High Wind Warning 16, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1146 AM PDT Fri Oct 27 2023

.SYNOPSIS...27/805 AM.

Mild temperatures through Saturday with areas of gusty northwest
to north winds. Daytime warming and drying Sunday through
Wednesday, with moderate to strong Santa Ana winds likely across
Los Angeles and Ventura Counties with fire weather concerns.
Interior valleys are expected to see freezing temperatures each
morning Sunday through Wednesday.

&&

.SHORT TERM (TDY-SUN)...27/818 AM.

***UPDATE***

Low clouds are making a late surge into Los Angeles County from
the southeast and look poised to push into parts of Ventura County
as well. Updated the forecast to account for that. These clouds
should clear out by this afternoon. Otherwise the forecast looks
good for today, with low-end Wind Advisory level northwest winds
over southwest Santa Barbara County and the I-5 Corridor the only
concern. The challenge for today will be the handling of the winds
for Saturday through Tuesday, and what to do with the High Wind
Watch. The current forecast remains on track in general. At this
point, the High Wind Watch period looks to be more of a strong
Wind Advisory with the appreciable probabilities of gusts over 60
mph fairly localized. Will assess further today. That is a small
nuanced detail however. Regardless, this is a high confidence
very significant event for Saturday Night through Monday,
especially from a Fire Weather standpoint. Residents in the Fire
Weather Watch area, especially those in high fire danger locales,
need to get ready now in case a fire starts or power loss
materializes.

***From Previous Discussion***

Skies were mostly clear across much of the region, except random
patches of low clouds have developed, especially across L.A.
County and eastern Ventura County. Clouds may increase a bit more
in coverage by daybreak. However, with a very weak inversion in
place, and with weakening onshore gradients between KLAX and KDAG,
and offshore N-S gradients, expect any low clouds to clear by
mid or possibly late morning. There will be some high clouds
later today, possibly enough to go with a partly cloudy forecast
of the afternoon, especially across SLO and SBA Counties.

A broad trough was in place across the western United States this
morning and it will deepen some today. Although heights will fall
a bit, decreasing W-E onshore flow and increasingly offshore N-S
gradients should offset cooling west of the mountains, so expect
max temps today to be similar to those on Thu in most areas.

N-S gradients and some upper level support should allow NW to N
winds to reach at least low end advisory levels across the
southwestern coast of SBA County, the western Santa Ynez
Mountains, and the I-5 Corridor late this afternoon or evening,
continuing through early Sat morning.

A rather strong and positively-tilted short wave trough will drop
southward through Idaho tonight, then into northeastern Nevada
and Utah on Sat, with the trough axis extending southwestward
into CA. Heights and thicknesses will drop tonight and Sat, and W
to NW flow aloft will increase. With so much cooling aloft, and
the northerly flow, there is some question as to whether there
will be much in the way of low clouds tonight and Sat morning. At
this point, the high res models and some short term ensembles are
indicating some stratus late tonight/Sat morning across coastal
sections of L.A. County and the San Gabriel Valley, and southern
portions of the Central Coast and Santa Ynez Valley. Any low
clouds should clear rather quickly on Sat. There will be some
gusty NW-N winds Sat afternoon and evening, probably strong
enough to require advisories in some areas, especially across
portions of southern SBA County, and through the I-5 Corridor.
Max temps should change little in most areas of Sat.

The upper trough will dig southward Sat night and Sun, with the
trough axis losing its negative tilt, and leaving strong
northerly flow aloft across the region. Wind advisories may again
be needed for much of southern SBA County late Sat afternoon
through late Sat evening.

Low level flow will turn northeasterly late Sat night and Sun,
with gradients between KLAX and KDAG getting to between 6 and 7 mb
offshore Sun morning. With the good offshore gradients, and
strong upper level support in the form of strong north to
northeast winds aloft, good subsidence and some cold air
advection, the ingredients seem to be coming together for a
moderate to strong Santa Ana Wind event across the region. A High
Wind Watch was already in effect for the L.A. County mountains.
Have extended the High Wind Watch to the southern mountains of
Ventura County, the Ventura County Valleys (mainly for eastern
sections), the Santa Clarita Valley, the San Fernando Valley
(mainly for northern and western sections), and the Santa Monica
Mountains. This watch is in effect from late Sat evening through
Mon. Wind advisories will be needed for coastal sections of
Ventura County and for the Malibu Coast, and there could be an
argument for a High Wind Watch on the Malibu Coast. Wind gusts
could reach 60 mph in the areas covered by the Watch, and would
not be surprised if there are some gusts to 70 mph in the windiest
locations Sun morning.

Where it is not windy, it will be very chilly Sat night, with
temps possibly dropping to freezing in the Antelope Valley and the
interior valleys of SLO and SBA Counties. Max temps on Sun will be
lower in the Antelope Valle and in the mountains due to the cold
advection. However, west of the mountains, NE flow will warm as
it descends, leading to some warming, especially across the
coastal plain. Max temps could exceed 80 degrees across coastal
sections of Ventura and L.A. Counties on Sun.

It will become very dry across the region late Sat night and
especially Sun, with widespread relative humidity values in the 5
to 10 percent range.

.LONG TERM (MON-THU)...27/511 AM.

The trough will begin to move eastward Sun night and Mon, and a
highly amplified ridge in the eastern Pacific will push into the
West Coast. There will still be some decent northerly winds aloft
across the region Sun night and Mon morning, but upper support
will weaken a bit as flow aloft gradually decreases and cold air
advection subsides. However, low level offshore gradients will
across be a bit steeper Sun night and Mon morning, so the
potential for High Wind level gusts to 60 mph will continue, at
least through the early Mon afternoon. Where it is not windy, it
will be very cold Sun night, with minimum temps near or possibly
even a few degrees below freezing in portions of the Antelope
Valley and the interior valleys of SLO and SBA Counties. On Mon,
rising heights, plenty of sunshine, offshore flow and weakening
cold advection should lead to warming everywhere, with the best
warming in coastal and valley areas. Max temps should climb above
80 degrees in many coastal and lower valley areas on Mon.

Beyond Monday, the deterministic runs of the GFS and EC show some
differences, with the EC maintaining a strong ridge across the
area Tue through Thu, briefly flattening out a bit on Tue as a
weak trough moves briefly into the West Coast to the north of the
region. The GFS also keeps the ridge across the region on Tue,
albeit a bit weaker, then shows a weak trough dropping through CA
Wed, evolving into a weak upper to the southwest of the forecast
area Thu. However, both models show high heights across the
region. In addition, both show offshore flow continuing Tue
through early Thu, with locally gusty winds especially Tue. Max
temps will likely rise in most areas on Tue, then depending upon
the model of choice, there will be slight cooling or little change
on Wed. It will again be very chilly both Mon night and Tue
night, with freezing or sub- freezing temps a good bet in the
Antelope Valley and in the portions of the interior valleys of SLO
and SBA Counties. For Thu, weakening offshore flow may bring some
slight cooling, especially to the coastal plain, and some low
clouds may return to coastal areas by Wed night/Thu.

&&

.AVIATION...27/1845Z.

At 1804Z at KLAX, the moist layer was 3900 feet deep with the
inversion top at 4600 feet and 11 degrees C.

Moderate confidence in TAF package. LA county coastal sections
are the main focus for a return of low clouds tonight, although
there is a 30 percent chance of MVFR CIGS pushing into the LA
valleys as well. Elsewhere, VFR conditions are expected through
the period.

KLAX...Moderate confidence in TAF. There is a 30 percent chance of
CIGS arriving plus or minus two hours from the forecast arrival
time. Light east winds under 5 kts are expected after 07Z
Saturday, but high confidence that any east wind component will
not reach 8 kts.

KBUR...Moderate confidence in TAF. There is a 30% chance of MVFR
CIGS late tonight into Sat morning.

&&

.MARINE...27/924 AM.

For the Outer Waters, moderate to high confidence in current
forecast. Now through Saturday night, high confidence in
widespread Small Craft Advisory (SCA) level winds and choppy seas
with a 20% chance of Gale force winds this afternoon and Saturday
afternoon. For Sunday through Tuesday, winds and seas are
generally expected to remain below SCA levels except there is a
50-60% chance of SCA level northeast winds on Sunday across
PZZ676.

For the Inner Waters north of Point Sal, high confidence in
current forecast. SCA level winds are expected until late tonight
and then during the afternoon and evening Saturday. For Sunday
through Tuesday, winds and seas are expected to remain below SCA
levels.

For the Inner Waters south of Point Conception, moderate
confidence in current forecast. Starting this afternoon lasting
through Saturday there is a 60-70% chance of SCA level winds
across the western half of the Santa Barbara Channel with high
confidence in winds and seas remaining below SCA levels elsewhere.
Late Saturday night through Monday afternoon, there is a 60-70%
chance of SCA level northeast winds from Ventura southward with
winds around 20 kt potentially impacting Catalina Island. Winds
will be the strongest on Sunday and in the morning hours. For
Tuesday, winds and seas are expected to remain below SCA levels
for all the southern Inner Waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Watch in effect from Saturday evening through
      Monday afternoon for zones 88-358-369>372-374>376-378. (See LAXNPWLOX).
     Wind Advisory in effect from 5 PM this afternoon to 9 AM PDT
      Saturday for zones 349-351-378. (See LAXNPWLOX).
     High Wind Watch in effect from Saturday evening through
      Monday evening for zones 379-380. (See LAXNPWLOX).
     Fire Weather Watch in effect from late Saturday night through
      Monday evening for zones 288-354-355-358-362-369>380-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Saturday for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Saturday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Sunday for
      zones 670-673-676. (See LAXMWWLOX).

&&

PUBLIC...DB/Kittell
AVIATION...Sweet/Schoenfeld
MARINE...Sweet/Schoenfeld
SYNOPSIS...RK/Schoenfeld

weather.gov/losangeles

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