Wind Advisory 78, NWS Los Angeles, CA

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
903 AM PDT Tue Oct 29 2019

.SYNOPSIS...29/857 AM.

It will be sunny today with cooler than normal temperatures.
Strong Santa Ana winds are expected for LA and Ventura Counties
early Wednesday through Thursday. Temperatures will remain cool
through the week, especially across the far eastern sections.

&&

.SHORT TERM (TDY-THU)...29/851 AM.

***UPDATE***

Have ended the frost advisory that was in effect until 9am for
the Antelope Valley and interior San Luis Obispo valley as
temperatures have sufficiently warmed. The cold air will return
this evening with the Santa Ana and there is a fairly widespread
freeze watch associated with that. Will be transitioning the High
Wind Watch to a Warning this morning as it is close enough in time.
No other impactful changes from the previous discussion for the
short term forecast.

***From Previous Discussion***

Not much going on at the moment. Skies are clear and it seems
likely that the low clouds will stay away. Winds are down and
humidity is up which is assuredly good news for the firefighters.

A dry cool atmosphere will allow min temps to fall into the lower
to mid 30s across the wind sheltered locations in the interior
SLO vly and the Antelope Vly these areas are under a frost
advisory through 900 am.

Not too much to say about today. The state is covered by the base
of a pos tilt conus spanning trof. Hgts are a lower than normal
575 DM. Max temps will be 4 to 8 degrees blo normal today under
sunny skies.

All eyes are on the major Santa Ana wind event that will start
late this evening. The trof axis will rotate through the state and
will bring another round of cold air advection to Srn CA as well
as turning the upper level flow to the east. At the same time a
cold dome of sfc high pressure will build into the inter mtn
region and will kick off a remarkable reversal of the easterly
pressure gradients. There KDAG grad will go from near zero all the
way to -10 MB From 5am today to 5am tomorrow. Mdls have been
consistent forecasting this gradient to come in somewhere between
-9.5 MB and -11 MB which will likely break the Late Oct/Early Nov
record of -10.1 MB.

Thus all three elements of a strong Santa Ana (Offshore flow,
Upper level support, Cold air advection) are in place.
Additionally most of these elements are also very strong and all
of this will add up to be high end dangerous event. If everything
comes together as forecast there will be 60 mph to 70 mph gusts
not only in the mtns but in some of the vlys as well. There could
well be 80 mph gusts in the mtns. These winds will result in a
significant threat of large fast-moving fires and wind damage
(Downed trees, power lines, and broken signs or overhangs). Power
outages are likely. Wind-driven wave damage is possible over
Catalina Island including Avalon Harbor. High Wind Watches and Red
Flag Warnings have been posted to cover this remarkable and
dangerous event.

Typical of Day 1 Cold Santa Ana event, temperatures across the
interior will fall due to the cold air advection. The Antelope Vly
will only see highs in the upper 50s or about 15 degrees blo
normal. Milder temps mostly in the mid 70s will occur across the
csts and vlys as the cold air is modified by downsloping
compressional warming.

This will not be a one day event...the winds will power through
the afternoon and continue only slightly diminished in the evening.
All the cold dry air will allow for rapid cooling in wind sheltered
areas and more than a few areas across the interior will see a
hard freeze.

There will be 2 MB onshore trends on Thursday and much less upper
support and no cold air advection. The winds will not be as strong
as Wednesday but will still likely produce warning level wind gusts
in the mtns and strong advisory level gusts across most of the
coasts and vlys of LA and VTA counties. Unlike Wednesday there
will be a significant drop off in winds during the

Since the cold air advection will no longer exist all areas will
see about 4 to 8 degrees of warming.

Decent weather for trick or treating Thursday evening with
temperatures generally between 53 and 63 degrees under clear
skies. Winds will be much lighter and will not pose much of any
problems.

Thursday night will be the coldest of next 7. Freeze Watches are
in effect as a result, with danger to vegetation and exposed water
lines. Some wind sheltered coastal valleys will also drop to near
freezing, including the Santa Ynez and Ojai Valleys. Frost
Advisories or Freeze warnings will probably be needed.

.LONG TERM (FRI-MON)...29/308 AM.

Weak high pressure then builds in over the area and remains in
some form through the four day extended period. Weak offshore flow
remains in place and will keep the marine layer away through at
least Sunday. From Monday on there will be an increasing chance of
low clouds each morning. Max temps will come within a few degrees
of normal each day.

Neither the EC nor the GFS has any hint of another offshore event
in the day 4 to 10 period.

&&

.AVIATION...29/1201Z.

At 1129Z at KLAX, there was no marine layer. There was a weak
surface based inversion with a top of 1300 ft with a temperature
of 16 C.

High confidence in 12Z CAVU TAFs. There is a chance of turbulence
and LLWS over higher terrain once gusty northeast winds arrive
tonight.

KLAX...High confidence in 12Z TAF. There is a 10% chance of an
east wind component exceeding 8 kt through 18Z.

KBUR...High confidence in 12Z TAF.

&&

.MARINE...29/334 AM.

For all of the coastal waters, good confidence that winds will
remain below Small Craft Advisory (SCA) level today. There is a
40% chance of localized SCA level gusts across the waters from
San Nicolas Island northward towards the Channel Islands this
afternoon and evening.

For tonight through Thursday, there is moderate confidence in the
forecast. Gusty northeast winds are expected to impact the
nearshore waters from Ventura to Malibu and out towards the
Channel Islands, as well as through the San Pedro Channel out
towards Catalina Island late tonight through Thursday afternoon.
Gale force gusts are likely nearshore from Ventura to Malibu on
Wednesday, but confidence is lower whether they will reach the
waters farther offshore. Also low confidence whether Gales will
continue into Wednesday night and early Thursday, so have left the
Gale Watch effective through Wednesday afternoon. Otherwise, SCA
level winds are likely late tonight through Thursday afternoon
over these waters. Good confidence that SCA level winds will
impact Avalon Harbor and Two Harbors on Catalina Island.

After Thursday and through the weekend, high confidence that
winds and seas will remain below SCA thresholds.

&&

.FIRE WEATHER...29/902 AM.

Onshore winds continue today (Tuesday), but still confidence that
these winds will rapidly shift to offshore tonight. A strong Santa
Ana wind event is still on target for tonight through Thursday, with
one of the strongest pressure gradients and moderate upper level
support. Damaging wind gusts between 50 and 70 mph are expected over
most of Los Angeles and Ventura Counties, with isolated gusts to
around 80 mph likely. Santa Barbara and San Luis Obispo Counties will
also see localized east wind gusts between 20 and 40 mph. Although
the air is cold, humidities will lower to the single digits nearly
everywhere by Wednesday or Thursday, and down to 1 or 2 percent in
the driest windiest locations. Overnight recoveries Wednesday night
will be near zero over Los Angeles and Ventura Counties, and poor
everywhere else. This all adds up to an extreme fire weather
threat. A Red Flag Warning is on course for 11pm tonight through
6pm Thursday for Los Angeles and Ventura Counties. Santa Barbara
and San Luis Obispo Counties will be elevated to briefly critical,
but not planning on needed a Red Flag Warning there.

Very dry conditions will persist through the week, and could continue
into Monday. The offshore flow will weaken quickly Thursday
afternoon into the evening, but could stay breezy in some mountain
and foothill spots into Friday leading to the possibility of localized
critical conditions and the extension of the Red Flag Warning for
one or two spots. There is NO indication of another Santa Ana
next week.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Freeze Watch in effect from late tonight through Friday
      morning for zone 37. (See LAXNPWLOX).
     High Wind Watch in effect from this evening through Thursday
      morning for zones 40-44>46-53-54-88-547. (See LAXNPWLOX).
     Hard Freeze Watch in effect from late tonight through Friday
      morning for zone 59. (See LAXNPWLOX).
     Red Flag Warning in effect from 11 PM this evening to 6 PM
      PDT Thursday for zones 240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Gale Warning in effect from 2 AM to 5 PM PDT Wednesday for
      zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

Moderate to strong Santa Ana winds will contine through Thursday.
These winds pose a very high risk for dangerous fire weather
conditions along with downed trees and powerlines, and hazardous
nearshore boating conditions.

&&

PUBLIC...Rorke/jld
AVIATION...Stewart
MARINE...Stewart
FIRE...Kittell
SYNOPSIS...30

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