Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 232120
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
220 PM PDT Sat Sep 23 2017

.SYNOPSIS...
A high along with offshore winds, breezy at times, should bring a
significant warming trend and mostly clear skies into Friday.

&&

.SHORT TERM...(TDY-TUE)

Upper level trough digging into Great Basin today, with northwest
flow aloft across Southwest California. Near the surface, high
pressure will build into the Great Basin and set the stage for
a prolonged weak to locally moderate offshore flow pattern through
much of the 7 day forecast period. Already seeing a warming and
drying trend underway today and this warming and drying will
become more pronounced during the next few days. Warmest valley
locations could reach 90 degrees by tomorrow, then more widespread
temperatures in the 90s on Monday and Tuesday.

For Sunday morning, LAX-Daggett gradient expected to be around -3
mb, and could increase to between -3.5 and -4 mb range on Monday
and Tuesday mornings. Upper level wind support is fairly weak
on Sunday and Monday but could nudge up a bit by Tuesday/Tuesday
night as models trending towards a closed low developing somewhere
in the deserts of southeast California. For the most part,
expecting winds to remain below advisory levels during the short
term, with gusts generally ranging between 30 and 40 mph in the
mountains (including the Santa Monicas), and between 20 and 30 mph
in the valleys.

The combination of gusty offshore winds along with warmer
temperatures, lowering humidities, and very dry fuels will bring
an extended period of elevated fire danger, with brief crticial
fire weather conditions possible. Please see fire discussion below
for more details.

.LONG TERM...(WED-SAT)

Long range models still showing potential for northeast winds
gusting between 30 and 40 mph across the mountains on Wednesday,
then offshore winds should diminish slightly for Thursday and
Friday. Despite the weakening of the offshore flow, temperatures
away from the coast are expected to continue climbing well into
the 90s on Wednesday and Thursday, with the potential for
isolated triple digit readings in the valleys by Friday. While
earlier model runs had suggested that next Saturday could bring
the warmest temperatures of the next 7 days with a slight
resurgence of offshore flow, the latest 12z ECMWF has thrown a
bit of a curve ball with a weak cutoff low digging down the
coast. If this solution were to verify, we would have to begin
a cooling trend for next Saturday, but for now, will ride the
course til we get some better model convergence. Also of note,
there will be some very warm overnight low temperatures in
the foothills and wind exposed locations.

&&

.AVIATION...23/12z.

At 11z at KLAX... There was no notable inversion.

Overall... Moderate to high confidence in the current TAFs. There
is a thirty percent chance of LIFR/IFR conditions through 20z at
Central Coast terminals and a twenty percent chance of MVFR
conditions through 20z at coastal Los Angeles County terminals.
There is a less than ten percent chance of MVFR conditions through
18z at KBUR and KVNY. Otherwise and elsewhere VFR conditions will
prevail.

KLAX... Moderate to high confidence in the current TAF. There
is a twenty percent chance of MVFR conditions through 20z.
Otherwise VFR conditions will prevail. There is a fifteen percent
chance of east winds up to ten knots through 16z.

KBUR... High confidence in the current TAF. There is a less than
ten percent chance of MVFR conditions through 18z. Otherwise VFR
conditions will prevail.

&&

.MARINE...23/100 PM.

Confident in low-end Small Craft Advisory (SCA) level winds
forming again this aftenroon and continuing over the outer waters
through tonight, especially between Point Conception and San
Nicolas Island. The western Santa Barbara Channel will also see
low-end SCA winds this evening, but should be localized and brief
enough to not need an Advisory. All areas will experience a short-
period chop from these winds into Sunday. No expecting SCA winds
through the middle of next week, but afternoon and evening onshore
winds will get into the 10 to 20 KT range everywhere starting
Sunday with the warm conditions inland. Pockets of dense fog
cannot be ruled at times through next week...please stay aware of
your immediate environment.

&&

.FIRE WEATHER...23/200 PM.

A long duration of elevated fire danger expected across portions
of Southwest California beginning Sunday, and potentially
persisting through next weekend. Weak to occasionally moderate
offshore flow is expected during this period which will bring a
prolonged period of very warm and dry conditions to the region.

In the short term, offshore pressure gradients are expected to
peak Monday through Wednesday when LAX-Daggett gradients are
expected to range between -3 and -4 mb each morning. There is the
potential for a slight ramp up in upper level support on Tuesday
and Wednesday as computer models are showing the potential for
a cutoff low pressure system to develop near the Colorado River
Valley. At this time, current thinking is that offshore winds
will be strongest across the mountains Monday through Wednesday
(including the Santa Monica Mountains) when gusts between 30
and 40 mph are expected, with lighter offshore winds surfacing
across the valleys. Offshore flow is expected to be somewhat
weaker on Thursday and Friday, then potentially nudge upward
again next weekend.

This prolonged period of offshore flow will bring widespread
minimum humidities between 10 and 20 percent for areas away
from the coast through next weekend, along with poor overnight
recoveries across the mountains, foothills, and wind exposed
locations. In addition, there will be a prolonged period of very
warm temperatures with highs climbing into the 90s much of next
week in the valleys, and triple digit readings possible Friday
through next weekend. The above mentioned fire weather conditions
combined with fuels becoming critically dry will support this long
duration of elevated fire danger, with an increased risk for
large fires and erratic fire behavior. There is also the potential
for brief critical fire weather conditions each day from Monday
through Wednesday (especially in the mountains) as well as next
weekend.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Sunday for
      zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
A prolonged period of elevated fire danger is expected Tuesday
through Saturday due to the combination of gusty offshore winds,
very warm temperatures, low humidities, and very dry fuels.
Brief critical fire weather conditions will be possible at times
especially in the mountains.  There will be potential heat
impacts by Friday and Saturday as triple digit temperatures
are possible for warmest valley locations.

&&

$$

PUBLIC...Gomberg
AVIATION...Kj
MARINE...Kittell
FIRE...Gomberg
SYNOPSIS...STu

weather.gov/losangeles


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