Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 140429
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
829 PM PST Wed Dec 13 2017

.SYNOPSIS...
Gusty offshore winds, above normal afternoon temperatures, and
very dry conditions will persist through Friday. There is a chance
that the flow will turn onshore for a few hours on Saturday
bringing some cooling. The offshore flow will likely return by
Saturday night.

&&

.SHORT TERM...(WED-SAT)

It was another warm and extremely dry day across the region today,
at least west of the mountains. High temperatures were mostly in
the upper 70s to mid 80s in coastal areas, and in the mid to upper
80s in the valleys. The high temperature of 88 degrees at Fillmore
was the highest temperature in the United States today. Temperatures
were also well above normal in the mountains and in the Antelope
Valley. Skies were clear across the entire region this evening.
However, there was plenty of smoke across portions of Ventura and
Santa Barbara Counties as the Thomas fire continued to burn.

An upper low in Utah will drop southward into Arizona overnight
and early Thu. This will cause winds aloft to increase across the
region late tonight and early Thu, with strong northeast winds
developing at 850 mb across Los Angeles and Ventura Counties by
morning. A large area of good subsidence through a deep layer
of the atmosphere will spread westward into L.A. and VTU Counties
early Thursday, and there will be decent cold air advection as
well. A 1037 mb high will drop into northern Utah, and surface
pressure gradients will turn offshore, although not hugely
so--about 5mb offshore between KLAX and KDAG. Despite the modest
offshore gradients, the strong upper level support will likely
cause another moderate to strong Santa Ana wind event to unfold
across much of Los Angeles and Ventura Counties, as the relentless
offshore flow pattern continues.

Winds should increase in the mountains overnight, spread into the
valleys very late tonight, then across coastal sections of VTU
County and coastal sections of L.A. County from Malibu to the
Hollywood Hills by daybreak. Winds will peak from mid morning
through early to mid afternoon. Peak wind gusts of 45 to 55 mph
are expected in the mountains with local gusts to between 60 and
65 mph are likely in the windiest mountain locations. In the
valleys and coastal areas, wind gusts will generally be in the 35
to 45 mph range, with local gusts to 50 mph in the windier
locations. Some of the gusty winds will likely affect portions of
the Thomas fire, especially in Ventura County. Max temps should
be down several degrees in most areas on Thu, but will still be
well above normal in most locations. Winds will begin to diminish
in most areas by evening.

Expect another round of gusty NE winds late Thu night and Friday
morning, but although gradients will be more strongly offshore,
support will be minimal, so expect winds to be weaker, and remain
mostly below advisory levels. Max temps may be up a bit on Friday,
especially inland, but cooling is expected near the coast as
gradients turn onshore in the afternoon.

*** from previous discussion ***

Saturday`s forecast is complicated by poor model consistency and
thus confidence is pretty low going into the weekend. There is the
potential for a rapid increase in northerly winds Saturday as
another trough moves through the western portion of the US, but
models are struggling with the exact location and this has huge
ramifications on the resulting winds. The GFS and NAM would
certainly support a ramp up of northerly winds Sat, including the
SB area, while the ECMWF is much weaker as it takes the trough
much further east. Favoring some blend of the GFS/NAM solution at
this point as the ECMWF is more of an outlier.

Smoke plume as shifted more to the south and east today as the
winds aloft backed around to the north. However, expecting the
smoke to shift back to the west and north Thursday as the Santa
Ana kicks in, so look for increasing smoke again for the western
and northern parts of SB county by Thu afternoon and evening.

.LONG TERM...(SUN-WED)

Low confidence with the temps/wind forecast for Sunday as well for
the same reasons, however if something like the GFS/NAM solution
is correct then Sunday should be a solid advisory level Santa Ana
wind day as gradients and upper support shift to the northeast.
This means less wind in the SB area but increasing winds in
LA/Ventura counties. Then dropping off early next week as the
upper low moves into New Mexico and Texas and a weak upper level
ridge redevelops over CA. Could be some very cold overnight lows
in wind protected locations Sunday and Monday. Winds probably
dropping below advisory levels by Monday with no significant winds
expected through the middle of the week. Highs will be warming
back up into the upper 70s to lower 80s by Tuesday and Wednesday.

&&

.AVIATION...14/0200Z.

At 00Z, there was a surface based inversion at KLAX. The top of
the inversion was at 2500 feet with a temperature of 24C.

VFR conditions are expected throughout the period, except for
local MVFR conditions in smoke at some coastal and valley
terminals south of Point Conception overnight.

KLAX...Moderate to high confidence in the 00Z TAF with VFR
conditions expected through the period. There is a 20-30% chance
of MVFR vsbys in smoke. Any east winds should remain less than 7
knots.

KBUR...High confidence in the 00Z TAF with VFR conditions
expected through the period.

&&

.MARINE...13/700 PM.

Across the inner Waters south of Point Conception, SCA winds will
not occur until Thursday, when there is a 70-80% chance of SCA
level NE wind gusts from Ventura to Santa Monica. Another offshore
wind event capable of causing SCA wind speeds is possible Sunday
morning. The confidence level is low due to run to run model
inconsistency.

Across the inner waters north of Point Sal, moderate confidence
that winds and seas will remain below SCA levels through Sun.

Across the outer waters, moderate confidence in the forecast.
There is a 40% chance of SCA winds across the northern two thirds
of the outer waters tonight. SCA conditions are possible across
the entire outer waters Sat, possibly starting Fri night.

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 2 nautical
mile or lower at times through Thu.

&&

.FIRE WEATHER...13/130 PM.

Red Flag Warnings in effect for much of Los Angeles and Ventura
counties through 10 am Friday.

The extended period of offshore flow will continue into
Friday...focused over Los Angeles and Ventura Counties. Light
offshore winds and single digit humidities will continue across
interior portions of Los Angeles and Ventura counties through
tonight. A more pronounced sea breeze pattern will overspread
coastal and valley areas of Ventura county this afternoon and
evening increasing the likelihood of greater smoke coverage
across central and eastern portions of Ventura county.

Strengthening offshore winds are expected Thursday into Thursday
night, when gusts between 35 and 55 mph can be expected. The LAX-
Daggett gradient expected to peak in the -6 to -6.5 mb range on
Thursday coupled with increasing upper level wind support and some
cold air advection. The humidities will remain incredibly dry
with extremely poor overnight recoveries nearly everywhere. This
will result in continued critical fire weather conditions into
Friday morning, especially Thursday into Thursday 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
this afternoon, resulting in a greater risk for large fire plumes
which will locally enhance winds near the fire and cause erratic
fire behavior.

Conditions will change Friday afternoon into Friday night, with
the flow becoming more west to northwest. By late Friday night
into Saturday, there is the potential for gusty north winds with
continued low humidities across Santa Barbara county (including
the Thomas Fire area) and the I-5 corridor. There is still
considerable uncertainty in these northerly winds and the
associated low humidities for Saturday. For Ventura and Los
Angeles counties, the flow may begin to shift to the northeast
again during the day Saturday, with a greater threat of increased
Santa Ana winds again on Sunday when gusts between 35 and 50 mph
will be possible.

If fire ignition occurs, very rapid spread, long range spotting,
extreme fire behavior is likely especially Thursday into Thursday
night. 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...Wind Advisory in effect from 6 AM to 3 PM PST Thursday for
      zones 40-41. (See LAXNPWLOX).
     Wind Advisory in effect from 6 AM to 9 PM PST Thursday for
      zones 44>46-53-54-88-547. (See LAXNPWLOX).
     Red Flag Warning in effect from 6 AM Thursday to 10 AM PST
      Friday for zones 240-241. (See LAXRFWLOX).
     Red Flag Warning in effect until 10 AM PST Friday for zones
      244>246-253-254-288. (See LAXRFWLOX).
     Red Flag Warning in effect from 4 AM Thursday to 10 AM PST
      Friday for zone 547. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect from 6 AM to 6 PM PST Thursday
      for zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
Critical fire weather conditions possible Saturday and Sunday with
gusty north to northeast winds and very low humidities.

&&

$$

PUBLIC...MW/DB
AVIATION...Sweet
MARINE...Sweet/Sirard
FIRE...Gomberg/Kittell
SYNOPSIS...jld

weather.gov/losangeles


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