Duarte Weather

000
FXUS66 KLOX 191822
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1022 AM PST Tue Nov 19 2019

.SYNOPSIS...19/828 AM.

An upper level low pressure system approaching the California
Coast will bring significantly cooler conditions today. Then,
periods of rain and mountain snow will be likely tonight through
Thursday. Drier and warmer conditions will return to the area
going into the weekend.

&&

.SHORT TERM (TDY-THU)...19/920 AM.

Two upper lows are moving towards our area, one from the north and
one from the south. The southern system generated some
thunderstorms over nrn Baja this morning but those dissipated
and/or shifted east as they moved into more stable air to the
north. The key word for today`s weather is cooler as west/east
gradients are trending strongly onshore and colder air aloft moves
in with the upper low to the north. It no longer looks like we`ll
see any precip in LA County today as it`s going to take another
12-18 hours for the air mass to moisten up and for the cooler air
aloft to destabilize things enough to generate rain.

Northerly winds are expected to increase across the western
portion of the forecast area as the low moves south. This will
generate some gusty winds in parts of SLO/SB Counties, strongest
in and around the western Santa Ynez Range this evening, possibly
strong enough for low end advisory level winds in the Gaviota
area.

Remainder of the forecast looks more or less on track, though it`s
a very tricky pattern with a mixture of northerly winds,
cold/unstable air aloft, and some uncertainty on how much moisture
the system will entrain from the upper low to the south. Best
chances for precip still in LA County as there is less of a
northerly downslope component, especially the southeastern part of
the county. High confidence in at least some precip falling but
low confidence on timing and amounts. Likely quite variable and
mostly on the benign side but with the potential for some locally
heavy precip from thunderstorms.

***From previous discussion***

This evening the upper trof to the north will develop a cut off
low at its base. This upper low will be over Pt Conception at 12Z.
This low will bring increasing PVA LA and parts of VTA county this
evening and overnight and there will be an increasing chance of
showers through the period. As the cold core of the upper low
moves down through SLO and into SBA counties it will bring a chc
of instability showers to those areas after midnight.

The best chc for rain showers will be Wednesday as the upper low
swings through SBA VTA and then LA counties during the morning and
afternoon. SLO county and western SBA county will largely be left
out of this rain event. The best PVA and upper level divergence
will occur over LA county and the areas coast side of the mtns
will see the greatest amounts of rain. The core of the low has 500
mb temperatures of -24 degrees C, which will destabilize the
atmosphere as it rolls over the area. LI`s will drop to minus 2 to
minus 4. If convection does kick off CAPE values of 200 to 500
J/kg will support decent convection and a slight chance of
thunderstorms looks good.

Snow levels will drop to around 6,000 feet over Ventura and Los
Angeles Counties. Up to 6 inches of snow is possible for the Los
Angeles County mountains and about half that amount in the
Ventura County mountains. The eastern San Gabriel mountains will
see the highest amounts. A Winter Storm Advisory may be needed to
handle this snow.

Rainfall amount estimates call for less than a tenth of an inch
across SLO and most of SBA counties. Up to a quarter inch of rain
is possible for VTA county. The LA county coast from the coastal
slopes to the coast will likely8 recieve a quarter to a half inch
of rain. The eastern San Gabriel foothills will see the most rain
and nearly an inch of rain is possible.

It will be cool day for sure with 546 DM hgts, clouds and showers
all conspiring keep max temps down. Max temps across all of the
coasts and vlys will struggle to reach the mid 60s.

The upper low exits the area Wednesday evening and will leave
clearing skies and only a diminishing chc of a shower or two over
LA county.

Dry NW flow sets up Thursday behind the trof. Hgts will rise some
and skies will be partly cloudy at worst. temperatures will
remain below normal on Thursday under the trough and clouds. Max
temps will warm 2 to 4 degrees but will remain below normal.

.LONG TERM (FRI-MON)...19/206 AM.

The GFS and EC are not in good agreement at all for the extended
period. The GFS has a brand new solution for Friday with a cool
upper low moving into the area to the west of Pt Conception while
the EC has a weak ridge moving into the state. Have discounted the
abrupt change indicated by the GFS and used the EC soln which
calls for slightly warmer temps and mostly clear to partly cloudy
skies.

The mdl disagreement will not affect the weekend forecast that
much. Both mdls agree that the state will be under some sort of
cull area with week to moderate offshore flow. The offshore push
will keep the low clouds away, but skies will be mostly clear or
partly cloudy depending on if the cloudier GFS verifies or clearer
EC. The EC forecast warmer temps on Sat then cooler on Sun while
the GFS says the exact opposite. Took a blended approach to the
max temps for both days with max temps rising both days with the
biggest warm up on Saturday.

Bigger differences for the Monday forecast with the GFS
advertising a sharp inside slider while the EC keeps the state on
the backside of broad CONUS spanning trof. The ensembles favor the
EC and tilted the forecast in that direction with just slightly
cooler temps.

&&

.AVIATION...19/1822Z.

At 1653Z, there was a surface-based inversion at KLAX. The top of
the inversion was 1500 feet with a temperature of 23 degrees
Celsius.

Moderate confidence in 18Z TAF package. Mostly VFR conditions are
expected through today. For tonight, moderate confidence in
timing of VCSH/SHRA development south of Point Conception,
although the latest model data suggests a slight delay in the
onset of precipitation.

There is a slight chance of TSTMs this evening through Wednesday
morning across Ventura and Los Angeles counties.

KLAX...Moderate confidence in 18Z TAF. Good confidence in VFR
conditions through this evening. For tonight/Wednesday morning,
good confidence in development of -SHRA, but low confidence in
timing of -SHRA and associated MVFR CIGs. There is a 15% chance of
a TSTM 10Z-18Z. Easterly wind component this morning is expected
to remain 5 knots or less.

KBUR...Moderate confidence in 18Z TAF. Good confidence in VFR
conditions through this evening. For tonight, good confidence in
development of -SHRA, but low confidence in timing and associated
CIG category.

&&

.MARINE...19/833 AM.

For the Outer Waters, low confidence in current forecast. There
is a 50% chance of Gale force gusts late this afternoon and
tonight. In fact, the 12z models were showing stronger winds in
the outer waters, so decided to raise a GALE WARNING for this
afternoon and tonight. On Wednesday, SCA level winds are expected
through the day. For Thursday through Saturday, winds and seas
are expected to remain below SCA levels.

For the Inner Waters north of Point Sal, moderate confidence in
current forecast. SCA level winds and seas are expected this
afternoon through tonight. For Wednesday through Saturday, winds
are expected to remain below SCA levels with a 30% chance of SCA
level seas continuing through Wednesday.

For the Inner Waters south of Point Conception, moderate
confidence in current forecast. There is a 70% chance of SCA level
winds this afternoon through Wednesday morning across the western
half of the Southern California Bight. On Wednesday, there is a
30 percent chance for SCA level winds across western sections. For
Thursday through Saturday, high confidence in winds and seas
remaining below SCA levels.

There is a slight chance of thunderstorms south of Point
Conception tonight through Wednesday afternoon. Thunderstorms will
be capable of producing locally rough seas, brief heavy rain,
gusty/erratic winds and frequent lightning.

&&

.BEACHES...19/843 AM.

A cold storm system will move over the region between tonight
through Thursday. As this storm moves through, it will generate
high surf conditions along the coast from this evening through
Thursday morning. For the Central Coast, peak surf heights between
7 and 10 feet are expected while surf height of 4-8 feet are
expected south of Point Conception.

A HIGH SURF ADVISORY is in effect for the Central Coast as well as
the coasts of Ventura and Los Angeles counties from this evening
through Thursday morning. For west-facing beaches along southern
Santa Barbara County, a BEACH HAZARDS STATEMENT is in effect from
this evening through Thursday morning. Along with the high surf
conditions, strong and dangerous rip currents can be expected.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect from 6 PM this evening to noon
      PST Thursday for zones 34-35. (See LAXCFWLOX).
     Beach Hazards Statement in effect from 9 PM PST this evening
      through Thursday morning for zone 39. (See LAXCFWLOX).
     High Surf Advisory in effect from 9 PM this evening to noon
      PST Thursday for zones 40-41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PST Wednesday for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect from 6 PM this evening to 9 AM
      PST Wednesday for zones 650-655. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PST Wednesday for zones
      670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

A cooler and showery weather pattern is expected Wednesday
through Friday. There is a slight chance of brief heavy downpours
in thunderstorms, especially Wednesday to Thursday morning. High
surf is possible at west-facing beaches late Wednesday through
early Thursday.

&&

$$

PUBLIC...MW/Rorke
AVIATION...Sweet/RAT
MARINE...Sweet/RAT
BEACHES...Sweet/RAT
SYNOPSIS...Smith/Delerme

weather.gov/losangeles

NWS LOX Office Area Forecast Discussion