Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 090410
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
810 PM PST Mon Mar 8 2021

.SYNOPSIS...08/227 PM.

A cooler air mass will remain in place through Tuesday due to
onshore flow, then a cold storm system will dig south into the
region between late Tuesday and Thursday night. Rain and low
elevation snow is expected for the region, along with gusty winds
at times. Drier weather and a modest warming trend is expected
for next weekend as high pressure builds in.

&&

.SHORT TERM (MON-THU)...08/807 PM.

Latest satellite imagery showing a mixture of high and low level
clouds across the region this evening. Wind advisory will be
allowed to expire at 9 pm for the Antelope Valley, while the
advisory for southern Santa Barbara county continues until 3 am
Tuesday, where wind gusts of 35 to 45 mph will be common.

A cold upper-level low pressure system centered off the coast of
northern Oregon will dig slowly south-southeast into the region
through Thursday. Rain chances will begin across the Central Coast
by Tuesday afternoon, then spreading into the LA basin later
Tuesday night. The bulk of the precipitation with this system is
expected to occur from Tuesday night through Thursday. As
discussed earlier, this system is associated with some very cold
air aloft with 500 mb temperatures falling as low as -34 degrees
Celsius. The very cold air aloft and associated instability in the
atmosphere combined with some jet dynamics will bring the
potential for convective showers and isolated thunderstorms,
capable of producing brief heavy downpours and small hail. Model
soundings showing shallow based convection with surface based
CAPE`s in the 200-400J/kg range. In fact, this is the type of
pattern that is favorable for accumulating small hail and isolated
waterspouts in heavier storms. Due to the showery/convective
nature of this system, rainfall amounts are expected to be widely
varying. Current projections still look on track for most areas to
see between 0.50 and 1.00 inches, with local amounts up to 1.50
inches in the foothills/mountains. Current snow levels and snow
amounts in the forecast look on track, with only modification
being upgrade of Winter Storm Watches to Winter Storm Warnings
for the mountains of Los Angeles, Ventura, and Santa Barbara
counties starting at 10 pm Tuesday. As the snow levels fall to
2500 to 3000 feet in the Wednesday-Thursday time frame, low
elevation mountain passes could see some significant travel
impacts, especially Interstate 5 near Tejon Pass, and the Highway
14 corridor through Soledad Canyons. In addition, there will be
the potential for small accumulations of snow in the foothills of
the Antelope Valley and Cuyama Valley, which may requite winter
weather advisories as we draw closer.

*** From previous discussion ***

A cold frontal boundary will move over the region
between Tuesday night and Wednesday and bring should bring a
period of steady stratiform rainfall, then the colder and more
unstable air aloft will punch in behind it, and the storm system
will turn convective, or showery in nature. Snow levels will drop
quickly to between 3000 and 3500 feet behind the frontal boundary,
then drop lower as a colder air mass is ushered in on Wednesday
night and Thursday. 700 mb temperatures drop to
-12 degrees Celsius on Wednesday night which will lower the snow
levels to between 2500 and 3000 feet. Local snow levels could drop
to around 2000 feet in heavier showers, possibly bring a mix of
rain and snow, small hail, or graupel to the foothill areas of
southern California, such as the Canyon Country in the Santa
Clarita Valley and Tujunga and La Canada Flintridge in the San
Fernando Valley areas. A dusting of snow is possible on the San
Marcos and Cuesta Passes with heavier showers as the coldest air
punches in.

500 mb temperatures will likely drop to between -25 and -34
degrees Celsius as the cold air mass spreads in. There is a
possibility of isolated thunderstorms. The best time for any
convective showers looks to be Wednesday night through Thursday
night, when a -34 degree 500 mb cold pocket digs south across the
area. Brief heavy rainfall, possibly mixing with hail or graupel
is possible during this time along with dangerous cloud-to-ground
lightning.

The low snow levels will very likely affect travel throughout the
mountains of southern California, such as the Interstate 5
Corridor between Los Angeles and Kern Counties and the portion of
Highway 14 between the Santa Clarita Valley and the Antelope
Valley. Travel should be avoided on mountain roadways, if
possible, between Tuesday night and Friday morning. Motorists
needing to travel across mountain roadways are advised to prepare
for treacherous wintry driving conditions. Prepare a proper
winter safety kit, bring extra food, clothes, and blankets, and
have snow chains for your vehicle. A Winter Storm Watch remains in
effect between Tuesday night and Thursday night as heavy snow
with dangerous travel conditions is likely with this storm system.
Blowing and drifting snow is possible as gusty winds are expected
with this storm system, especially along the frontal boundary
between Tuesday night and Wednesday and across the interior
portions of the area at times.

.LONG TERM (FRI-MON)...08/227 PM.

The storm system will linger into Thursday and Thursday night,
keeping showers in the forecast. ECMWF solutions continue to hold
to the possibility of wraparound moisture, possibly lingering
shower activity into Friday morning.

A warming and drying trend is forecast over the weekend as high
pressure aloft builds in over the region, then model solutions are
hinting at a possibility of returning a windy pattern with
trough`s brushing the area.

&&

.AVIATION...08/2332Z.

At 2315Z, there was a moist layer up to 6000 feet at KLAX.

Overall, moderate confidence in 00Z TAF package. Overnight,
there is a 30% chance of CIG/VSBY restrictions at coastal and
valley sites south of Point Conception. Moderate confidence
in MVFR CIGs at KPRB/KSMX overnight. For Tuesday, expect
increasing clouds ahead of cold front with a chance of light
rain north of Point Conception after 21Z.

KLAX...Moderate confidence in 00Z TAF. There is a 30% chance of
MVFR CIGs 08Z-18Z. No significant easterly winds are anticipated.
However, gusty west winds can be expected after 20Z.

KBUR...Moderate confidence in 00Z TAF. There is a 30% chance of
MVFR CIGs 08Z-18Z. Southerly winds Tuesday afternoon will be
gustier than usual (gusts to 20 knots possible).

&&

.MARINE...08/747 PM.

For the Outer Waters, high confidence in current forecast. A
combination of Small Craft Advisory (SCA) level winds and seas
will continue through Tuesday evening. For Wednesday through
Friday, there is a 60-70% that SCA level winds and seas will
continue.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. High confidence in SCA level seas
continuing through Tuesday evening. On Wednesday, there is a 50%
chance of SCA level seas continuing. For Thursday through
Saturday, high confidence in winds and seas remaining below SCA
levels.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Through Tuesday evening, high
confidence in SCA level winds with the strongest winds during the
afternoon and evening hours. For Wednesday through Saturday,
generally high confidence in winds and seas remaining below SCA
levels with a 30% chance of SCA level winds across the western
half of the Santa Barbara Channel Thursday and Friday.

A moderately strong storm will move through the coastal waters
Tuesday through Thursday bringing a chance of rain and slight
chance for thunderstorms.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 9 PM PST Thursday for
      zones 34-35. (See LAXCFWLOX).
     Wind Advisory in effect until 3 AM PST Tuesday for zones
      39-52. (See LAXNPWLOX).
     Winter Storm Warning in effect from 10 PM Tuesday to 10 PM
      PST Thursday for zones 52>54. (See LAXWSWLOX).
     Wind Advisory in effect until 9 PM PST this evening for zone
      59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 9 PM PST Tuesday for
      zones 645-650-655-670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

High surf and dangerous rip currents are likely through Thursday.
Gusty west to northwest winds will create driving and boating
hazards at times through Thursday. A winter storm will bring rain
and low elevation snow and isolated thunderstorm with brief heavy
downpours to the area through Thursday night.

&&

$$

PUBLIC...Gomberg/Hall
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Hall

weather.gov/losangeles


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