Wind Advisory 32, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1239 PM PDT Mon Mar 13 2023

Updated Aviation and Marine Sections

.SYNOPSIS...13/941 AM.

It will be cloudy today with maybe some partial clearing this
afternoon. A powerful storm will bring widespread heavy rain to
the region Tuesday into early Wednesday, possibly resulting in
flooding and gusty winds at times. Generally dry and cool weather
is expected Wednesday night through Thursday night. There is a
slight chance of rain Friday into into the weekend, mainly in
northern areas.

&&

.SHORT TERM (TDY-WED)...13/937 AM.

***UPDATE***

Another damp night last night with widespread drizzle and even
some light rain across the Central Coast. All that has pretty much
ended and expecting a dry but cloudy day today.

The forecast for the next AR storm system remains more or less the
same. Models still focusing on Santa Barbara and western Ventura
Counties but all areas will see significantly higher rainfall
totals than what a typical winter storm would bring. Snow levels
will be above 8500 feet for the vast majority of the storm so
virtually no snow with this storm in the local mountains.

***From Previous Discussion***

All eyes then turn to the potent major winter storm that will
affect the region beginning in northern areas late tonight, all
areas Tue and Tue night, with its effects possibly lingering into
Wednesday morning in southern and eastern sections. Confidence in
fairly high that this storm will bring significant widespread
heavy rainfall to the region. In fact, it will be a big surprise
if it does not do so.

An upper level trough moving through the eastern Pacific will
begin to amplify a bit as it moves toward the West Coast later
today and tonight, causing the flow aloft across the region to
turn more southwesterly. This will tap into a long fetch of
subtropical moisture, setting up another atmospheric river event
for the forecast area. Models clearly suggest warm frontal
development tonight as low level southerly flow increases, and
rain will develop across SLO County and SBA County overnight,
possibly becoming heavy at times.

The deterministic runs of the EC, GFS, and WRF and just about
all of their ensemble members show a significant rain event
across the entire area Tue and Tue night. The models have come
into better agreement with timing, with the WRF having sped up
some, and the EC and GFS having slowed down a bit. The GFS and the
WRF are now in rather good agreement with the timing and
placement of the heaviest rain.

As the trough and its surface frontal system approach the coast,
it will transport copious amounts of subtropical moisture thru a
deep layer of the atmosphere on Tue. Precipitable water values
will rise to between 1.25 and 1.30 inches. A very strong jet in a
favorable position will only enhance the good dynamics associated
with the system, with a very deep and strong layer of lift within
the moisture rich atmosphere. At 850 mb, south-southwesterly flow
will increase to 40-50 knots, with decent southerly flow at the
surface. This will provide good, efficient orographic enhancement
of the rain on and below south and southwest facing slopes.

Even without thunderstorms in the forecast (and they cannot be
ruled out), rainfall rates of one half to one inch per hour are
likely during the peak of this storm. Flood Watches are in effect
for San Luis Obispo County from 4 am PDT Tue thru 10 pm PDT Tue
evening, and for Santa Barbara County from 7 am PDT Tue thru late
Tue night. Based on the latest model runs, it appears that the
Flood Watch may have to be expanded to the mountains of Ventura
County and possibly the mountains of L.A. County.

The current thinking for the timing of the peak of the storm is
very late tonight thru Tue afternoon in SLO County, from about
daybreak or shortly thereafter to late Tue evening in SBA County,
mid morning to just after midnight in Ventura County, and mid to
late Tue afternoon thru late Tue night in Los Angeles County.

Rainfall totals are expected to average 2 to 4 inches in most
coastal and valley areas with 3 to 6 inches in the foothills,
and up to 1 inch in the Antelope Valley. It appears that the
mountains of southeastern SBA County and western Ventura County
may end up with the absolute highest totals, possibly over 7
inches in some locations.

There will likely be widespread and significant roadway flooding
across the region from this storm, but there may also be more
significant flooding, with mud and debris flows, rock slides, and
some flooding of creeks and rivers.

Strong south winds will also be an issue with this storm. High
Wind Watches have been issued for Tue for the mtns of SLO and SBA
County and the higher valley locations of those counties. Many
other locations around the region will likely need wind advisories
as the event draws closer.

Rain is likely across southeastern portions of the forecast area
Wed morning, with a chance of rain elsewhere, then there will be a
decrease chance of showers during the day. In fact, skies may
become partly cloudy in the afternoon.

.LONG TERM (THU-SUN)...13/532 AM.

A weak ridge will build into the eastern Pacific Thu, with broad
NW flow aloft over the region. It should be dry and cool. For Fri
thru the weekend. there is low confidence in the forecast, as
deterministic runs of the GFS and EC are in poor agreement and
ensembles show a wide range of solutions. In general, it appears
that a couple of weak fronts will attempt to push into the area,
but they will be quite weak. Still, enough ensemble members show
measurable rain to keep at least a slight chance of light rain Fri
thru Sun in northern areas, with the slight chance extending
southward across the entire forecast area late Fri/Fri night, and
again late Sat night/Sun. At this point, amounts look very light.

&&

.AVIATION...13/1938Z.

At 1740Z at KLAX, the marine layer depth was around 1600 ft deep,
with an inversion with a top at 2000 feet and a max temp of 12 C.

High confidence in the onset of rain and gusty S winds overnight,
starting with the northern sites and moving south through the
region. Flight categories will generally stay in the IFR/MVFR
OVC008-015 for most of the rain period, but could lower to LIFR
briefly under heavier cells anywhere. Moderate confidence in the
timing of flight cat changes and wind shifts, with may be off by
up to +/- 3 hours.

There is a 30% chance dense fog and LIFR conditions tonight at
KOXR KSBA and KSBP.

KLAX...Moderate confidence in SCT025 through 23Z, with BKN015-025
reforming sometime between 23-03Z. With the onset of rain around
16Z, BKN008-15 will be most common, with a 20 percent chance of
brief OVC004 during heavier cells. Moderate confidence of extended
period of southeast winds over 8 knots from Tue 15-17Z through
Wed 05-08Z, but easterly component might stay around 6 knots.

KBUR...Moderate confidence in SCT025 through 23Z, with BKN015-025
reforming sometime between 23-03Z. With the onset of rain around
16Z, BKN008-15 will be most common, with a 20 percent chance of
brief OVC004 during heavier cells.

&&

.MARINE...13/957 AM.

High confidence conditions staying under Small Craft Advisory
(SCA) through this evening. A storm system will approach the
Central Coast later tonight into Tuesday, then spread through
southern California into early Wednesday. Gale Force south wind
gusts 35-40 knots likely north of Point Conception, including the
nearshore Central Coast. There is a 30 percent chance of low-end
Gales between Point Conception and San Nicolas Island. Otherwise
high confidence that SCA south winds 25-30 knots will be common
at times on Tuesday. Periods of heavy rain with low visibilities
expected also expected. Winds will abruptly turn to west to
northwest on Wednesday. High confidence in widespread SCA winds
25-30 knots. 20 percent chance of reaching Gale force. Moderate
confidence in fairly benign conditions to follow Thursday and
Friday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 6 AM to 10 PM PDT Tuesday for
      zones 38-340-341-343-346>349. (See LAXNPWLOX).
     Flood Watch in effect from Tuesday morning through late
      Tuesday night for zones 38-346>353. (See LAXFFALOX).
     Flood Watch in effect from late tonight through Tuesday
      evening for zones 340>345. (See LAXFFALOX).
     High Wind Warning in effect from 6 AM to 10 PM PDT Tuesday
      for zones 342-344-345-353. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 10 PM this evening to 4
      AM PDT Tuesday for zones 645-670-673. (See LAXMWWLOX).
     Gale Warning in effect from 4 AM to 10 PM PDT Tuesday for
      zones 645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect from 4 AM Tuesday to 4 AM PDT
      Wednesday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 10 AM Tuesday to 4 AM PDT
      Wednesday for zone 655. (See LAXMWWLOX).
     Gale Watch in effect from Tuesday morning through late
      Tuesday night for zone 676. (See LAXMWWLOX).

&&

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

weather.gov/losangeles

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