Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 230540
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1040 PM PDT Sun Oct 22 2017

.SYNOPSIS...
Gusty offshore winds will strengthen and become more widespread
through Tuesday as many locations have record heat on Monday and
Tuesday. Winds ease on Wednesday and temperatures begin to cool
and possibly drop to near normal by Friday.

&&

.SHORT TERM...(SUN-WED)

Forecast on track with little changes this evening. Northeasterly
winds are continuing to affect SLO and northern SBA Counties, but
they are on their down and nothing is near to advisory
strength...so will be canceling the wind advisory there.

The latest guidance shows little change to the Monday through
Wednesday hot Santa Ana wind event...with the 00Z NAM
strengthening the peak Tuesday 12Z LAX-DAG pressure gradient
to -8.3 MB (12Z run had -7.6 MB) and showing 45-55KT of winds
between 1000 and 5000 feet. Expecting the winds to start
strengthening after midnight in the mountains, with the winds
working their way down into the valleys early Monday morning
(4-9AM) then down into the coasts (6-10AM). The current wind
advisory keeps it simple with everything starting after midnight
tonight, which is admittedly way too early for the valleys and
coasts, but communicates the general idea of the winds increasing
Monday morning and matches up with our SGX neighbors to the south.
These winds will likely weaken a little late Monday afternoon and
evening, then strengthen again Monday night and Tuesday
morning...likely 5-10 mph stronger than Monday. Agree with the
previous shift that the winds could touch warning strength in
isolated spots on Tuesday, but this looks like a typical
advisory-type event for Monday and a strong advisory-type event
for Tuesday.

With the offshore winds in solid form, and a 593 DM (at 500 MB)
ridge currently nosing into northern California...pushing over the
entire West Coast by Tuesday...all signs point to record heat in
the day Monday and Tuesday (highs 95-105 common) and somewhat
uncomfortably warm nights (lows 65-75 common). The actual
temperatures would not be extreme if this were August, but being
the end of October with the population beginning to acclimate to
the cooler conditions we had earlier this morning, such heat poses
a concern for heat- related illnesses...especially for sensitive
populations like the elderly and children. The current suite of
heat warnings/advisories communicate this threat and look good as
is. Major fire concerns exist as well (see fire weather section
below).

***From Previous Discussion***

One caveat is that there is a deck of high clouds well
to the southwest of the area. If these hold together as they
approach the area Tuesday it could lead to some areas being a few
degrees cooler than currently forecasted.

Weakening offshore flow is expected Wednesday with weak onshore
flow expected for many coastal areas by late in the afternoon.
This along with rapidly lowering heights (596 to 588 DM) will
bring cooling to most areas, most significant north of Point
Conception and near the coast. However, temperatures will remain
well above normal with widespread 80s and 90s. The extended period
of heat Monday and Tuesday has prompted the continuing of
excessive warnings for most areas to the west of the mountains
south of Point Conception with newly issued heat advisories to
the north (see NPWLOX for the latest heat and wind products).

.LONG TERM...(THU-SUN)

Overall there is moderate confidence in the forecast through
Friday with lower confidence for next weekend. A short-wave trough
is expected to drop south and east through the Central Rockies
into the High Plains Thursday into Friday. Behind this feature a
weak and elongated high pressure may build near the Pacific
Northwest. This setup would bring continued cooling with
temperatures approaching normals and near neutral pressure
gradients Thursday and possibly Friday with warming for the
weekend. Confidence in the latter of this solution is low as any
shift east or west of the long wave pattern could bring
considerable different temperatures to Southwest California. The
If weak onshore flow persists into the weekend low clouds and
patchy fog would likely make a return to many coastal areas.

&&

.AVIATION...

23/0535Z

At 05Z at KLAX, there was a 400 ft deep marine layer, with an
inversion up to 1400 ft at 29 degrees Celsius.

High confidence in CAVU conds for next 36 hours. Strong NE winds
developing Monday and peaking Tuesday...with gusty surface wind or
LLWS concerns at KSBP KOXR KCMA KLGB KBUR KVNY.

KLAX...High confidence in CAVU TAF. There is a 20 percent chc of a
9KT east wind from 11Z-17Z.

KBUR...High confidence in CAVU TAF. NE winds will develop aloft
on Monday with minor LLWS +/- 10KT possible 12Z-20Z. There is a 30
percent chc of N to NE winds from 11Z-17Z.

&&

.MARINE...

22/900 PM

High confidence in Small Craft Advisory (SCA) N winds ending
tonight over the outer waters from the Central Coast to San
Nicolas Island. Moderate confidence in SCA NE winds from Ventura
to Santa Monica forming Monday morning by 9AM and continuing
through Tuesday afternoon, except for a few hours of weaker winds
late Monday morning and evening. Tuesday morning should be the
strongest, with localized gale force winds possible, though the
coverage of such strong winds will likely not be strong enough to
warrant an actual gale warning.

There is a chance that the NE winds will reach Avalon and Two
Harbors on Tuesday, but since this is a hot Santa Ana event the
likelihood of significant winds out that far is small. 30 percent
chance of 10-20 KT winds in the harbors, 10 percent chance of
15-25 KT.

&&

.FIRE WEATHER...

22/400 PM

From today through Wednesday, a prolonged duration of gusty Santa
Ana winds with hot and very dry conditions is expected. Northeast
winds are expected to gust between 30 and 45 mph across wind
favored passes and canyons of Los Angeles and Ventura through
tonight. The Santa Ana winds are expected to increase in areal
coverage and intensity Monday into Tuesday, with strongest winds
on Tuesday gusting between 45 and 60 mph across wind favored
portions of Los Angeles and Ventura counties. During the peak of
the offshore wind event on Tuesday, the LAX-Daggett gradient is
projected to peak at around -7.5 mb with better upper level wind
support up to 850 mb level. Moderate offshore winds and low
humidities are now expected to continue Tuesday night into
Wednesday morning, prompting the extension of the Red Flag Warning
through Wednesday. The hottest and driest conditions are expected
on Monday and Tuesday, when triple digit heat and humidities
lowering into single digits and lower teens will be common across
coastal and valley areas. Northeast to east winds gusting between
25 and 40 mph will also bring elevated to brief critical fire
weather concerns to portions of San Luis Obispo and Santa Barbara
counties through Tuesday, especially across the Central Coast. Of
particular concern will be Monday when the gusty offshore winds
combine with humidities lowering into the teens(locally single
digits) and temperatures rising to between 90 and 100 degrees
along the Central Coast.

Very warm and dry conditions during the overnight hours in the
mountains, foothills, and wind prone areas will add to this long
duration fire weather threat. The duration, strength, and
widespread nature of this Santa Ana wind event combined with the
extreme heat and very dry fuels will bring the most dangerous fire
weather conditions that Southwest California has seen in the past
few years. If fire ignition occurs, there will be the potential
for very rapid spread of wildfire with long range spotting and
extreme fire behavior that could lead to a threat to life and
property.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory in effect from 10 AM Monday to 8 PM PDT Tuesday
      for zones 34>36-39-88. (See LAXNPWLOX).
     Wind Advisory in effect until 2 PM PDT Wednesday for zones
      40-41-44>46-53-54-88-547. (See LAXNPWLOX).
     Excessive Heat Warning in effect from 10 AM Monday to 8 PM
      PDT Tuesday for zones 40-41-44>46-87-547. (See LAXNPWLOX).
     Red Flag Warning in effect until 6 PM PDT Wednesday for zones
      240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for
      zones 645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect from 6 AM Monday to 3 PM PDT
      Tuesday for zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(WED-SUN)
Heat advisory level temperatures may continue for many areas into
Wednesday. Gusty northeast winds will likely impact wind prone
areas of Los Angeles and Ventura Counties through early
Wednesday. Hot, dry and windy conditions will likely bring
continued critical fire weather conditions to some areas
Wednesday.

&&

$$

PUBLIC...Munroe/Kittell
AVIATION...Rorke
MARINE...Kittell
FIRE...Gomberg
SYNOPSIS...Kj

weather.gov/losangeles



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