Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 241754
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1054 AM PDT Sun Sep 24 2017

...Aviation and Fire discussions update...

.SYNOPSIS...

Gradually strengthening offshore flow will bring a warming and
drying trend with mostly clear skies to the area through next
week. There will be some gusty north to northeast winds at times
over the region, especially below and through passes and canyons.
Temperatures will be a few degrees above normal for many areas by
early next week, then increase to much above normal by late in
the week.

&&

.SHORT TERM...(TDY-TUE)

Satellite imagery showing mostly clear skies across the district
this morning as extended period of offshore flow has begun.
Already seeing some offshore breezes (with gusts in the 20 to 30
mph range) across the mountains, foothills, and locally into the
valleys this morning which has resulted in a rapid warming and
drying trend. As of 9 am, many observation sites are trending
5 to 10 degrees warmer than this time yesterday.

LAX-Daggett gradient peaked at -2.4 mb this morning, and is
expected to increase to around -3.5 mb on Monday and Tuesday
mornings. In addition, latest 12z NAM cross section and high
resolution 2km WRF model showing a solid 5-10 knot increase
in offshore winds through the boundary layer in eastern Ventura
and western Los Angeles counties on Monday as compared to today.
This will likely result in offshore winds ramping up in coverage
and speed on Monday and Tuesday, with gusts in the 30 to 40 mph
range across the mountains and valleys of Los Angeles and Ventura
counties. While these offshore winds are generally expected to
remain below advisory levels, there could be some isolated wind
gusts up to 45 mph on Monday morning across the Los Angeles county
mountains (including the Santa Monica range) and near Wiley Ridge
in eastern Ventura County. In addition, high resolution models
showing an increased probability of brief offshore winds surfacing
on Monday morning across the Ventura county coastal plain, and
from Malibu to county line, although wind speeds expected to be
lighter. This ramp up in offshore flow for Monday expected to
bring further warming and drying to much of the region, including
coastal areas which could see a dramatic warmup if offshore winds
materialize.

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 this week, with brief
critical fire weather conditions likely on Monday and Tuesday.

*** From previous discussion ***

On Monday offshore gradients will be about 1 MB strong in each
direction. This will lead to a little strong canyon winds in the
morning and there may be a need for some low end advisories along
the LA/VTA county line in the morning. Max temps will rise another
3 to 5 degrees. The cst/vly areas will all be 2 to 4 degrees above
normal except the central coast with the max temps will be 8 to 10
degrees warmer than normal. Despite the warming the interior will
remain blo normal.

Gradients actually relax a little on Tuesday...but there is a
little better upper air support and Tuesday might be the best day
for wind advisories. Again the best area for the winds will be 10
miles either side of the LA/VTA county line. Look for another 2 to
4 degrees of warming across the area and the interior will finally
reach normal max temps. Max temps across the vlys will all be in
the 90s and the coasts will be mostly in the 80s with a smattering
of readings in the lower 90s.

.LONG TERM...(WED-SAT)

An upper low near Yuma will bend the upper level flow pattern into
a NNE direction on Wednesday. Earlier mdl runs had the low a
little further to the west which would have added a little more
oomph to the offshore winds. The current solution would support
low end advisory winds in the usual places. Max temps will be very
similar to Tuesdays much above normal values.

Offshore flow weakens (but still remains offshore) on Thu. The
upper support vanishes as well as the state will be under a large
cull area. Winds will not be a problem. There will be little
change across the interior but the coasts and vlys will cool some
exp the coast.

On Friday the GFS is a little more aggressive with a pop up ridge
and a warming trend than the EC. This looks like a warm pattern
and will favor the warmer GFS.

Although the GFS indicates a weak trof over the state on Saturday
while the EC forecasts a weak ridge the 500 mb hgts only differ by
1 DM. Offshore flow is forecast to increase and max temps should
head up again to warmer readings.

&&

.AVIATION...24/18z...

At 17z at KLAX... there was no notable inversion.

Overall... High confidence in the current TAFs. VFR conditions
will prevail through the forecast period.

KLAX... High confidence in the current TAF. VFR conditions will
prevail through the forecast period. There is a fifteen percent
chance of east-southeast winds up to ten knots 09z-12z.

KBUR... High confidence in the current TAF. VFR conditions will
prevail through the forecast period.

&&

.MARINE...24/830 AM...

There is a thirty percent chance of Small Craft Advisory (SCA)
level winds across the northern and central outer waters Monday
afternoon and evening. Otherwise winds and seas will remain below
SCA levels through Thursday. There will likely be some light
north to northeast winds each night and morning across the
nearshore waters from Ventura south to Santa Monica through
Thursday.

&&

.FIRE WEATHER...24/1045 AM.

A long duration of elevated fire danger expected across portions
of Southwest California through this upcoming 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 and Tuesday 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 continue to show 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 local gusts to around 45 mph. Gusts
between 25 and 35 mph are likely for wind prone valley and
interior coastal areas. Offshore flow is expected to be weaker on
Thursday and Friday, then potentially nudge upward again by
Sunday.

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 this
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. Brief critical fire weather
conditions are likely for areas with strongest winds Monday,
Tuesday and possibly Wednesday. Local or brief critical fire
weather conditions may return by Sunday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.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...Rorke
AVIATION...Kj
MARINE...Kj
FIRE...Munroe/Gomberg
SYNOPSIS...Sweet

weather.gov/losangeles



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