Winter Storm in California, December 30, 2021

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
946 AM PST Thu Dec 30 2021

.SYNOPSIS...30/220 AM.

A strong storm system will affect most of the region through
this evening. It will bring rain, wind, and low elevation mountain
snow to the area. The system will have much less effect across the
Central Coast. Temperatures will remain significantly below
average. Dry weather is expected Friday through the upcoming
holiday weekend and likely beyond.

&&

.SHORT TERM (TDY-SAT)...30/944 AM.

Moderate to heavy rain is widespread through Ventura and Los
Angeles Counties at this hour. Very impressive totals so far with
24 hour totals between 3 and 5 inches in the Santa Monicas
and surrounded by an area of 2 to 3 inch amounts. There's another
max axis of 2 to 6 inch amounts from around Santa Monica NE through
eastern San Fernando and western San Gabriel Valleys and into the
Bobcat burn area. Ventura County will also likely get quite a bit
of additional rainfall. SBA County could get some showers move by on
the fringe of the main precipitation axis today but with low rates,
while SLO County is pretty much done for this event. So far peak
one hour rates have been between 0.50 and 0.80 inches. More
impressive are the peak 30 minute rates of around 0.50 inches and
15 minute rates of 0.25 inches. As long as we don't see too much
of an increase in these rates then this system will turn out to be
more beneficial than impactful.

The main upper low has closed off about 200 miles SW of Point
Conception and slipping southward. There remains a steady
southwesterly stream of moisture and dynamics on its southern side
and into LA and VTU Counties for most of today. Orographics are
also an added factor. Then as the low continues to move further
south and even southwest away from the area and the southwest flow
over us goes away, those contributing factors will end as will
the precipitation. Models and their ensembles depict this as model
reflectivities gradually shrink between 4 and 6 pm, and rather
than the area translating eastward. System will therefore need to
be closely monitored through day and possibly early evening,
especially in LA County and parts of Ventura County where an
additional 2 to 4 inches of rainfall is possible. Satellite
imagery still showing active cells moving northeast towards the
coast. All considered, there's still the risk of higher rates and
the potential for mud and debris flows from the recent burn areas
of LA County. Thus foresee leaving the flash flood watch for these
areas going through its scheduled expiration time of 4 pm this
afternoon.

There's also been some decent snow totals coming in this morning
including up to a foot and a half in the eastern San Gabriels,
about 3 to 6 inches in the Frazier Park area. More important, and
with impact, has been some accumulation on the Grapevine with
pretty efficient snow processes as Sandberg is reporting 35
degrees on the last observation. Short range ensembles gradually
end snow in the VTU mountains and the Grapevine later this
morning or early afternoon. The WSW for that area is up until 2
pm and will likely leave it until that time.

Next up will be the wind, first with gusty northwest winds from
the Grapevine west into the Santa Barbara south coast starting
tomorrow and continuing into tomorrow night. Wind advisories will
eventually be needed for these areas. These northerly winds will
also pile some moisture up on the north slopes of the VTU County
mountains and the Grapevine which could bleed over a bit into our
area. Dry conditions will prevail on Friday for sunny skies
elsewhere across the area and a very weak warmup, although still
plenty below normal for this time of year.

*From previous discussion***

2022 will dawn sunny as a weak ridge moves into the state. A weak
Santa Ana will develop as offshore flow continues from the north
and develops from the east. It does not look like a big event with
only localized advisory gusts in the most wind prone areas.
Unfortunately the offshore flow will bring in enough cool
interior air that there will be no warm up.

.LONG TERM (SUN-WED)...30/330 AM.

Both the GFS and EC deterministic mdls as well as their respective
ensembles show a cold 505 dam upper low moving out of the Gulf of
Alaska. This will push the ridge out of the state and set up fast
moving WSW flow over the state. The energy from the system will
push slowly south and will bring a slight chc of rain to SLO
county of Tuesday. But other than that it will be dry. Skies will
likely be partly cloudy as there will be plenty of moisture
embedded in the flow aloft. Hgts will rise some and max temps
will rise into the 60s but will still remain several degrees blo
normal through the period.

&&

.AVIATION...30/1156Z.

At 12Z at KLAX, there was no marine layer. There was a 10000 ft
deep moist layer.

Low confidence in TAFs. Cigs and vis will vary frequently. LIFR
conds are unlikely at most TAF sites and if they do occur they
will not persist for more than 30 minutes. Heaviest rain, and
likely worst flight conditions, will occur across Ventura and LA
counties through Thursday 18Z. KPRB has a 30% chance of VLIFR
conds thru Thursday 18Z.

KLAX...Low confidence in TAF. Cigs and vis will vary frequently.
There is a 20 percent chc of brief BKN004 +RA conds thru 16Z.
Improving conditions are likely after 00Z. A persistent east to
southeast wind will continue through 20Z with speeds 10-15 knots
likely. Then, moderate confidence in east wind component of 7 kt
or less.

KBUR...Low confidence in TAF. Cigs and vis will vary frequently.
There is a 20 percent chc of brief BKN004 +RA conds thru 16Z.
Improving conditions are likely after 00Z.

&&

.MARINE...30/842 AM.

A slowly moving storm system will continue to generate southeast
winds over 20 knots over the nearshore waters south of Point
Conception through most of today, strongest around the San Pedro
Channel. Small Craft Advisories (SCA) have been extended as
result. These winds will generate locally steep seas at very short
periods. There is a very slight chance for thunderstorms today
south of Point Conception, but the chances are dropping.

High confidence that winds will turn to northwesterly tonight,
reaching SCY from offshore of the Central Coast to San Nicolas
Island by Friday morning. There is a 40 percent chance of winds
reaching low-end Gales Friday afternoon and evening. These winds
will also impact the Santa Barbara Channel, with a 40 percent
chance of reaching SCY Friday afternoon and evening. Same goes for
the waters from Point Mugu to Catalina Island. Expect widespread
choppy seas at short periods with these winds. The winds will turn
northerly later Friday night, with a chance of 20 knot winds
south of Malibu. Northeast Santa Ana winds will follow Saturday
(New Years Day) and Sunday, with SCA gusts 20-30 knots likely
from Ventura to Santa Monica. These winds will also reach Avalon,
with a 20 percent chance of exceeding 20 knots.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning in effect until 2 PM PST this afternoon
      for zone 53. (See LAXWSWLOX).
     Winter Storm Warning in effect until 8 PM PST this evening
      for zone 54. (See LAXWSWLOX).
     Flash Flood Watch in effect until 4 PM PST this afternoon for
      zones 54-59. (See LAXFFALOX).
PZ...Small Craft Advisory in effect until 2 PM PST this afternoon
      for zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 6 PM this evening to 6 AM
      PST Friday for zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SAT-WED).

No significant hazards expected.

&&

PUBLIC...Jackson/Rorke
AVIATION...Rorke/Lund
MARINE...Kittell
SYNOPSIS...Rorke

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