Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 161559
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
759 AM PST Tue Jan 16 2018

Updated Aviation and Marine Sections

.SYNOPSIS...
An approaching upper level trough from the northwest of the area
will increase the onshore flow for more clouds and cooler
temperatures today. A storm system will spread the possibility of
precipitation from the northwest Thursday afternoon and evening,
then spreading into all areas by early Friday. A few lingering
showers and low snow levels expected through early Saturday. Cool
and dry conditions expected through the remainder of the weekend.
Potential for periods of additional rain early next week.

&&

.SHORT TERM...(TDY-THU)

Latest GOES-16 satellite imagery indicated plenty of high clouds
moving through the region this morning. Below the high clouds,
some patchy low clouds were occurring. However, with the
approaching upper trough to the north, the inversion was a bit
deeper and not much in the way of dense fog reports across coastal
areas. There will be some patchy dense fog showing up in some of
the coastal valley locations early this morning, most likely for
the Santa Ynez and Ventura County Valleys as well as the Salinas
River Valley through mid morning hours. Mostly cloudy skies will
become partly to mostly cloudy by this afternoon.

Synoptically, zonal aloft will persist across the region today
with only slightly stronger onshore flow expected. Therefore high
temps should be a degree or two cooler compared to yesterday with
more cooling expected across some interior areas such as SLO and
SBA counties and the local mountains. Highs will still remain a
few degrees above normal for this time of year. A shortwave ridge
will build over SoCal ahead of an approaching upper trough for
Wednesday. Weak offshore flow will set up Wednesday morning as
well which will bring 4 to 6 degrees of warming to the LA/Ventura
Coast and valleys, and a few degrees warmer just about everywhere
except for a couple degrees of cooling across the Antelope Valley
which is typical with this type of pattern. A few valley locations
should reach the lower 80s. Some weak winds also across the San
Lucia Mtns to the Central coast are expected. Low clouds should
remain off the coast tonight into Wednesday with the continuation
of plenty of mid to high level clouds through Thursday.

For Thursday, the shortwave ridge will begin to breakdown as the
upper trough approaches southern California. Latest models have
slowed the approach of what will be a very weak system. But once
it arrives, it will push through quickly as 850 mb winds will be
out of the NW. Light rain will move in across the NW portion of
the Central Coast initially by late Thursday morning or afternoon,
then overspread to the south into LA County by late Thursday
night or early Friday morning. Snow levels will be between 7000 -
8000 ft through Thursday night then drop to around 4,000 ft by
Friday mid morning. With the wind profile mainly out of the NW
direction, rain totals will be quite light. Have lowered the QPF
to less than a quarter inch for SLO county and much lighter to
the south. High temps will be around 4 degrees cooler from
Wednesday for most areas, with a few degrees of warming across the
Antelope Valley.

Rainfall is expected to remain light across Montecito Thu/Fri
with a tenth of an inch or less expected.

.LONG TERM...(FRI-MON)

Both the GFS and ECMWF continue to be in decent agreement with
large scale features through Monday before going out of phase on
Tuesday. For Friday morning, a weak cold front will continue to
bring some light rain across the region. As the front moves
through, The snow level will drop from 7000 ft Thu evening to
4000 ft by Friday mid morning. Most the showers will be very
light. but the northerly winds will help to pile up clouds across
the northern slopes above the Grapevine, which should cause snow
flurries to continue through much of Friday into Friday evening.
It will be cold enough to potentially cause some hazardous
driving conditions across the I-5 Corridor above the Grapevine.
Snow amounts should be 2-4 inches for areas above 5,000 ft with
this next system.

Temperatures will be plummeting on Friday with valleys and
interior areas lowering 10 to 20 degrees with coastal areas down 8
to 10 degrees from Thursday. Highs across the low-lands will be
in the upper 50s to lower 60s on Friday, with just a few degrees
of warming on Saturday. Sundowner winds are expected to develop
Friday afternoon through early Saturday morning across SBA County
and across the LA/VTU County Mtns and Antelope Valley as well. It
will remain quite cool on Sat with a few lingering showers across
the northern slopes of the Ventura County Mtns. Weak offshore flow
will set up briefly Sat evening into Sunday morning, with some
limited warming expected. It will continue to be quite cool
overall, but highs should be in the upper 60s for coast and
valleys with partly cloudy skies.

Both models are hinting at another system to move into the
northern portion of the forecast area Monday then spreading into
the entire forecast area. The GFS is much more bullish and keeps
the trough over the region longer, allowing precip to continue
through Wed. It is too early to get a good handle on what this
system will actually going to do, but as we draw closer to the
weekend, we should start getting an idea of some preliminary
precip amounts to expect. People in the Montecito area should
remain aware for the potential of more extended periods of rain.

&&

.AVIATION...16/12Z.

At 11Z at KLAX, there marine layer was around 600 ft deep. The top
of the inversion was around 1300 ft with a temperature of 18C.

Areas of low clouds and fog with LIFR/VLIFR conds will affect
coastal sections through mid morning, then conds will improve,
although IFR cigs may linger near the beaches into early afternoon.
There will be patchy dense fog with vsbys one quarter mile or less.
VFR conds are expected tonight as weak offshore flow develops.

KLAX...Moderate confidence in the 12Z TAF. There is a 20 to 30%
chance that cigs will linger through 19Z. There is a 20 to 30%
chance of IFR cigs between 07z and 15Z Wed.

KBUR...High confidence in CAVU TAF.

&&

.MARINE...16/400 AM.

For the Outer Waters, high confidence in current forecast. Seas
will remain above Small Craft Advisory (SCA) levels thru Wed night,
then may briefly fall below SCA levels for a 6-12 hour period late
Wed night and early Thu. Then, seas will build Thu, increasing thru
at least Fri, with wave heights of 18 to 20 ft possible, with high
seas continuing into early next week.

Winds will be near SCA levels today and tonight, especially across
the southern two thirds of the outer waters. SCA level winds are
likely across the entire outer waters Fri thru Sat, possibly
beginning as early as Thu night.

For the northern Inner Waters, high confidence in the forecast.
SCA conditions for Hazardous Seas will continue thru Wed night.
After slightly subsiding, seas will build to SCA levels again Thu
and Thu night, with high seas continuing into early next week. SCA
level winds are expected Fri through Sat.

For the waters south of Point Conception, winds will remain below
SCA levels thru Thu. Seas could get close to SCA levels later today
and tonight across western portions of the SBA Channel and western
portions of the southern inner waters. Even larger seas are expected
Thu night thru Sat. SCA level winds are expected Fri thru Sat.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 6 AM PST Thursday for
      zones 34-35-40-41-87. (See LAXCFWLOX).
     Dense Fog Advisory in effect until 9 AM PST this morning for
      zones 39-41. (See LAXNPWLOX).
PZ...Small Craft Advisory for hazardous seas in effect until 3 AM
      PST Thursday for zones 645-670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Thursday for
      zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(THU-MON)
Possible winter weather issues for the I-5 near the Grapevine
Friday night. Otherwise, no significant hazards expected.

&&

$$

PUBLIC...Kaplan
AVIATION...DB
MARINE...DB
SYNOPSIS...JLL

weather.gov/losangeles


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