Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 151844
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1044 AM PST Fri Dec 15 2017

.SYNOPSIS...
Very dry and warm conditions will persist today. Some cooling is
expected this weekend as a trough moves through the region. Northerly
winds will return on Saturday, turning into another Santa Ana
wind event late Saturday night into Sunday. Gradual warming early
next week with weaker offshore winds expected.

&&

.SHORT TERM...(TDY-SUN)

Forecast on track today, with similarly warm conditions compared
to yesterday and mostly light winds during the day as we quickly
turn from offshore flow to light onshore. This light wind regime
will allow smoke from the Thomas fire to become more concentrated
and spread more into Ventura and possibly west Los Angeles County.
Updated the forecast to better reflect that. Northerly winds for
tonight into Saturday are still on target to strengthen, including
Sundowner winds over Santa Barbara County and the Thomas Fire.
These winds should stay just below wind advisory however.

***From Previous Discussion***

Synoptically, An upper level disturbance will ride down the
eastern side of the Eastern pac upper ridge through California
tonight into Saturday. Earlier models had this shortwave becoming
cutoff across the southern SoCal Bight early Saturday morning, but
the GFS, ECMWF and the high resolution NAM-WRF now quickly move
this digging trough towards San Diego or Baja before it becomes
cutoff Saturday morning into So Arizona. Areas to the south will
have the best chance of seeing a few showers develop, or off the
coastal waters of Orange County and San Diego. But this is still a
slight chance. With the height falls, clouds should develop
tonight across LA and VTU Counties then clear out by Saturday
afternoon.

The main story for Saturday will be the northerly winds
developing behind the frontal passage. Sundowner winds are
expected to develop by late tonight into Saturday morning. By late
Saturday into Sunday the pattern shifts to a Santa Ana. For
Ventura/LA Counties north to northeast winds will start up during
the day Saturday and increase overnight and peaking Sunday morning
with solid advisory level winds and possibly local gusts to 60
mph in the mountains as good upper support and cold air advection
will enhance this wind event Sunday. The additional cold air
coming in Saturday night and Sunday will more than offset any
warming from the downslope flow so highs at lower elevations
expected to top out in the lower 70s.

.LONG TERM...(MON-THU)

Both the GFS and ECMWF remain in good agreement with large scale
features through the Thursday. Offshore winds will be rather weak
both Monday and Tuesday with a gradual warming trend. By
Wednesday, another upper level trough begins to drop out of the
PAC NW and become an inside slider as it moves into the Great
Basin on Thursday. Both models then retrograde the upper low
towards northern Baja by Thursday night. By Wednesday night into
Thursday morning, northerly winds develop affecting the Santa
Barbara Mtns and adjacent south coast as well as the I-5 Corridor,
then turn offshore into another Santa Ana event. The placement of
the upper low over nrn Baja and more cold air advection could
bring another strong Santa Ana Wind event. OF course, this
scenario is still several days away, and models can flip-flop like
your buttermilk pancakes, so stay tuned.

&&

.AVIATION...15/1755Z...

At 1709Z, there was a weak surface-based inversion at KLAX. The top
of the inversion was near 1100 ft with a temperature of 22 deg C.

Generally hi confidence in the 18Z TAFs. VFR conditions are expected
at most of the airfields thru Sat morning. However, there is a 10-20
percent chance low clouds may move into KLAX or KLGB Sat morning
with LIFR/IFR cigs. For KSBA, CMA and OXR, there is a 40-50 percent
chance that smoke from the Thomas Fire may reduce vsbys to MVFR at
times thru Sat morning. Low level wind shear will be possible at
KSBA aft 12Z Sat due to gusty N canyon winds in the vicinity.

KLAX...Generally hi confidence in the 18Z TAF with VFR conditions
expected thru Sat afternoon. However, there is a 10-20 percent
chance low clouds may move into the airfield Sat morning with
LIFR/IFR cigs.

KBUR...Hi confidence in the 18Z TAF with VFR conditions expected
thru Sat morning.

&&

.MARINE...15/830 AM.

Across the outer waters, fairly good confidence that NW winds
will increase to SCA levels this evening, then to gale force
conditions by late tonight. There is a 30% chance that seas will
build to SCA levels as early as this afternoon, at least across
the northern two thirds of the outer waters. Gale force winds
will continue through late Sat night, then SCA conds will continue
for a few hours thereafter. Seas will possibly remain above SCA
thresholds for hazardous seas through around noon Sunday.

Across the inner waters north of Point Sal, moderate to high
confidence that winds and seas will reach SCA level winds
from late tonight and continue through late Saturday night. There
is a 20-30% chance that SCA conds will arrive before midnight
tonight.

Across the inner Waters south of Point Conception, winds and seas
are expected to be below SCA levels through Tue, except for a
40-50% chance of SCA level NE winds late Sat night into Sun
morning from Ventura to Anacapa Island south to Santa Monica,
and possibly from San Pedro to Catalina Island.

There will be areas of smoke from the Thomas fire over portions
of the coastal waters today, mainly from the waters west of
Ventura County through the Santa Barbara Channel and over the
outer waters. Visibilities could locally drop to 2 NM or
lower at times today.

&&

.FIRE WEATHER...15/1030 AM.

Red Flag Warnings allowed to expire this morning for Los Angeles
and Ventura counties. Red Flag Warning in effect from 2 am to
10 pm Saturday for the Santa Barbara County Mountains and adjacent
foothills. Fire Weather Watch in effect from late Saturday night
through Sunday evening for the Los Angeles and Ventura county
mountains and valleys.

Offshore winds continue to weaken this morning allowing
for the expiration of the Red Flag Warnings for Los Angeles and
Ventura counties. Onshore winds will overspread most coastal
and valley areas this afternoon and evening. There is expected
to be some improvements in humidity along the coast, but will
remain very dry across inland areas.

After the passage of an upper level disturbance, there will be
increasing upper level wind support and cold air advection that will
bring strengthening north to northeast winds across the mountains
and adjacent foothills of Santa Barbara county (including higher
elevations of the Thomas Fire) late tonight through Saturday evening,
with gusts of 30 to 40 mph being common. Due to the stronger winds
in the upper atmosphere, there is the potential for locally stronger
gusts across exposed ridgetops on Saturday. The combination of these
gusty winds with humidities generally ranging between 10 and 20 percent
will likely result in critical Red Flag conditions across the Santa
Barbara county mountains. As a result, there will be an increased
threat of very rapid fire spread, long range spotting, and extreme
fire behavior. Gusty north to northeast winds up to 30 mph will
also filter down into the Santa Barbara south coast, which could
bring significant impacts to lower elevations of the Thomas Fire.
In addition, there will be increasing northerly winds on Saturday
across the Ventura county mountains, especially near the Highway
33 and Interstate 5 corridors.

For the remainder of Ventura and Los Angeles counties, the flow will
likely shift to the northeast by Saturday evening, with a greater
threat of increased Santa Ana winds and low humidities late
Saturday night into Sunday when gusts between 35 and 55 mph will
be possible. A Fire Weather Watch is in effect for late Saturday
night through Sunday evening for the mountains and valleys of Los
Angeles and Ventura counties, with critical conditions possibly
extending to coastal areas. This upcoming Santa Ana wind event
has upper level wind support, cold air advection, and a fairly
strong offshore pressure gradient (projected to be around -7 mb
for LAX-Daggett on Sunday morning). An additional Santa Ana wind
event is possible next Thursday into Friday.

Those near current wildfires need to stay aware of and follow
official evacuation orders. Everyone needs to exercise extreme
caution when handling any potential fire ignition sources...such
as campfires, cigarettes, welding and brush clearing equipment.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 9 AM PST Sunday for zones
      34-35. (See LAXCFWLOX).
     Fire Weather Watch in effect from late Saturday night through
      Sunday evening for zones 244>246-253-254-288-547. (See LAXRFWLOX).
     Red Flag Warning in effect from 2 AM to 10 PM PST Saturday
      for zone 252. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect from 3 AM Saturday to 3 AM PST
      Sunday for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM this evening to 3 AM
      PST Saturday for zones 670-673-676. (See LAXMWWLOX).
     Gale Warning in effect from 3 AM Saturday to 3 AM PST Sunday
      for zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SUN-THU)
Critical fire weather conditions possible Sunday with gusty north
to northeast winds and very low humidities. Gusty offshore winds
and critical fire weather conditions possible starting Thursday.

&&

$$

PUBLIC...Kittell/Kaplan
AVIATION...Sirard
MARINE...DB/Sirard
FIRE...Gomberg
SYNOPSIS...Kaplan/RK

weather.gov/losangeles



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