Acton Weather

000
FXUS66 KLOX 221148
AFDLOX

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

.SYNOPSIS...22/308 AM.

Weak offshore flow with locally gusty morning winds will continue
across inland areas this morning. High temperatures will remain a
a few degrees warmer than normal as well. After a cooling trend
Monday, a moderate Santa Ana wind event develops. Temperatures
will warm significantly Tuesday and Wednesday and gusty winds will
return. Cooling should develop for late week along with a chance
of light rain starting late this week into Saturday.

&&

.SHORT TERM (TDY-TUE)...22/318 AM.

Latest fog product satellite imagery indicated no stratus across
the forecast area this morning, and feel pretty confident skies
will remain stratus free this morning. However a jet streak to the
west of the Southern California has caused some high clouds to
develop and should move through the southern two thirds of the
forecast area through this afternoon. Therefore have added partly
skies to all areas south of San Luis Obispo County today.

Synoptically,an elongated trough situated N-S just off the coast
between Northern Cal and the PAC NW will move over the west coast
later this afternoon. The upper level trough will then become cutoff
over NV/CA border just south of Reno NV by Monday morning. The cutoff
low will then continue to trek south towards southern CA/AZ border
around Yuma AZ by early Tuesday morning.

As far as today, there will continue to be weak offshore flow this
morning across the L.A. County Mtns, Santa Clarita Valley, western
San Fernando Valley, Eastern Ventura County Valleys. Winds will be
a bit weaker than yesterday and coastal areas should not see any
offshore flow. Overall, strongest offshore winds should be 15 to
25 mph across the mountains, and slightly weaker for the other
areas mentioned. Winds will turn onshore in the afternoon hours
which should bring some minimal warming to the interior areas
including the Antelope Valley. There will be some cooling across
the Central Coast today as the offshore flow will be shifting to
southerly flow today with little change for most other coastal
areas.

Both the GFS and ECMWF models continue to be in good agreement
with the track of the upper level trough that becomes cutoff with
a 576 DM Low around Reno Nevada Monday morning then slides south
over the eastern California desert just west of Las Vegas as mentioned
earlier. The inside track will bring little in the way of moisture
and will not be tapping into any of the hurricane remnants, at least
over Southern Cal. However, increasing onshore flow Monday along
with cooling aloft will drop highs 5-8 degrees from today. By tonight
a southerly surge will develop across the SoCal Bight and there will
be increasing low clouds across the L.A. County Coast and possibly
into the San Gabriel Valley. This southerly surge should make it
to Ventura County coastal areas and and outside chance into
Southern SBA County by early Monday morning.

Later Monday night into Tuesday, the GFS and EC runs are in good
upper level agreement with upper low sliding southeastward near
Yuma. The latest 06z run of the NAM now also puts the upper low
over the same area as the other models now. If these models stay
on track and keep the upper low over southern AZ/CA border, this
should help to enhance the NE upper level support for the wind
event for late Monday night into Tuesday. With the EC/GFS scenarios,
the winds would start off more northerly late Monday night then
turn northeast by Tuesday morning. The NAM/WRF solutions continue
to advertise the LAX-DAG pressure gradient near the surface, peaking
over -3 mb on Tuesday morning. With a relatively warm atmosphere
over the region, the added compressional warming through the passes
will be driving high temps well into the 90s in many coastal areas
and should reach 100 degrees around Woodland Hills and Van Nuys
Tue afternoon. For now, will continue to go with northeast winds
gusting between 25 and 35 mph across coastal/valley areas of LA/Ventura
counties late Monday night into Tuesday, with mountain areas potentially
gusting up to 45 mph. As a result, wind advisories may be issued as
we draw closer to the event. While there is some uncertainty in how
the offshore winds will develop and extent of warming into coastal
areas, the models are consistent with a significant warming and
drying trend across inland areas, with warmest valley areas approaching
100 degrees. The combination of gusty north to northeast winds, very
warm temperatures, low humidities, and dry fuels will bring elevated
to brief critical fire weather conditions late Monday night into
Tuesday. See fire discussion below for more details.

.LONG TERM (WED-SAT)...22/320 AM.

On Wednesday, the GFS and EC models continue to spin the upper
low near the northern portion of Baja. while the NAM model
continues to track the system further east. There will likely be a
continued threat of gusty offshore winds on Wednesday morning,
but not as strong as Tuesday. And by afternoon, onshore flow is
expected to strengthen, resulting in some cooling across
coastal/valley areas.

On Wednesday night into Thursday morning, onshore flow and marine
layer depth are expected to rapidly increase, 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
5000 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, latest runs of GFS, ECMWF, and National
Blend models trending higher on pops from Thursday night into
Saturday, and have bumped up Pops accordingly into the 20-40
percent range. As a result, Southwest California could see its
first light rain event of the season later in the week.

&&

.AVIATION...22/1143Z.

At 11Z, the marine layer at KLAX was 500 ft deep. The top of the
6inversion was around 1600 ft with a temperature of 27C.

Except for some high clouds across the region, skies are expected
to be mostly clear with VFR conds thru early tonight. Low clouds
with IFR conds are expected to affect coastal sections of L.A,
County late tonight, possibly reaching coastal sections of VTU
County by daybreak Monday. There is also a chance of LIFR to VLIFR
conds on the Central Coast late tonight.

KLAX...High confidence in the 12Z TAF through midnight with VFR
conds, then confidence only moderate. There is a 30% chance that
skies will remain clear overnight/Mon morning with conditions
remaining VFR. There is a 20% chance of east winds of 7-9 knots
between 11Z and 16Z Mon.

KBUR...High confidence in the 12Z TAF with VFR conds thru the pd.

&&

.MARINE...22/447 AM.

Moderate to high confidence in the current forecast.

For the Outer waters, expect Small Craft Advisory (SCA) level
winds and seas across the northern two outer waters zones
(PZZ670/PZZ673) this afternoon thru Monday evening. There is a
30-40% chance of SCA level NW winds across the southern outer
waters zone (PZZ676) late tonight thru Mon evening.

Across the inner waters north of Pt. Sal, there is a 30-40%
chance of SCA level NW winds during the afternoon/evening
hours today and Mon.

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

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 9
      PM PDT Monday for zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

On Tuesday into early Wednesday, there is the potential for gusty
Santa Ana winds bringing very warm and dry conditions to the
region, especially Los Angeles and Ventura Counties. As a result,
there will likely be elevated to brief critical fire weather
conditions.

&&

$$

PUBLIC...Kaplan/Gomberg
AVIATION...DB
MARINE...DB
SYNOPSIS...Kaplan

weather.gov/losangeles