the potential for steep waves
between 4 and 6 feet.

There also is a 40%-50% chance of SCA level NW winds over parts
of the central outer waters Mon night into early Tue. Otherwise
SCA winds and seas are not expected across the coastal waters Tue
thru Thu.


.FIRE WEATHER...24/344 PM.

Late Sunday night into Monday, a cold upper level trough of low
pressure will drop into the Great Basin bringing a moderate to
strong Santa Ana wind event across Los Angeles and Ventura
counties. The strongest winds are expected late Sunday night into
mid afternoon Monday, when damaging gusts of 60 to 80 mph can be
expected in the mountains, gusts of 50 to 60 mph in the wind prone
valleys, and gusts of 35 to 50 mph across the Ventura coast and
Malibu to Hollywood Hills. The downslope winds will bring rapid
drying, with humidities falling into the teens late Sunday night
and early Monday morning, with widespread single digit humidities
from late Monday morning through Tuesday.

The combination of a strong offshore pressure gradient, strong upper level
wind support, and significant cold air advection will likely bring the
strongest and most widespread Santa Ana wind event we have seen so
far this season. The moderate to strong Santa Ana winds coupled with
very low humidities and very dry fuels will likely bring widespread critical
fire weather conditions across Los Angeles and Ventura counties late Sunday
night through Tuesday, resulting in the issuance of the Red Flag Warning.
The very dry air with single digit humidities is expected to impact interior
portions of San Luis Obispo and Santa Barbara counties by Tuesday. New fire
ignitions in Los Angeles and Ventura counties will have the potential for
rapid fire growth, extreme fire behavior, and long range spotting, resulting
in a threat to life and property. Light offshore flow and very low humidities
are expected the rest of next week but Monday and Tuesday are the primary
concerns for fire weather.


CA...High Wind Warning in effect from midnight Sunday night to 3
      PM PDT Monday for zones 44>46-53-54-88-547. (See LAXNPWLOX).
     Freeze Watch in effect from late Sunday night through Monday
      morning for zone 59. (See LAXNPWLOX).
     Red Flag Warning in effect from 5 AM Monday to 6 PM PDT
      Tuesday for zones 240-241. (See LAXRFWLOX).
     Red Flag Warning in effect from midnight Sunday night to 6 PM
      PDT Tuesday for zones 244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Gale Watch in effect from late Sunday night through Monday
      afternoon for zones 650-655-676. (See LAXMWWLOX).



Gusty Santa Ana winds are expected Tuesday creating critical fire
weather conditions. Very cold overnight conditions are expected
through at least Wednesday over the interior areas, with frost
and freeze concerns likely.



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