Southwestern California Discussion

FXUS66 KLOX 242127

227 PM PDT Sun Sep 24 2017

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.



Prolonged period of weak to locally moderate offshore winds
expected through next weekend. Already seeing some offshore
breezes (with gusts in the 20 to 30 mph range) across the
mountains, foothills, and valleys today which has resulted in a
rapid warming and drying trend. Many locations across the forecast
area seeing temperatures 5 to 10 degrees warmer than this time

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, 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 areal 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 indicating
that brief offshore winds will likely surface on Monday and
Tuesday mornings across the Ventura county coastal plain, and
from Malibu to county line, although wind speeds expected to be
lighter than valleys and mountains. This ramp up in offshore flow
for Monday expected to bring further warming and drying to much
of the region, including coastal areas which will likely see
a dramatic warming where offshore winds materialize. In fact,
expecting locations like Oxnard and Camarillo to soar to between
90 and 95 degrees on Monday.

While offshore gradients are expected to weaken slightly by late
Tuesday and Wednesday, the models are trending towards a slight
increase in upper level wind support during this time as a closed
low pressure system dives southward across the Colorado River
Valley. Continued very warm temperatures with many valley and
inland coastal plain locations climbing well into the 90s on
Tuesday and Wednesday.

The combination of gusty offshore winds along with very warm
temperatures, low humidities, and very dry fuels will bring an
extended period of elevated fire danger this week, with brief
critical fire weather conditions likely at times Monday
through early Wednesday. For more information, please refer to
the latest fire weather discussion below.


Offshore flow weakens (but still remains offshore) on Thursday.
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 in temperatures across interior areas but the coasts
could see a few degrees of cooling. Latest 12z GFS and ECMWF
model runs coming into better agreement for the Friday through
Sunday time frame, advertising a building upper ridge of high
pressure with increasing heights/thicknesse during period, along
with offshore flow ramping up slightly. As a result, have trended
temperatures up slightly during this period, with the potential
for triple digit temperatures across warmest valleys next weekend.



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...24/200 PM...

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


.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

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.




A prolonged period of elevated fire danger is expected Wednesday
through Sunday 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 on
Wednesday and again next weekend. There will be potential heat
impacts next weekend as triple digit temperatures are possible
for warmest valley locations.




NWS LOX Office Area Forecast Discussion