Flash Flooding and Winter Storm in California, February 1, 2024

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
319 AM PST Thu Feb 1 2024

.SYNOPSIS...01/155 AM.

The first in a series of storms will bring rain and mountain snow
today (mostly in the afternoon). Dry weather is expected most
areas Friday and Saturday except for some scattered light showers,
mainly north of Point Conception and in the mountains. A second
and likely much stronger storm will arrive Sunday with periods
moderate to heavy rain and mountain snow through at least Tuesdau
and perhaps Wednesday.

&&

.SHORT TERM (TDY-SAT)...01/312 AM.

Today's strong cold front is currently over SLO county. The
frontal dynamics and strong southerly flow ahead of the front is
producing moderate to heavy rainfall. Current rainfall rates are
between a quarter inch and a half inch per hour (.15"-.25"/hr SLO
county) except across the south slopes of the Santa Ynez range
where rates are a half inch to near an inch per hour are occuring.
This high intensity rainfall will continue to move to the east
with the rain peak arriving in LA county around 7 am. The front is
moving along at a decent clip and this will minimize the flash
flooding threat - still advisory level flooding is almost certain
in and around the front. When the front exits the eastern edge of
LA county around noon the steady rain will turn to showers and the
amount of rain will decrease. The front will bring about 2 inches
of rain to most of the coasts and vlys (perhaps a little less
across the Central Coast). The south slopes south of Pt Conception
will likely see 3 to 5 inches of rain.

Heavy snow accumulations are expected in the higher mountain
elevations, mainly above 7000 feet, where 8-16 inches of snow are
likely (60 percent) Periods of strong southerly winds also
expected in the mountains with gusts up to 50-60 mph, especially
on the higher peaks.

There are some lightning strikes behind the main front and there
will be a 10-20 percent chance of TSTMs this afternoon. Any TSTM
that develops could produce rain rates in excess of 1 inch per
hour. These rates could be high enough to require a flash flood
warning.

Moist cyclonic flow with some instability will keep a chance
(20-30 percent) of showers going into this evening. After midnight
the shower threat will be confined to SLO county and the north mtn
slopes. Snow levels will fall to about 4500 ft which is still
above the Grapevine pass level.

Showers will continue to subside Friday morning. Snow levels will
bottom out in the morning and there there is a 30 percent chc of
some snow on the Grapevine but only a 10 percent chc of a 1 inch
accumulation. By afternoon it will dry and skies will be partly
cloudy with the exception of the north slopes near the Kern County
line.

Certainly not a warm day Friday with max temps struggling to
reach 60 degrees. Max temps will crawl up into the lower 60s.

A chance of rain 20 to 40 percent will develop over the area in
the evening with rain becoming likely after midnight.

.LONG TERM (SUN-WED)...01/318 AM.

Not much has changed at all in the long term forecast and the
large and dangerous atmospheric river storm remains on track and
little change is needed for the long term discussion.

The largest storm of the season will begin late Saturday night
into Sunday as an upper low is expected to spin down the coast
and merge with another upper low moving in from the west. This
will create a powerful upper low just west of the Bay Area that
will tap into an atmospheric river across the Pacific Ocean. As it
stands now, the heaviest portion of the storm is expected to be
Sunday night into Monday as the front stalls from around Pt
Conception south. Where this stalls could easily shift 50-100
miles either direction so there is still some uncertainty, though
models have been consistent the last several runs indicating
higher rain amounts south of Pt Conception. This system will
likely produce 24 to 36 hours (or more) of continuous rain.

Over the Sun-Wed period the early estimates call for widespread
rain amounts of 2-4 inches for lower elevations and likely twice
those amounts in the south facing mountains. There are more than a
few ensemble solutions that call for much higher amounts than
those. Rain rates around an inch per hour are possible at times
during the peak of the storm, especially in upslope areas.
Citizens and authorities need to be aware of the likelihood of
significant hydrologic issues, including: rock and mud slides in
the mountains and flooding of small streams and rivers as well as
significant rises on some of the larger rivers and streams.

Significant and hazardous snowfall amounts will be possible above
6000 ft through Monday night. Lower elevation snow down to around
4000 feet is possible by later Tuesday into Wednesday as the upper
low works its way to southern California bringing cooler air into
the area.

Everyone across the southland should keep abreast of the latest
developments in the forecast of this very significant storm.

&&

.AVIATION...01/1100Z.

At 1045Z, there was no marine inversion or marine layer at KLAX.
However, there was a moist layer up to around 10,000 feet.

Overall, low confidence in 12Z TAF. A front will continue to move
across the area this morning, bringing moderate to locally heavy
rain. CIGs and VSBYs will vary dramatically between VFR and IFR
levels for all sites. For this afternoon and tonight, there will
continue to be a chance of light showers with MVFR or IFR CIG and
VSBY restrictions possible. Gusty southeasterly winds can be
expected ahead of the front, impacting Ventura/LA counties through
18Z.

KLAX...Low confidence in 12Z TAF. CIGs and VSBYs will vary
frequently through 20Z between VFR and IFR levels. For this
afternoon and tonight, low confidence in timing of changes of
flight categories between VFR and MVFR. High confidence in
easterly wind component greater than 10 knots through 20Z with
winds shifting to a west direction after 20Z and weakening.

KBUR...Low confidence in 12Z TAF. CIGs and VSBYs will vary
frequently through 20Z between VFR and IFR levels. For this
afternoon and tonight, low confidence in timing of changes of
flight categories between VFR and MVFR.

&&

.MARINE...01/300 AM.

For the Outer Waters, moderate to high confidence in current
forecast. Today through Saturday, high confidence in Small Craft
Advisory (SCA) level seas with wave heights peaking in the 12 to
17 foot range with a 30-50% chance of SCA level westerly winds.
For Sunday and Monday, high confidence in SCA level southerly
winds and seas with a 30-50% chance of Gale force winds.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Today through Saturday, high
confidence in SCA level seas with wave heights peaking in the
12 to 16 foot range with a 20-30% chance of SCA level winds.
For Sunday through Monday, high confidence in SCA level southerly
winds and seas with a 30-50% chance of Gale force winds.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. For today, Gale force
southeasterly winds will shift to a westerly direction by this
afternoon while seas remain at SCA levels. For tonight through
Saturday, there is a 40-60% chance of SCA level westerly winds
and SCA level seas. For Sunday and Monday, high confidence in SCA
level southeasterly winds with a 30-50% chance of Gale force
winds.

Along with the winds and high seas, there will be a 10-20% chance
of thunderstorms today through this evening. Any thunderstorms
that develop will be capable of brief heavy rain, gusty and
erratic winds, steep and rough seas, and dangerous cloud to
surface lightning. Also, isolated waterspouts will be possible.

&&

.BEACHES...01/300 AM.

An extended period of high surf will impact the beaches. Surf will
continue to build along the Central Coast today and persist
through Sunday or Monday. The largest waves are expected today and
Friday. At this time, confidence is high for all west facing
beaches to see surf over 8 feet, with the Ventura Coast peaking at
12 feet, and the Central Coast at 18 feet. This puts most coasts
into High Surf criteria, with hazardous surf for most people and
dangerous waves on rocks and jetties. There is a 30 percent chance
that waves will reach warning levels this afternoon and evening
over the Central Coast (20+ feet) and the Ventura Coast (15+
feet). If waves reach these levels, there is a risk for pier
damage as well as significant beach erosion and inundation of
sandy beach areas despite high tides not being very high (4.5 to
5.0 feet). Further complicating the situation, during the peak of
this current storm and the Sunday/Monday storm, a 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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until 7 AM PST this morning for zones
      38-340>349-351. (See LAXNPWLOX).
     High Surf Advisory in effect until 6 AM PST Saturday for
      zones 87-349-350-354-362-366. (See LAXCFWLOX).
     Wind Advisory in effect until 11 AM PST this morning for
      zones 87-366>368. (See LAXNPWLOX).
     Wind Advisory in effect until 9 AM PST this morning for zones
      88-354>358-362-369>375. (See LAXNPWLOX).
     High Surf Advisory in effect until 5 PM PST Saturday for
      zones 340-346. (See LAXCFWLOX).
     Wind Advisory in effect until 6 AM PST early this morning for
      zones 350-352. (See LAXNPWLOX).
     High Wind Warning in effect until 5 AM PST early this morning
      for zone 353. (See LAXNPWLOX).
     Winter Weather Advisory in effect until 4 PM PST this
      afternoon for zone 376. (See LAXWSWLOX).
     Winter Storm Warning in effect until 4 PM PST this afternoon
      for zone 377. (See LAXWSWLOX).
     Wind Advisory in effect until 5 AM PST early this morning for
      zones 378>380. (See LAXNPWLOX).
     Winter Weather Advisory in effect until 10 PM PST this
      evening for zones 378-379. (See LAXWSWLOX).
     Winter Storm Warning in effect until 10 PM PST this evening
      for zone 380. (See LAXWSWLOX).
     Wind Advisory in effect until 10 AM PST this morning for
      zones 381-382. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 9 PM PST Friday for
      zones 645-670-673. (See LAXMWWLOX).
     Gale Warning in effect until 9 AM PST this morning for zones
      650-655-676. (See LAXMWWLOX).

&&

PUBLIC...Rorke/MW
AVIATION...RAT
MARINE...RAT
BEACHES...Hall/RAT
SYNOPSIS...MW

weather.gov/losangeles

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