Flash Flooding and Flooding in California, January 9 to 10, 2023

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1037 PM PST Sun Jan 8 2023

.SYNOPSIS...08/1025 AM.

An unsettled weather pattern will continue through tonight, then
a strong Pacific storm system will move into the region through
Wednesday. A brief lull is expected by Thursday as a drying trend
establishes. There is potentially another storm system expected
over next weekend.

&&

.SHORT TERM (SUN-WED)...08/813 PM.

***UPDATE***

After a fairly quiet (if not cloudy) afternoon, the prelude to two
days of storms has begun. Currently a warm front is setting up
ahead of the main system and light rain is falling or developing
across the south coastal slopes from Pt Conception to Malibu. All
deterministic mdls as well as short range ensembles have converged
on a two day solution for the Mon/Tue storm. This rain could
really add up and by mid morning Monday many areas across these
coastal slopes will recieve around two inches of rain. Fortunately
the rain rates will not pose any threats with this warm frontal
rain. Periods of rain are likely for the rest of the area esp
after midnight but rainfall amounts and rates will not be that
worrisome.

The real action will start around mid morning when an cold front
and atmospheric river enters into the Central Coast. Steady
moderate to heavy rain will develop esp over SLO county and
most impressively over the Santa Lucia range. In the afternoon the
Central Coast will still be fully involved but the heavy rain will
spread into southern SBA county. The peak rain with this event
will occur across the W/E facing coastal slopes in southern SBA
county and Ventura County. Peak rainfall rates are expected to
range between 0.50 to 1.00 inch per hour across the region, but
local rates in excess of 1 inch per hour are certainly possible.

All of this rain on top of last weeks rain will produce responses
on the areas major rivers. The Ventura River is forecast to reach
flood stage late Monday evening. The Salinas, Sisquoc and Santa
Clara Rivers are forecast to reach monitor stage. Along with the
major rivers many smaller streams will be full or overflowing.

A very rare 15 mb gradient between KLAX and KSFO develops and this
will bring warning level winds to most all of SLO and SBA counties
as well as the mtns of VTA and LA counties and the Antelope Vly
(esp the foothills). Catalina Island was added to the wind
advisory and the western portion of the SBA county south coast.
Since the ground will be soaked these winds have a stronger than
normal possibility of toppling trees.

Tuesday continues to look very convective with a possibility of
severe TSTMs.

The forecast is in good shape and aside the from the addition of
Catalina Island to the wind advisory and upgrade to wind warning
for the portion of SBA county no changes are planned.

***From Previous Discussion***

A warm frontal boundary is  developing and moving toward the
California coast currently. PoPs have been increased over most
coastal and valley areas through Tuesday as almost all of the high
resolution multi-model ensemble members and EPS, GEFS, and CMC
ensemble members all indicate rain developing over the region.
While light to moderate rainfall will develop through tonight and
Monday morning, the rain will turn much heavier during the day on
Monday as favorable dynamics develop for thunderstorms and the
frontal boundary approaches the region.

The wettest part of the system will move through the region
between Monday night and Tuesday. Precipitable water values climb
to between 1.1 and 1.3 inches across the region Monday night then
remain around 1.0 inch Tuesday. Favorable dynamics improve and
develop a very deep area of strong omega values extending into the
middle levels of the atmosphere in a couple of different waves
between Monday night and Tuesday. Thunderstorms are possible at
any time Monday night or Tuesday as model solutions indicate a
difluent flow pattern developing late Monday, then extending into
Tuesday as the colder air aloft moves into the region.

To highlight the heavy rainfall potential, NWS WPC now has a
moderate risk for much of the area between Monday and Tuesday,
and Tuesday through Wednesday across the Transverse Range. Peak
rainfall rates are expected to range between 0.50 to 1.00 inch
per hour across the region, but local rates in excess of 1 inch
per hour are certainly possible in favored orographic areas and
near thunderstorms. Rainfall amounts have been increased from 2-5
inches for coasts and valleys to 5-9 inches for the mountains and
foothills. Local amounts to 12 inches are possible along the
south-facing slopes and foothills. Given the recent wet pattern,
some rivers will likely reach at least minor flood stage with the
expected rainfall amounts.  Snow levels will be above 8000 feet
for the most significant precipitation, although they will drop
rapidly behind the front Tuesday afternoon and evening.

.LONG TERM (THU-SUN)...08/214 PM.

A progressive pattern is in place heading into this weekend as the
ridge axis slides east of the region by Thursday promoting
increasingly moist southwest flow aloft into Friday. Best moisture
and lift is deflecting mainly north of the region through early
Friday, but with increasing cyclonic flow aloft shower activity
becomes more likely through the day from north to south. Ensemble
guidance continues to support another more significant storm with
widespread rain across the region this weekend. Guidance has
trended away from the idea of a single significant storm sometime
late in the week or early next week, with another prolonged
period of active weather possible beginning this weekend and
continuing into next week. Considering the recent wet conditions
and significant widespread rainfall Monday into Tuesday, flooding
may once again become a concern for much of the region should
periods of moderate to heavy rain return as suggested by over half
of ensemble guidance. Snow levels will likely be high on the front
end of the active weather, but may fall to between 5000 and 7000
feet by Sunday or Monday. The cold air aloft and proximity of
upper lows to the region may also support isolated thunderstorms
at times. Gusty and potentially damaging winds onshore to
northwest winds may be possible at times, depending on exact
storm(s) evolution.

&&

.AVIATION...09/0630Z.

At 0455Z at KLAX, there was no marine layer nor any inversion.

Moderate confidence in TAFs through 12Z then lower confidence due
to uncertainty with rain timing and lower cig possibilities. MVFR
cigs may arrive 2 hours later than fcst. Low confidence in rain
forecast. LA county...steady rain may hold until 14Z, although
short periods of rain are possible at any site at any time.

KLAX...Moderate confidence in TAF. There is a 20% chance of BKN025
conds 06Z-09Z. There is a 30 percent chc that BKN025 conds will
not arrive until 11Z. There is a 30 percent chc that steady rain
will not develop until 14Z. There is a 30 percent chc of a 10 kt
east wind component 08Z-14Z.

KBUR...Moderate confidence in TAF. There is a 20% chance of BKN025
conds 06Z-09Z. There is a 30 percent chc that BKN025 conds will
not arrive until 11Z. There is a 30 percent chc that steady rain
will not develop until 14Z.

&&

.MARINE...08/1036 PM.

For the Outer Waters, moderate to high confidence in current
forecast. For PZZ670/673, high confidence in Gale force winds
this evening through Monday evening. For Monday night through
Wednesday, high confidence in SCA level winds and seas (with a 30%
chance of Gale force winds on Tuesday). On Thursday, high
confidence in SCA level seas with a 50% chance of SCA level winds.
For PZZ676, high confidence in SCA level winds and/or seas from
this afternoon through later tonight. A Gale Warning has been
posted for Monday, mainly for the northwest half of the zone where
frequent gusts to 35 knots will occur most of Monday. Winds are
expected to subside to SCA levels Monday night and continue into
Tuesday. For Wednesday and Thursday, high confidence in SCA level
seas with winds remaining below SCA levels.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. High confidence in SCA level
winds and seas tonight. For Monday and Monday night, high
confidence in Gale force winds and SCA level seas. On Tuesday and
Tuesday night, high confidence in SCA level winds/seas (with a 30%
chance of Gale force winds on Tuesday). For Wednesday and
Thursday, high confidence in SCA level seas with winds remaining
below SCA levels.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Gale force winds appear likely
for the East Santa Barbara Channel on Monday. Therefore, the Gale
Watch was converted to a GALE WARNING, in effect from 9 AM to 6
PM Monday. Elsewhere, a SCA has been issued for zone 655 from
Monday afternoon through Tuesday evening with a 20-30% chance of
Gale force winds Monday and Monday night. Should wind gusts reach
gale force, they will likely be short-lived. For all the southern
Inner Waters, high confidence in winds/seas remaining below SCA
levels Wednesday and Thursday.

For all the coastal waters, there is a threat of thunderstorms
developing Monday night and Tuesday. Any thunderstorms that
develop will be capable of producing gusty outflow winds, rough
seas, heavy rainfall, dangerous cloud to surface lightning, and
possible waterspouts.

&&

.BEACHES...08/1027 PM.

The High Surf Advisory for both the Central Coast and the Ventura
Coast through Tuesday looks on track. However, will likely need to
extend this as high surf should continue into the weekend. Los
Angeles County beaches and Santa Barbara county south coast will
continue to see elevated surf of 3 to 6 feet with strong rip
currents through Monday, resulting in the Beach Hazards Statement.
However, later shifts may need to update LA County if morning surf
observations are higher.

The train of strong storms continues on Monday night into Tuesday
at least, and High Surf Advisories will likely (60-70 percent
chance) for the entire coastline into next weekend. As a matter
of fact, the highest waves will impact west and northwest facing
beaches from Wednesday through next Saturday.

For more details about the High Surf Advisories, please refer to
the Coastal Flood Advisory product (LAXCFWLOX). Stay tuned to
National Weather Service forecasts during this very active
situation.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Wind Warning in effect until 4 PM PST Tuesday for zones
      53-353. (See LAXNPWLOX).
     Flood Watch in effect from 9 AM PST Monday through Tuesday
      afternoon for zones 53-354>359. (See LAXFFALOX).
     High Wind Warning in effect from 10 AM Monday to 7 PM PST
      Tuesday for zones 54-59. (See LAXNPWLOX).
     Flood Watch in effect from Monday evening through Tuesday
      evening for zones 54-59-88-362>365-547-548. (See LAXFFALOX).
     Beach Hazards Statement in effect through Monday afternoon
      for zones 87-349-350-362-364. (See LAXCFWLOX).
     Wind Advisory in effect from 10 AM to 10 PM PST Monday for
      zones 88-349-350-354>359-362>365-547-548. (See LAXNPWLOX).
     High Surf Advisory in effect until 4 PM PST Tuesday for zones
      340-346-354. (See LAXCFWLOX).
     High Wind Warning in effect until 10 PM PST Monday for zones
      340>348-351-352. (See LAXNPWLOX).
     Flood Watch in effect from 5 AM PST Monday through Monday
      afternoon for zones 340>345. (See LAXFFALOX).
     Flood Watch in effect from 7 AM PST Monday through Tuesday
      morning for zones 346>353. (See LAXFFALOX).
PZ...Small Craft Advisory in effect until 1 AM PST Monday for zone
      645. (See LAXMWWLOX).
     Gale Warning in effect until 9 PM PST Monday for zones
      645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PST Monday for zone
      650. (See LAXMWWLOX).
     Gale Warning in effect from 9 AM to 6 PM PST Monday for zone
      650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM Monday to 7 PM PST
      Tuesday for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Monday for zone
      676. (See LAXMWWLOX).
     Gale Warning in effect from 3 AM to 3 PM PST Monday for zone
      676. (See LAXMWWLOX).

&&

PUBLIC...Hall/Rorke
AVIATION...Rorke
MARINE...Phillips/Sweet
BEACHES...Phillips/Sweet
SYNOPSIS...Hall

weather.gov/losangeles

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