781
FXUS66 KLOX 230527
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1027 PM PDT Sun Sep 22 2019

.SYNOPSIS...22/746 PM.

Temperatures will be cooler on Monday then warm significantly on
Tuesday as gusty Santa Ana winds develop. A notable cooling trend
begins by Thursday and there is a chance of light rain late in the
week.

&&

.SHORT TERM (SUN-WED)...22/907 PM.

The latest satellite imagery shows an upper-level trough of low
pressure over northern California and developing high amplitude
ridging building into eastern Pacific Ocean behind it. The trough
will brush the area as it passes by to the north through Monday
night, then it will settle into the lower Colorado River Basin and
the Gulf of California on Tuesday. There is some variation in the
trough movement on Monday night, but the general idea of the
weather pattern is similar through Tuesday. An onshore flow
pattern developing this evening will bring some cooling on Monday,
then offshore flow will develop on Monday night. A few tweaks
were made to temperatures for Monday in the latest updates, but
overall, the major details still remain on track.

***From Previous Discussion***

This low in combination with an amplifying upper level ridge of
high pressure off the California coast will bring some upper level
wind support for a Santa Ana wind event across Southern
California late Monday night into Tuesday. While the surface
offshore gradient is fairly weak (LAX-Daggett gradient peaking at
around -3 mb on Tuesday morning), the upper level support will
help drive this offshore wind event, with projected wind gusts of
30 to 40 mph in the mountains and foothills of LA/Ventura
counties, and gusts 25 to 35 mph in the valleys. At this point,
still questionable if offshore winds will reach coastal areas of
Ventura county and Malibu, but if it does, winds should only be in
the 10 to 20 mph range.

The Santa Ana wind event is expected to bring significant warming
and drying to much of the region, with warmest valley areas now
expected to top 100 degrees, while portions of the coastal plain
climb into the 90s. With current forecast bumping up some of the
valley temperatures for Tuesday, as well as minimum temperatures
in the foothills for Tuesday night, a heat advisory may need to
be considered for portions of the valleys as we draw closer to
the Tuesday heat event. Some lighter offshore breezes are
expected to linger into Tuesday night and Wednesday morning
across the mountains and valleys, but wind speeds expected to
be less than Tuesday morning due to diminished upper level wind
support. Some cooling expected across coastal areas on Wednesday
due to earlier arrival of the sea breeze, however inland areas
will see another warm and dry day, with temperatures climbing
well into the 90s. The main impact of this Santa Ana wind event
will be an extended period of elevated fire weather concerns from
late Monday night through Wednesday, with the potential for brief
critical fire weather conditions on Tuesday when the strongest
offshore winds and driest conditions are expected to occur. Please
see fire discussion below for more details.

.LONG TERM (THU-SUN)...22/229 PM.

On Wednesday night into Thursday morning, onshore flow and marine
layer depth are expected to increase rapidly, bringing a surge of
low clouds well into the lower coastal slopes. GFS model cross
section showing a deep layer of moist south-southeast flow up to
4000 feet by Thursday morning, which will likely result in areas
of drizzle. An unusually strong upper level trough will continue
to deepen across California Thursday  through Saturday. This will
result in significant cooling across Southwest California during
this period, along with gusty onshore winds in the mountains and
deserts. In addition, long range models showing moist layer
continuing to deepen from Thursday night into Saturday, which
could bring periods of drizzle or light rain showers through the
period. At this time, still have 20-40 percent chance of
measurable light rain from Thursday night through Saturday,
followed by a 20 percent chance of showers for areas north of
Point Conception on Sunday.

&&

.AVIATION...23/0527Z.

At 05Z, the marine layer was around 600 ft deep at KLAX. The top
of the marine inversion was near 1900 ft with a temperature of 24C.

Expect low clouds to push into most coastal areas tonight with
generally IFR to LIFR conds, with locally VLIFR conds. The
exception may be the south coast of SBA County and the SLO County
coast where skies may remain clear. Some clouds with LIFR Io
VLIFR conds could push into the Santa Ynez Valley and the lower
valleys of VTU County as well. Skies should clear by mid morning,
except possibly late morning near the beaches.

KLAX...Moderate confidence in the 06Z TAF. There is a 20-30%
chance that cigs will arrive as early as 08Z. There is a 20%
chance that skies will remain clear overnight. There is a 20%
chance that east winds will reach 7-9 knots between 10Z and 17Z.

KBUR...High confidence in the 06Z TAF. There is a 20% chance of
MVFR vsbys between 13Z and 16Z.

&&

.MARINE...22/846 PM.

Moderate to high confidence in the current forecast.

For the Outer waters, Small Craft Advisory (SCA) level winds across
the northern two outer waters zones along the Central Coast from Point
Conception northward are likely tonight through Monday evening.
There is a 30% chance of SCA level winds across the southern outer
waters zone south of Point Conception (PZZ676) late tonight
through Monday evening.

Across the inner waters north of Pt. Sal, there is a 30% chance
of SCA level NW winds during the afternoon/evening hours on
Monday.

Across the inner waters south of Pt. Conception, conditions are
expected to remain below SCA level through Friday. However, there
is a 30% chance of SCA level NE winds across the eastern Santa
Barbara Channel and nearshore from Pt. Mugu to Santa Monica
Tuesday morning.

Patchy dense fog with visibility 1 nm or less may impact much of
the coastal waters late tonight through Monday morning.

&&

.FIRE WEATHER...22/910 PM.

An upper level cutoff low is forecast to move south through
Nevada on Monday, reaching a location just south of Las Vegas by
Monday evening, then shifting into southwest Arizona by Tuesday
morning. This system has the potential to bring northeast wind
gusts of 25 to 40 mph to the mountains and valleys of Ventura and
Los Angeles Counties Monday night through Wednesday morning,
strongest Tuesday morning. Widespread minimum humidities of 8 to
15 percent will be possible Tuesday afternoon, with the lowest
humidities lagging just a few hours behind the peak winds.
Elevated to brief critical fire weather conditions are likely as
this occurs. There is better confidence this morning as to this
system`s track, although there is a chance that model runs may
shift somewhat as the event draws closer in time.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 9 PM PDT Monday for
      zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Continued warm and dry conditions will bring elevated fire
weather conditions across inland areas into Wednesday.

&&

$$

PUBLIC...Hall/Gomberg
AVIATION...DB
MARINE...Sweet/Stewart
FIRE...Hall/Smith
SYNOPSIS...Kj

weather.gov/losangeles




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