Duarte Weather

000
FXUS66 KLOX 220409
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
909 PM PDT Mon Oct 21 2019

.SYNOPSIS...21/845 PM.

A weak to locally moderate Santa Ana wind event will affect
portions of Ventura and Los Angeles counties through at least
Tuesday, bringing hot and dry conditions to the region. After
some relief on Wednesday, a stronger wind event will develop
Thursday and Friday. Weekend temperatures will be cooler.

&&

.SHORT TERM (MON-THU)...21/903 PM.

*** Dangerous fire weather conditions possible Thursday into Friday
    due to potential for moderate to strong Santa Ana winds, single
    digit humidities, hot temperatures, and very dry fuels ***

This will likely be one of our most critical weeks of the fall
season for fire weather due to the hot temperatures, low
humidities, and periods of Santa Ana winds. Weak to locally
moderate Santa Ana winds will continue through Wednesday,
reinforcing the hot and dry conditions across the Southland.
Weak Santa Ana wind conditions earlier today, with gusts generally
in the 25 to 35 mph range across wind prone passes and canyons
of LA/Ventura counties. For Tuesday, there will be a slight bump
up in offshore pressure gradients and upper level wind support,
resulting in the low-end wind advisories issued for the Santa
Clarita Valley and Ventura County valleys, where wind gusts
will generally be in the 30 to 40 mph range. The LAX-Daggett
gradient peaked at -4.5 mb this morning, and is expected to
be around -5.5 mb on Tuesday morning.

The other short term concern in addition to the Santa Ana winds
will be the hot temperatures. Some of the warmest locations today
were Long Beach at 96 degrees, and Camarillo/Filmore at 95
degrees. With some additional rises in heights and boundary
layer temperatures, we are expecting high temperatures to
be a few degrees warmer in most locations, meaning warmest
coastal/valley areas will range between 95 and 100 degrees
on Tuesday. Moderate heat risk can be expected on Tuesday during
daytime hours, and later shifts may need to consider issuing a heat
advisory for this late season heat, especially for coastal
areas where some areas do not have air conditioning.
Offshore flow weakens slightly for Wednesday, but still
expecting widespread temperatures into the 90s. The offshore
winds and warm air mass will also maintain very warm overnight
temperatures in the foothills and breezy canyon locations.

The combination of weak to locally moderate Santa Ana winds,
hot temperatures, and low humidities will maintain elevated to
brief critical fire weather conditions through Wednesday. Then
all eyes turn to Thursday/Friday time period when there is the
threat for dangerous fire weather conditions due to the potential
for moderate to strong Santa Ana winds, single digit humidities,
hot temperatures, and very dry fuels. Fire Weather Watches have
already been issued for much of Los Angeles and Ventura counties.
Please see fire discussion below for more details.

*** From previous discussion ***

Heights are suppressed to the west slightly on Wednesday as an
upper trough carves southward into the northern Rockies. 950mb
temperature though are just as warm, so it will be another warm
start to the day and a hot afternoon. LAX-DAG pressure gradients
indicate at least 1 mb less offshore for Wednesday, however, that
will still result in northeast winds 10-20 mph with local gusts to
30 mph for the eastern Ventura and western LA County areas.
Temperatures should once again peak in the 90s again for valleys
and many coastal areas...possibly a little cooler due to return
onshore winds for part of the Oxnard Plain and immediate western
LA coast.

Main headlines for Wednesday night to Thursday will be a
significant Santa Ana wind event on the heels of a hot and dry
period leading up to it. There continues to be small differences
between GFS and ECM/NAM model runs, however, ECM ensembles remain
consistent with a moderate to possibly strong Santa Ana. Since the
warm air mass already established will then have downsloping
winds on top of it, there doesn`t appear to be enough cold air to
impact temperatures on this side of the mountains. Would
anticipate winds increasing over the mountains and foothills late
Wed night, then making it to the ocean early Thu morning. Although
not as strong, northeast winds will occur along the SLO County
coast as well. Humidities are likely to be widespread in the
single digits (see fire weather discussion below) for critical
fire weather conditions in Ventura/LA Counties. Winds will
decrease Thursday night into Friday, but it will remain very warm
and dry through Friday afternoon.

.LONG TERM (FRI-MON)...21/204 PM.

As described above, Friday will be on the tail end of the Santa
Ana event but temperatures will remain hot due to a large high
pressure center directly overhead. Dry conditions will remain
also for continued elevated to critical fire weather conditions.
Saturday looks like a transition day as the high pressure begins
to break down and heights fall. Onshore winds will bring in cooler
and higher humidity for many valleys and coasts during the start
of the weekend. From Sunday to Monday, another large upper level
trough over the northern Rockies rotates southward for continued
cooling. Yesterday it was looking like the potential for light
rain, but newer model data suggests the trough will remain too far
inland. Will keep mention of a little rain for the foothills for
now, however, it`s possible we`re just looking at another inside
slider that only brings additional wind to southwest California
early next week.

&&

.AVIATION...21/2354Z.

At 23Z, there was no marine layer depth at KLAX.

High confidence in the current forecast. VFR conditions are
expected through the period. Periods of moderate wind shear and
turbulence are possible at all terminals through 16Z.

KLAX...Any east winds after 05Z will likely be less than 7
knots. Otherwise, VFR conditions are expected through the period.

KBUR...There is a 30 percent chance of moderate wind shear and
turbulence through 14Z. Otherwise, VFR conditions are expected
through the period.

&&

.MARINE...21/844 PM.

For the Outer Waters... There is a forty percent chance of Small
Craft Advisory (SCA) level winds on Wednesday, otherwise
conditions will remain below SCA levels through the forecast
period.

For the Inner Waters north of Point Sal... Conditions will remain
below SCA levels through the forecast period.

For the Inner Waters south of Point Conception... There is a
thirty percent chance of SCA level winds across the nearshore
waters on Tuesday and a seventy percent chance on Thursday. The
strongest winds will occur between Point Mugu and Santa Monica.
Otherwise conditions will remain below SCA levels through the
forecast period.

&&

.FIRE WEATHER...21/909 PM.

Through Wednesday afternoon, elevated to brief critical fire
weather conditions will persist due to weak to locally moderate
Santa Ana winds, hot temperatures, low humidities, and very dry
fuels. Tuesday will be the closest to critical over Los Angeles
and Ventura Counties, when the Santa Ana winds get a slight bump
upward due to a slight increase in offshore pressure gradient and
upper level wind support. Peak wind gusts between 20 and 40 mph
are expected, strongest on Tuesday. Expecting some areas (like the
mountains, and Santa Clarita/eastern Ventura Valleys/Foothills)
to see 2 to 4 hours of critical fire weather conditions on
Tuesday.

Late Wednesday night through Friday, a more significant Santa Ana
wind event will likely bring widespread critical fire weather
conditions to Los Angeles and Ventura Counties, and elevated to
brief critical conditions for Santa Barbara and San Luis Obispo
Counties. A Fire Weather Watch has been issued, with a high
probability of being upgraded to a Red Flag Warning as we draw
closer to the event. There is still some uncertainty in the
strength of the winds, but peak gusts between 40 and 60 mph is
the most likely outcome. With projected LAX-Daggett gradients
potentially reaching -7 to -8 mb and fairly strong upper level
wind support, there is the potential these wind speeds will
need to be ramped up even higher. Humidities will likely lower
to 2 to 9 percent, with temperatures into the 90s. The extended
period of hot and dry conditions leading into the Thursday-Friday
Santa Ana event will make fuels very dry, adding to the potential
for increased fire ignitions and rapid fire growth. This has the
potential to be a very dangerous Red Flag event with very critical
fire weather conditions due to the strength and duration of the
Santa Ana winds, hot temperatures, very low humidities, and
critically dry fuels. Fire weather conditions could be as severe
as we saw on October 10th-11th during the Saddleridge Fire.

The expected fire weather conditions will create an environment
that will bring the potential for very rapid fire spread,
long range spotting, and extreme fire behavior which could
threaten life and property. We urge everyone to be extremely
cautious with any potential fire ignition sources. Fires have
started from things like cigarettes, camp fires, welding and brush
clearing equipment, and dragging towing chains.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until noon PDT Tuesday for zones
      44-45-88. (See LAXNPWLOX).
     Fire Weather Watch in effect from late Wednesday night
      through Friday evening for zones
      240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

Gusty northeast winds and very dry air will impact much of Los
Angeles and Ventura Counties Thursday and Friday. Critical fire
weather conditions are possible in these areas along with elevated
fire weather for the Central Coast.

&&

$$

PUBLIC...Gomberg/EB
AVIATION...Hall
MARINE...Kj
FIRE...Gomberg/Kittell
SYNOPSIS...Kj

weather.gov/losangeles

NWS LOX Office Area Forecast Discussion