Scientific Forecaster Discussion
fxus66 klox 222353
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
453 PM PDT sun Oct 22 2017
Updated aviation discussion
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.
A 595 ridge of high pressure aloft continues to build just west of
the baja of California. To the north, a shortwave trough continues
to push through the Pacific northwest towards the central rockies.
The building ridge aloft with moderate to strong offshore
gradients (lax-dag peaked at -5.8 and lax-bfl peaked at -8.1) has
resulted in temperatures rising well into the 80s for many areas
west of the mountains with the warmest areas climbing into the 90s
so far today.
The warming trend continues heading into Monday with offshore
flow continue to shift from the north to the east. This trend will
lead to advisory level winds of about 10 mph stronger than
experienced today across typical areas near the Los Angeles and
Ventura County border from the mountains to the coast with the
Wind Advisory continuing through early Wednesday afternoon. A
Wind Advisory will continue for wind prone foothills of the
central coast into the adjacent mountains tonight into early
Monday afternoon. Wind prone areas will only see low temperatures
in the 70s south of point conceptions and 60s to near 70 north of
Point Conception. Temperatures will continue to be the big story
Monday with widespread 90s to low 100s west of the mountains with
several locations forecasted to be near or excess of their daily
records (see the record heat weather story for some examples).
The ridge of high pressure aloft will likely peak around 597 dm
over northeastern California late Monday into early Tuesday.
Clockwise flow around this feature along with surface high
pressure peaking around 1035 mb will support another jump in winds
Monday night into Tuesday with moderate to locally strong Santa
Ana winds during this time. The stronger and more widespread
overnight winds with continuing warming near the surface will
bring even warmer overnight lows for many areas for Monday night.
Record breaking heat is likely again for many areas Tuesday due to
the strengthening offshore flow and high pressure aloft. Widespread
90s to low 100s can be expected again to the west of the
mountains. 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).
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.
At 23z at klax, there was a 400 ft deep marine layer, with an
inversion up to 1400 ft at 29 degrees celsius.
High confidence in VFR everywhere through at least Monday. Strong
NE winds developing Monday and peaking Tuesday...with gusty
surface wind or low level wind shear concerns at ksbp koxr kcma klgb kbur kvny.
Klax...high confidence in VFR through at least Monday. NE winds
will develop aloft on Monday with minor low level wind shear +/- 10kt possible
12-20z. Any east wind at the surface likely under 8 knots (20 percent
chance of 8+kt).
Kbur...high confidence in VFR through at least Monday. NE winds
will develop aloft on Monday with minor low level wind shear +/- 10kt possible
12-20z. Low confidence on surface wind forecast for Monday 12-00z,
with northwest to southeast winds possible.
Speeds in zone 645 have been lowered a bit, but a Small Craft Advisory remains in
effect through late tonight. Similarly, zones 670-676 will have an
Small Craft Advisory in effect through late tonight for gusty north winds.
Additionally, portions of 676 will also be subject to northeast
winds that will increase across the inner waters south of Point
Conception late tonight and Monday morning. Therefore, the Small Craft Advisory
across 676 will likely be extended into Monday morning.
The strongest winds for Monday and Tuesday will be found directly
below passes and canyons in the southern inner waters from
Ventura to Santa Monica Monday and Tuesday mornings.
The eastern portion of Santa Catalina may also see some local
gusty winds to 25 or 30 knots along with choppy seas Tuesday
morning. This situation will be evaluated more thoroughly as we
approach the critical period.
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.
California...Wind Advisory in effect until 5 am PDT Monday for zones
34-35-51. (See laxnpwlox).
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 from midnight tonight to 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 2 am Monday to 3 PM PDT
Tuesday for zones 650-655. (See laxmwwlox).
Small Craft Advisory in effect until 5 PM PDT Monday for zone
676. (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