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FXUS66 KLOX 171834
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1034 AM PST Sun Dec 17 2017

.SYNOPSIS...
Strong northeast winds will affect Ventura and Los Angeles
Counties today. Dry and more tranquil weather is expected Monday
through Tuesday. Another round of strong northerly winds may affect
the region by late Wednesday with Santa Ana winds possible
Thursday into the weekend.

&&

.SHORT TERM...(TDY-TUE)

Strong and gusty N to NE winds prevailed across VTU/L.A. Counties
this morning, with wind gusts up to 65 to 70 mph in the mtns, 45 to
55 mph for the vlys, and 40 to 50 mph for coastal areas from Ventura
thru Santa Monica and the Hollywood Hills. The pressure gradient for
LAX-DAG at 16Z was -5.5 mb. The winds are forecast to continue into
early to mid afternoon before diminishing. High Wind warnings will
continue thru 3 PM in the L.A./VTU County mtns including the Santa
Monica range, with high-end Wind Advisories thru 3 PM for the
VTU/L.A. County vlys, VTU County coast and the L.A. County coast
from Malibu thru the Hollywood Hills. Please see the latest Non-
Precipitation Weather Message (LAXNPWLOX) for further details.

Areas of smoke continued this morning over SW VTU County, the SBA
County S coast and Santa Ynez Mtns. The strong winds also stirred up
ash from the Sand and Creek burn areas and spread ash and dust into
the San Fernando Vly to the L.A. Coastal plain this morning. The
areas of smoke and ash will likely persist into the afternoon.
Otherwise, mostly sunny skies will prevail across swrn CA thru this
afternoon. Temps today with the offshore flow will turn warmer for
the coast and vlys to several degrees above normal, while the
interior vlys, mtns and deserts will turn slightly cooler but highs
there should still be near to slightly above normal overall.

An upper low level over nwrn Mexico this morning will slowly move E
and NE thru Mon. At the same time,upper level ridging off the CA
coast will build into the region. The upper level ridge will then
persist over the area Mon night and Tue.

Gusty sub-advisory NE winds will prevail across VTU/L.A. Counties
tonight into Mon morning, then weak to increasing onshore flow can
be expected Mon afternoon thru Tue. Skies will be mostly clear
across the region thru Tue, except for areas of smoke persisting in
the vicinity of the Thomas Fire.

Temps are expected to be slightly above normal for many areas on Mon
and Tue, altho Tue will be the cooler of the two days. The dry air
mass and lighter winds will allow for cold overnight lows for the
wind-sheltered locations way from urban areas. There may be the need
for frost advisories tonight and Mon night for the Central Coast,
the south coast of SBA County, and the Ojai Valley.

***FROM PREVIOUS DISCUSSION***

.LONG TERM...(WED-SAT)

There is less confidence than normal in the extended period
since the models have exhibited poor run to run consistency.
However, the models are is fairly good agreement this morning.
The general idea on the latest runs of the GFS and EC is that
an upper trough will drop southeastward through the Pac NW Wed
and evolve into a closed low as it pushes into Utah by early Thu.

Steepening north to south gradients, decent subsidence, and strong
northerly flow aloft should bring another round of gusty north
winds to SBA County late Wed into Wed night, followed by gusty
northeast Santa Ana winds across L.A. and VTU Counties Thu into
Fri. At this time, it looks like at least a moderate wind event,
with Wind Advisory level winds in most wind prone locations.
However, models have been oscillating between this synoptic
pattern and a moderate wind event, and a synoptic pattern
with a strong upper low digging closer to the region, bringing
a very strong and damaging wind event for Thu into Fri. This will
continue to be monitored closely. In the current model solutions,
expect weakening off any northeast winds Fri afternoon, with
generally benign weather on Sat.

Skies will be mostly clear thru the period, except for some clouds
on northern slopes Wed night into Thu. Below normal temps are
expected Wed and Thu, with temps generally near normal
Fri, and a bit above normal Sat.

&&

.AVIATION...17/1800Z

At 1622Z at KLAX, there was no marine layer. There was an
inversion with a base near 2500 feet and a top near 4000 feet and
a max temperature of 16 degrees Celsius.

VFR conditions are expected throughout the period, except for a chance
of MVFR conditions in smoke at Santa Barbara and Ventura county
terminals. Periods light to moderate wind shear and turbulence are
possible at all terminals through 20Z.

KLAX...Good confidence in Cig/Vis portions of the TAF. Lower
confidence in the wind forecast. But only a 20 percent chc of east
winds greater than 8 kt. Lgt turbc at times DURGD.

KBUR...Moderate confidence in Cig/Vis portions of the TAF. Lower
confidence in the wind forecast. There is a 60 percent chc of
north to northeast winds at times through 20Z. Turbc at times
DURGD.

&&

.MARINE...17/900 AM

Across the outer waters...Small Craft Advisory (SCA) level will
continue into the afternoon. Large seas consisting of a longer
period NW swell and wind waves will subside below 10 feet later
this afternoon. There is a 30 percent chance that seas remain over
10 feet into the evening. The seas have subsided below 10 feet in
zone 645.

Across the inner waters south of Point Conception...Strong SCA
level NE winds are expected from Ventura to Anacapa Island south
to Santa Monica and from San Pedro to Catalina Island late through
this afternoon. These winds will create choppy seas and short
period wind waves which could affect Avalon and other east facing
harbors in the Channel Islands.

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

&&

.FIRE WEATHER...17/1015 AM.

A strong Santa Ana wind event is impacting Ventura and Los
Angeles counties today, with alignment of the offshore pressure
gradient, upper level wind support, and cold air advection.
The LAX-Daggett gradient peaked at -5.8 mb this morning.
Observations this morning already showing damaging gusts
of 60 to 70 mph in the mountains, and 50 to 60 mph across
portions of the coast and valleys. Critical Red Flag conditions
can be expected for much of Los Angeles and Ventura counties
through 8 pm this evening. The strongest winds are expected to
occur this morning through early afternoon. These powerful Santa
Ana winds will bring considerable blowing dust and ash across the
area from recent fire activity. The strong Santa Ana winds will
likely bring very rapid fire spread, long range spotting, and
extreme fire behavior on the Ventura county portion of the Thomas
Fire and any new ignitions.

Offshore winds will gradually diminish overnight into Monday morning. There
will be a return of more extensive onshore flow on Tuesday with some
improvements in humidity across coastal and valley areas. Another upper
level trough will dig through California on Wednesday, bringing cooler
conditions and higher humidities to much of the region during the day,
along with gusty west to northwest winds. Behind this upper level trough,
northwest to north winds are expected across the Santa Barbara county
mountains and south coast late Wednesday afternoon into Thursday morning,
when gusts of 40 to 55 mph will be possible. Gusty northwest to north winds
will also impact other mountain areas, including the highway 33 and
interstate 5 corridors. The increased winds will once again bring potential
fire weather impacts for both the Santa Barbara and Ventura county portions
of the Thomas Fire late Wednesday into Thursday. Later in the week, there is
low confidence in the potential Santa Ana wind pattern as computer models
continue to widely fluctuate on the timing, duration, and strength of
the Santa Ana winds.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 6 PM PST this evening for
zones 34-35. (See LAXCFWLOX).
Wind Advisory in effect until 3 PM PST this afternoon for
zones 40-41-44-45-88-547. (See LAXNPWLOX).
High Wind Warning in effect until 3 PM PST this afternoon for
zones 46-53-54. (See LAXNPWLOX).
Red Flag Warning in effect until 8 PM PST this evening for
zones 240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 PM PST this afternoon
for zones 650-655-670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Critical fire weather conditions are possible with gusty
Sundowner winds Wednesday night and gusty offshore winds
Thursday and Friday.

&&

$$

PUBLIC...Sirard/DB
AVIATION...Sweet
MARINE...Sweet
FIRE...Gomberg
SYNOPSIS...DB/Kj

weather.gov/losangeles
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