Wind Advisory 15, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
931 PM PST Fri Feb 2 2024

.SYNOPSIS...02/127 PM.

Except for isolated showers through this evening, mostly dry
weather is expected through Saturday morning. A strong and wet
storm system will arrive later Saturday or Saturday night. This
storm will bring heavy rain and high elevation mountain snow to
much of the region through at least Tuesday morning, with showers
and lowering snow levels through midweek.

&&

.SHORT TERM (FRI-MON)...02/918 PM.

***UPDATE***

Except for a few lone showers across the mountains of eastern Los
Angeles County and in the northern mountains of Ventura County,
shower activity has ended across the region. Skies were partly
to mostly cloudy across the forecast area, and will remain so
overnight. There were some locally gusty winds in the Antelope
Valley, through the I-5 Corridor and across southern SBA County,
but winds were below advisory levels and will diminish overnight.
For the most part, tonight and Saturday morning will be the calm
before the upcoming storm, with winds below advisory levels, and
mostly dry conditions across the region.

A strong and cold upper low was centered a couple of hundred miles
west of the coast of southern Washington state this evening. This
upper low will drop slowly southward overnight and Saturday, and
will be located about 250 nm west of the California/Oregon border
late Saturday afternoon. Strong westerly flow to the south of
this upper low will be aimed at the forecast area, which will
transport increasingly moist Pacific air into the region during
Saturday. Models strongly suggest warm frontal development along
the Central Coast during the day Saturday. This will likely cause
rain to develop in the afternoon across San Luis Obispo County
and most Santa Barbara County, extending into the mountains of
Ventura County. There is a chance of rain in coastal and valley
areas of Ventura County and in western Los Angeles County late in
the day.

Rain is expected to overspread the entire region Saturday night.
As the flow aloft becomes more southwesterly Saturday night, the
system will begin to tap into subtropical moisture as an
atmospheric river begins to approach the coast. At the same time,
flow from 925 mb through 850 mb will become increasingly strong
from the south. This will cause rain to become heavy at times
across San Luis Obispo County and much of Santa Barbara County
late at night, especially on and below south and southwest facing
terrain, where orographic lift will provide extra enhancement of
the rain, increasing rainfall rates.

The forecast is in very good shape and handles the overall
synoptic pattern very well. The main changes to the forecast will
be to upgrade High Wind Watches to Warnings in northern areas
and the higher terrain of Ventura and Los Angeles County (not
included in the Winter Storm Watches) for Sat night through Sun
night. Will likely issue wind advisories for other portions of
the forecast area with either the evening update, or the early
morning forecast on Sat.

In addition, will expand the chance of thunderstorms to all of
SLO and SBA Counties on Sunday, and extend the slight chance of
thunderstorms southward into at least Ventura County. Low level
wind profiles, with significant directional and speed shear, will
become increasingly favorable for low level rotation of any
convective cells Sunday. This will yield a slight chance of
waterspouts Sunday across the northern outer coastal waters, the
inner waters north of Point Sal, and across the Santa Barbara
Channel. It is not out of the question that some of these could
come ashore briefly as small tornadoes. Also, with very strong
winds from 950 mb and above, it will not take much to transport
strong winds to the surface in any convective activity Sunday,
especially from Ventura County northward.

This activity will continue to translate southward through the
coastal waters and across the forecast area Sunday night.

***From Previous Discussion***

...SIGNIFICANT THREAT OF WIDESPREAD AND LIFE-THREATENING FLASH
FLOODING SUNDAY INTO MONDAY...

Minimal changes to the forecast and primary messages. A strong
storm tapping into an atmospheric river will move into the region
later Saturday and remain through Monday. The period of most
concern is Sunday into early Monday as the heaviest part of the
storm moves slowly through the area. The latest forecast models
weren't quite as slow with the progression as earlier runs, but
this only reduces the the total rain amounts by maybe 10% and the
most likely outcome for rain totals through Monday are still 3-6"
for coast and valleys and 6-12 inches for the foothills and
mountains. The higher end totals are still expected to be from
southern Santa Barbara County to western Ventura County where
60-80kt of south wind will generate a huge orographic enhancement
on the south facing mountains. Rain rates during the peak of the
storm expected to be from a half to one inch per hour, with
isolated higher rates in upslope areas and near any thunderstorms.

Flooding issues will not be confined to just the foothills,
mountains, and burn ares. ALL AREAS, INCLUDING HIGHLY POPULATED
URBAN AREAS, WILL BE AT RISK FOR LIFE-THREATENING FLOODING.
Streams and small rivers, as well as the Los Angeles River through
the San Fernando Valley and metro LA will rise quickly and turn
into very dangerous raging rivers. Many roads will be impassable
due to flooding. Numerous mud and rock slides are expected in the
foothills and mountains. Evacuations may be required in some
areas, especially near recent burn scars.

In addition to the hydrologic issues, very strong winds will
accompany the storm, especially SLO and SB Counties and the
mountains and Antelope Valley. Wind gusts up to 80 mph are
possible in the mountains, including the Grapevine, and as high
as 60-70 mph across parts of SLO and Santa Barbara Counties and
the Antelope Valley. Elsewhere winds 30-50 mph are expected.

Heavy snow accumulations are possible at higher elevations and a
winter storm watch is in effect for the higher portions of the
Ventura and LA County mountains. Up to 2-4 feet of snow are
possible above 7000 ft with lesser amounts down to around 5000
feet. Snow levels will start out on the lower side, around 5000
feet before the warm core of the system moves in Sunday into
Monday and snow levels rise back to at least 7000 feet and
possibly higher. Then on the back side of the storm later Monday
into Tuesday snow levels will fall back down to 5000 feet.

.LONG TERM (TUE-FRI)...02/207 PM.

After the front moves east of the region, very moist westerly
flow will linger Mon night through early Tue night, keeping
periods of rain a possibility just about anywhere. There should be
some drying Wed into Thu. However, there will still be a chance
of showers at times, although there could also be some sunshine
during the days. Snow levels expected to lower to 4500-5000 feet
Tuesday and Wednesday and as low as 3500-4000 feet Thursday so
there is the potential for accumulating snow at lower elevations
towards mid week.

There is a chance of a more organized area of precipitation late
Thu into Fri, but that should be mostly on the light to possibly
moderate side if it indeed affects the region.

&&

.AVIATION...03/0229Z.

At 00Z at KLAX, there was no marine layer nor inversion.

Moderate confidence in the current forecast, with mostly VFR
conditions, with a 20% chance of patchy MVFR conditions at all
sites except but desert TAFs. Timing of next rain will occur as
soon as 20Z for the Central Coast sites, but for the rest
locations after 00Z.

KLAX...Moderate confidence in TAF. There is a 20% chance of
patchy BKN025 conditions through the period. There is a chance of
rain starting as early as 00Z, with a 30% chance of BKN025 conds.
Just a 15% chance of east wind component exceeding 6-8 kts from
12Z-17Z Sat, and 30% chance after 05Z.

KBUR...Moderate confidence with a 20% chance of patchy BKN025
conditions through the period.

&&

.MARINE...02/921 PM.

An extended period of Small Craft Advisory (SCA) conditions are
expected across the coastal waters into Saturday evening, then
dangerous marine conditions are likely to develop across the
entire coastal waters Saturday evening and lasting into Monday.
There is an 80-100 percent chance of GALE FORCE winds developing
Saturday night, with a 70-80 percent chance of STORM FORCE winds
west through northwest of the Channel Islands and for the
nearshore waters along the Central Coast. Very large southerly
wind swell will affect south- facing harbors with large breaking
waves Saturday night into Monday morning. Santa Barbara Harbor and
Port San Luis could see large waves entering harbors during this
period.

Along with the winds and high seas, there will be a 20 percent
chance of thunderstorms between Sunday through Monday. Any
thunderstorms that develop will be capable of brief heavy rain,
gusty and erratic winds, steep and rough seas, dangerous cloud to
surface lightning, and possible waterspouts.

Leading up to the major storm, hazardous seas over the majority
of the coastal waters will likely persist up until the onset of
the storm Saturday evening. Following the storm, marine
conditions are expected to only improve slightly with continued
unsettle weather and high swell.

&&

.BEACHES...02/356 PM.

An extended period of high surf will impact the beaches through
Monday. However, the swell direction will switch from west to
south Sunday, thus the greatest impacts Sunday and Monday will be
for SOUTH and SOUTHWEST facing beaches.

Currently the primary swell is northwest to west. Thus northwest
and west facing beaches are expected to see surf around 6 to 10
feet south of Point Conception and 10 to 16 feet along the
Central Coast, with the chance for a brief drop Saturday. Sunday
and Monday, surf will peak between 8 and 12 feet for south to
southwest facing beaches south of Point Conception, and the
Central Coast peak at 14 and 18 feet with local sets up to 20
feet.

This large wind swell will create breaking waves and
a possible push of water into unprotected SOUTH facing areas like
Port San Luis and Santa Barbara Harbor. As for coastal flooding,
for the Central Coast, moderate coastal flooding is expected for
SOUTH facing harbors on early Sunday morning extending though
Monday morning. For the Central Coast, impacts will occur even
through lower tides as the surf will build between Sunday and
Monday early high tides. South of Point Conception, moderate
coastal flooding is expected for SOUTH facing harbors mainly
during high tide early Monday morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Watch in effect from Saturday evening through
      Sunday evening for zones 38-340>353-549-550. (See LAXNPWLOX).
     Flood Watch in effect from late Saturday night through
      Tuesday afternoon for zones 38-340>358-369-371-374>377. (See LAXFFALOX).
     High Surf Advisory in effect until 9 PM PST Monday for zones
      87-340-346-349-350-354-362-366. (See LAXCFWLOX).
     Flood Watch in effect from Sunday afternoon through Tuesday
      afternoon for zones
      87-88-362-366>368-370-372-373-378>383-548. (See LAXFFALOX).
     Coastal Flood Advisory in effect from 1 AM to 9 AM PST Sunday
      for zones 340-346. (See LAXCFWLOX).
     High Wind Watch in effect from late Saturday night through
      Sunday evening for zones 376-378-379-381>383. (See LAXNPWLOX).
     Winter Storm Watch in effect from Saturday evening through
      late Monday night for zone 377. (See LAXWSWLOX).
     Winter Storm Watch in effect from Saturday evening through
      Tuesday afternoon for zone 380. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect until 6 PM PST Saturday for
      zones 645-670-673. (See LAXMWWLOX).
     Gale Warning in effect from 6 PM Saturday to 1 AM PST Sunday
      for zones 645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PST Saturday for
      zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM Saturday to 3 AM PST
      Sunday for zone 650. (See LAXMWWLOX).
     Gale Watch in effect from late Saturday night through late
      Sunday night for zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PST Saturday for
      zone 676. (See LAXMWWLOX).
     Gale Warning in effect from 9 PM Saturday to 3 AM PST Monday
      for zone 676. (See LAXMWWLOX).

&&

PUBLIC...MW/DB
AVIATION...Lund
MARINE...Schoenfeld/Lund
BEACHES...Hall/Schoenfeld
SYNOPSIS...MW

weather.gov/losangeles

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