670 
FXUS66 KLOX 240422
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA 
822 PM PST Sun Dec 23 2018

.SYNOPSIS...23/718 PM.

A developing low pressure system will bring increasing clouds to
the area on Monday, along with strong northerly winds, high surf,
and light precipitation, including mountain snow showers. Cooler
conditions are expected starting Tuesday, with below normal
temperatures through late in the week. 

&&

.SHORT TERM (SUN-WED)...23/813 PM.

***UPDATE*** 

Relatively quiet weather pattern this evening, with little change
noted since this time yesterday. Locally gusty northwest to 
north winds are affecting mainly the mountains, slightly weaker 
than last night but still strongest through the passes and canyons
of the SBA South Coast. High clouds continue to stream over the 
area in strong northerly flow aloft, preceding a storm system that
is affecting northern California and the Pacific Northwest. Winds
are expected to shift to the north to northeast later tonight, and
offshore gradients are expected to strengthen slightly after
midnight. The continued weak offshore flow on Monday will bring
another mild day to the area, but with clouds increasing at mid to
lower levels from the northwest during the day as a low pressure
system approaches. This storm is expected to bring mainly light
precipitation to areas north of Point Conception Monday evening,
then spreading southeast across the area Monday night into
Christmas Day. 
 

***From Previous Discussion***

The latest computer model runs have trended slightly further west
with the upper low that is expected to dive southward closer to
our forecast area later Monday night into Christmas morning. The 
track of this weather system will be critical for the precipitation
and wind forecasts. As it stands now, this slightly further west 
track of the system will result in a slight increase in over-water
trajectory. And perhaps most importantly, colder upper level 
temperatures (-23 to -25 degrees Celsius at 500 mb level) could 
result in a more unstable environment, thereby increasing the 
threat of showers. The latest NAM/GFS models do indicate some 
instability, especially over the coastal waters on Monday night. 
Have not introduced any thunderstorms at this time, but if this 
track materializes, there could be a threat of shower intensity be
slightly stronger than earlier thinking. 

At this time, still projecting highest probabilities of rainfall 
with this system for areas north of Point Conception, the north 
facing mountain slopes, and eastern LA county southward to San 
Diego county. While much of LA/Ventura county coast/valleys will 
be in more of a rain shadow with this northwest flow pattern, 
there appears to be enough dynamics and colder air aloft to warrant
20-40 percent pops for these areas. Storm totals are generally 
expected to remain below one quarter inch in most areas, with 
highest QPF values expected across areas north of Point Conception
and north facing mountain slopes. Local amounts greater than one 
quarter inch will be possible across the north facing slopes as 
well as any areas that see isolated heavier showers. 

The most significant aspect of this system will likely be strong 
northwest to north winds. Initially the flow will be out of the 
northwest on Monday night and early Christmas morning with wind 
advisories likely needed as we draw closer to the event. Then the 
strongest winds are expected from the north by late Tuesday 
morning through Tuesday evening as the closed low digs 
southeastward into the deserts. At this time, the strongest upper 
level northerly wind support and cold air advection are focusing 
over southern Santa Barbara county, with NAM model 500 mb winds 
projected to be over 100 knots, and 850-700 mb winds peaking 
between 50-60 knots. As a result, this type of pattern has the 
potential to bring damaging wind gusts up to 60 mph across the 
Santa Ynez Range and foothills of the SBA south coast, resulting 
in the issuance of a high wind watch from 8 am Tuesday to 10 pm 
Tuesday. Many other portions of Southwest California will likely 
see wind gusts in the 35 to 55 mph range on Christmas afternoon 
and evening which will likely result in several wind advisories 
being issued as we draw closer to the event. There is still some 
uncertainty in the timing and magnitude of this wind event due to 
the uncertainty in the track of the upper low.

Another threat from this system will be lowering snow levels. The
strong north flow will generate upslope showers late Monday night
into Christmas Day on north facing slopes. With the proximity of 
the cold core aloft, have lowered snow levels a bit more in 
current forecast, as low as 4000 feet by Christmas morning on the 
north facing slopes. This could result in a dusting of snow on 
Interstate 5 near the Grapevine on Christmas Day. Snow 
accumulations are expected to range between 1 and 3 inches with 
this system, mainly on north facing slopes. The snow combined with
strong winds could create reduced visibilities in blowing snow 
and winter weather advisories may need to be posted as we draw 
closer to the event. Motorists planning to travel along Interstate
5 for the holidays should check the latest weather forecasts and 
road conditions as snow and strong winds may impact this busy 
travel period.

.LONG TERM (THU-SUN)...23/242 PM.

Dry weather is in store for Southern California Thursday through 
Sunday. There will be dry northerly flow overhead. The northerly 
flow will keep the north canyon winds going through the period and
the I-5 corridor may see a few periods of advisory level winds. 
This flow pattern will advect quite a bit of cool air into the 
area from the interior and this will keep max temps a few degrees 
below normal through the period. There is the potential for
a stronger offshore wind event by next weekend.

&&

.AVIATION...24/0006Z.

At 2300Z at KLAX...the marine inversion was around 1200 ft deep. 
The top of the inversion was near 1800 ft with a temperature of 
17 deg C.

Moderate to high confidence in the 00Z TAFs. Lower confidence in LIFR/IFR
conditions for KPRB, KLAX and KLGB overnight into Monday morning.
The timing of the onset and dissipation of the low clouds may be 
off +/- an hour or two. There is also a 20%-30% chance that low
clouds do not form at these sites, or that there are periodic
reductions through the overnight hours. 

KLAX...Moderate confidence in the 00Z TAF. IFR/LIFR conditions are
most likely between 08z-16z, but may occur as early as 03z. East
winds are expected to remain below 10 kt. 

KBUR...High confidence in the 00Z TAF with VFR conditions expected
through Monday.

&&

.MARINE...23/813 PM.

For the Outer Waters, moderate to high confidence in the current 
forecast. Small Craft Advisory (SCA) level wind gusts can be 
expected for much of these waters through Monday afternoon. Winds
are expected to increase further late Monday night through 
Tuesday night, and a Gale Watch is in effect for that time. There 
is an 80% chance that the Gale Watch will be upgraded to a Gale 
Warning. Steep short period wind waves will prevail along with a 
large NW swell, with combined seas forecast to build up to 20 ft 
on Tuesday. There will then be a 50%-60% chance that SCA level 
winds and seas will persist Wednesday through Friday.

For the Inner Waters north of Point Sal, moderate confidence in 
the current forecast. Winds and seas will remain below SCA levels 
through Monday evening. There is a 60% chance of Gale force winds
and for late Monday night through late Tuesday evening, along 
with steep combined seas. A Gale Watch is in effect for that time. 
There is a 40% chance of SCA level wind gusts on Thu, otherwise 
winds and seas should remain below SCA levels Wednesday through 
Friday.

For the Inner Waters south of Point Conception, moderate to high 
confidence in the current forecast. Winds and seas will remain 
below SCA levels through Monday. There is a 60%-70% chance of
Gale Force winds Monday night through late Tuesday evening, with a
Gale Watch in effect for that time period. Building short period 
seas can be expected during the period as well. Conditions should 
be below SCA levels on Wed, then there is a 30%-40% chance of SCA 
level wind gusts on Thu and Fri.

&&

.BEACHES...23/813 PM.

A northwest swell building into the northern coastal waters will
bring high surf to the Central Coast beaches this evening through
Monday with surf building to 8 to 12 feet with sets up to 14 feet.  
Another long period west to northwest swell will move into the 
waters Monday night into Tuesday night, bringing larger surf to 
the Central Coast, with wave heights approaching High Surf Warning
levels. Breakers of 10 to 15 feet with peak sets up to 18 feet are
expected through Wednesday afternoon. 

This swell will also bring high surf conditions to beaches south 
of Point Conception starting Tuesday and continuing through 
Wednesday evening. Surf of 5 to 8 feet with sets to 10 feet is
expected on west facing beaches, mainly in L.A. and Ventura
Counties. Along the SBA South Coast, local sets up to 7 feet are
expected, with a 40% chance that surf advisory conditions are met.
Strong rip currents are expected on all beaches, with a risk of
sneaker waves and beach erosion. 

High tides are expected during the mid-morning to noon each day.
The combination of high tides and large waves will bring a risk of
minor to moderate coastal flooding during the time of peak surf,
Tuesday and Wednesday mornings.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 2 PM PST Wednesday for
      zones 34-35. (See LAXCFWLOX).
     Beach Hazards Statement in effect from Tuesday morning
      through Wednesday evening for zones 39>41-87. (See LAXCFWLOX).
     High Wind Watch in effect from Tuesday morning through
      Tuesday evening for zones 39-52. (See LAXNPWLOX).
PZ...Gale Watch in effect from Monday evening through Tuesday
      evening for zones 645-650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Monday for zone
      670. (See LAXMWWLOX).
     Gale Watch in effect from Monday evening through late Tuesday
      night for zones 670-673-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 10 PM PST Monday for
      zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

There is the potential for strong and damaging northerly winds on
Christmas Day across portions of Southwest California, especially
in the mountains and Santa Barbara south coast. High surf 
conditions and strong rip currents could impact the Central Coast 
much of this week, as well as the west facing beaches of 
LA/Ventura counties on Monday night and Tuesday. There is the
potential for a stronger offshore wind event by next weekend.

&&

$$

PUBLIC...Gomberg
AVIATION...Smith
MARINE...Smith
BEACHES...Smith
SYNOPSIS...TF

weather.gov/losangeles