Gale Warning 40, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
311 AM PDT Sun Oct 25 2020

.SYNOPSIS...25/310 AM.

Temperatures will remain cool through the weekend with mostly cloudy
skies and patchy drizzle or light rain across Los Angeles and
Ventura Counties through this morning. Strong and gusty Santa Ana
winds will develop tonight and Monday and possibly remain fairly
strong across the inland areas on 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)...25/307 AM.

Latest GOES-W satellite imagery indicated cloudy skies just about
everywhere, except for the Antelope Valley with clear skies. There
have been a few reports of light rain across the foothills of L.A.
and Ventura County with only a hundredth or two reported. As the
morning progresses, the best chance of any light precip will be
limited to L.A. County, mainly in the foothills but a few drops
could occur across the coast and valleys. There will be gusty SW
to W winds across the Antelope Valley today, locally reaching 45
to 50 mph around Lake Palmdale and adjacent foothills. Otherwise,
winds should be more widespread around 30-35 mph remaining below
advisory thresholds. High temps will bottom out today 2-6 degrees
cooler than yesterday, and 10-15 degrees across the mountains.

Synoptically, a relatively weak upper disturbance within the broad
troughy pattern will pivot farther south into Nrn Baja by this afternoon.
the vort-max associated with the disturbance will remain south of the
area, therefore the slight chance of rain is just that. Maybe a few
isolated light showers will occur across the coastal slopes of the
L.A. County Mountains this morning. Another much colder 549 DM upper
low over the Great Basin will continue to retrograde towards southern
Nevada and SE California by this evening. There is an outside chance
for a few light showers possible across the interior portions of
L.A. County as H5 heights crash, but models are not picking up on
much of any moisture due inland track of the low.

As the upper low moves into Utah/Nrn AZ Monday morning, cold air will
settle into the Great Basin including Nevada which will set up a strong
offshore gradient. The NAM-WRF is now showing the LAX- DAG gradient
at -7.4MB which is over a 10 mb trend from 24-hours earlier. With the
upper low expected to be over southern Arizona near the Calif border,
the upper level support will cause this Santa Ana wind event to be
quite strong. Initially, winds will develop Sunday night and strengthen
through Monday morning and continue to remain strong through the afternoon,
due to the synoptic pattern. Surface gradients do weaken on Tuesday,
but upper support will remain over the eastern portion of L.A. County
allowing for continued gusty offshore winds, especially across the
higher terrain of the San Gabriel Mountains.

The tough part of the forecast will be Monday night into Tue.
Much of the wind focus will shift over L.A. County with lighter
offshore winds expected over Ventura County. The San Gabriel
Mountains continue to look like the main focus of very strong
damaging winds late Monday afternoon into Tuesday. Therefore the
High Wind Warnings have been extended into Tue afternoon for not
only the L.A. County Mountains, but also Santa Clarita, San
Fernando Valley and the Santa Monica Mountains. Gusts are expected
to range from 60 to 70 mph, with isolated gusts to 80 mph across
the San Gabriel Mtns. The wind prone coastal areas of L.A. and
Ventura Counties will experience gusts between 40-50 mph.

The high resolution NAM-WRF model has remained consistent in
showing a secondary burst of strong N-NE winds across the San
Gabriel Mountains Monday late afternoon through Tuesday. Although
the San Gabriel Valley is usually protected to the gusty NE winds,
the combination of the colder air and some of the downward motion
with these winds are concerning for the potential of a mountain
wave to develop across the foothills of San Gabriel Valley and
possibly reaching the valley floor around Pasadena to Diamond
Bar. Mountain waves are difficult to forecast, but the potential
is there so a High Wind Watch will go into effect Monday afternoon
through Monday evening. Today's shift will look at this closer to
see if the watch needs to be upgraded to a warning. Advisory
level winds will affect the wind prone foothills starting late
tonight and likely continue through Tuesday morning. Wind
advisories have also been issued for coastal areas including
Catalina Island. Mariners should avoid Avalon Harbor and other
east facing harbors during the gusty easterly winds Monday into
Tuesday.

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 has been issued for that area. Other
areas like interior SLO county and the Ojai Valley may need frost
advisories early next week.

.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.

&&

.AVIATION...25/0538Z.

At 0505Z 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. There is a 20 percent chc of MVFR
cigs 10Z-15Z at sites with only VFR cigs fcst. There is a 30
percent chc of SCT conds 21Z-01Z at sites that have BKN-OVC conds
forecast.

KLAX...Moderate confidence in TAF. There is a 20 percent chc of
OVC030 10Z-15Z. There is a 30 percent chc of SCT conds 21Z-01Z.
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 09Z-15Z. There is a 20 percent chc of SCT conds 21Z-01Z.

&&

.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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 11 PM this evening to 3 PM PDT
      Monday for zone 40. (See LAXNPWLOX).
     Wind Advisory in effect from 11 PM this evening to noon PDT
      Tuesday for zones 41-87. (See LAXNPWLOX).
     High Wind Warning in effect from 11 PM this evening to 3 PM
      PDT Monday for zones 44-45-53. (See LAXNPWLOX).
     High Wind Warning in effect from 11 PM this evening to noon
      PDT Tuesday for zones 46-88-547. (See LAXNPWLOX).
     High Wind Warning in effect from 11 PM this evening to 3 PM
      PDT Tuesday for zone 54. (See LAXNPWLOX).
     Freeze Watch in effect from late tonight 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 tonight to 6 PM PDT
      Tuesday for zones 244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Gale Warning in effect from 3 AM to 3 PM PDT Monday for zones
      650-655-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

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.

&&

PUBLIC...Kaplan/MW
AVIATION...Rorke
MARINE...Sirard
FIRE...Gomberg
SYNOPSIS...Kaplan

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