Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 130041

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
441 PM PST Tue Dec 12 2017

Updated Aviation Section


Gusty offshore winds, warm afternoon temperatures, and dry
conditions will persist through the week. Smoke from the fires
will impact regional air quality through the week as well. Weak
onshore flow on Saturday will bring some relief.



High pressure aloft continues to dominate the upper level pattern
along the west coast with occasional inside slider type troughs
dropping south along the eastern flank of the high and into the
Great Basin. We have another one of these inside sliders coming
Wednesday which will initially weaken the offshore flow as surface
pressures drop across the interior. But then behind the trough the
high will strengthen and once again provide another little
offshore push Thu/Fri, possibly with some advisory level Santa Ana
winds across LA/Ventura counties. Still not seeing much wind at
all across SB/SLO counties and models continue to show winds
mainly less than 20 mph there the next few days.

Today was another warm day, pretty similar to Monday in most
areas. With the weaker offshore flow Wednesday there should be
some cooling along the coast but not much more than a few degrees.
Inland areas more or less the same as today. As the offshore flow
strengthens again Thu/Fri that should bump the temps back up for
coastal areas by a few degrees while far interior areas cool due
to the colder air over the Great Basin.


Gradients trend back onshore Saturday as another trough goes up
and over the ridge and into the Great Basin Saturday night. So
temps should cool a few degrees with weaker northeast winds and
and earlier sea breeze arrival for the coast. Models still flip
flopping on the strength of the offshore flow to follow behind
that trough. Today`s models cut the previously projected LAX-DAG
gradient Sunday by about half but could easily revert back and
forth the next several runs so very low confidence on the expected
winds for late in the weekend. Temps should at least bounce back
up Sunday with some offshore flow and rising heights.

Further uncertainty beyond Monday as the ECMWF splits off a semi
cutoff low to our west and brings that onshore Tuesday. This looks
odd and given the poor run to run consistency this solution is
being ignored and sticking with the more stable GFS which has
much higher heights but shows offshore flow weakening quite a bit
and thus cooler for most coast/valley areas.



At 00Z, there was no marine inversion at KLAX.

VFR conditions are expected throughout the period, except for a
chance of MVFR conditions in smoke at coastal and valley
terminals south of Point Conception. Periods of moderate low-
level wind shear and turbulence are possible at Ventura County
terminals between 06Z and 12Z.

KLAX...VFR conditions are expected throughout the period. There is
a 30 percent chance of MVFR conditions in smoke through 08Z. Any
east winds should remain less than 7 knots.

KBUR...VFR conditions are expected throughout the period. No wind
issues are expected at this time.


.MARINE...12/200 PM.

For the Inner Waters south of Point Conception, winds will remain
below SCA strength through Thursday. There will be local NE winds
near shore from Ventura to Malibu and S of Palos Verdes Peninsula
across the San Pedro Channel each morning into the early afternoon

The near shore waters north of Point Sal will remain below SCA
level through at least early Friday.

Elsewhere, winds and seas are expected to remain below Small
Craft Advisory levels through Thursday, and then rise to SCA level
from the northwest on Friday. There is a 30 percent chance of Gale
level winds for the Outer Waters North of Point Conception
sometime Friday or Saturday.

There will 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 the outer
waters. Local visibilities could possibly drop to near one
nautical mile or lower at times.


.FIRE WEATHER...12/200 PM.

The Red Flag Warnings have CHANGED. It has been EXTENDED through
10 AM Friday for the Los Angeles and Ventura County Mountains, the
Ventura Valleys, and the Santa Clarita Valley. A Fire Weather
Watch is in effect for Thursday 6 AM to Friday 10 AM over the Los
Angeles and Ventura County Coasts and the Santa Fernando Valley.

The extended period of offshore flow will continue into
Friday...focused over Los Angeles and Ventura Counties. Little
change expected through Wednesday with peak gusts in the 20 to 40
mph range in the wind prone areas. Strengthening is expected
Thursday into early Friday. The humidities will remain incredibly
dry with extremely poor overnight recoveries nearly everywhere.
This will result in continued critical conditions into Friday,
especially Thursday afternoon and night when the winds peak.

Santa Barbara and San Luis Obispo Counties will continue to see
periods of north to east winds (gusts of 15 to 30 mph) through
the week, including areas near the Thomas Fire...resulting in
elevated to near critical fire weather conditions. In addition,
the air mass will be fairly unstable above the Thomas Fire through
Wednesday, resulting in a greater risk for large fire plumes
which will locally enhance winds near the fire.

Conditions will change Friday afternoon into Saturday as the flow
quickly shifts to more west to northwest, but this shift might
result in gusty winds through southern Santa Barbara County and the
I-5 Corridor. Humidities should improve some as well but will remain
dry. More offshore flow is likely Saturday night and Sunday.

If fire ignition occurs, very rapid spread and extreme fire
behavior is likely. 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.


CA...Red Flag Warning in effect until 8 PM PST this evening for
      zones 240-244>246-288. (See LAXRFWLOX).
     Red Flag Warning in effect until 8 PM PST Wednesday for zones
      253-254. (See LAXRFWLOX).


Critical fire weather conditions possible Thursday and Friday
across LA/Ventura counties with an increase in Santa Ana winds
and continued very low humidities.



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