Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 220353
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
853 PM PDT Fri Oct 21 2016
A significant cooling trend is expected over the weekend as a
trough of low pressure develops off the coast, bringing a return
of the marine layer and onshore flow. There is a slight chance of
rain for some areas south of Point Conception Sunday night and
Monday, with a better chance for significant rain late in the
week. Slightly below normal temperatures are expected next week.
It was another hot day across much of the forecast area today,
with max temps mostly in the 90s in the valleys, in the mid 80s
to around 90 across the coastal plain, in the 80s in the Antelope
Valley and lower mountain elevations, and in the 70s in immediate
beach areas on the Central Coast and Catalina island. Max temps
were generally lower in most coastal and valley areas, with a few
exceptions, and slightly higher in foothills and Antelope Valley.
The leading edge of a large area of stratus has pushed into the
beaches of the Central Coast, and the WRF suggests it will
overspread the entire Central Coast, but shows it stopping short
of pushing into the Santa Ynez Valley. The marine layer is very
shallow, so there will likely be some dense fog. In fact, dense
fog will likely be widespread enough to require dense fog
advisories, at least for the Central Coast overnight into mid
morning Saturday, and one will be issued shortly. There will be
widespread dense fog across the outer and northern coastal waters
overnight and Sat morning, and a marine weather statement has
already been issued.
The WRF shows skies remaining mostly clear south of Pt Conception,
but it is possible that some low clouds/dense fog could push into
the SBA Channel into portions of the south coast of SBA County
and the VTU County coastal plain later tonight/Sat morning, and
less likely, some stratus could develop across immediate coastal
sections of L.A. County toward daybreak. Expect clearing by mid
morning in most areas, although some stratus could linger near the
Central Coast beaches into the afternoon.
A broad upper level trough in the eastern Pacific will move toward
the forecast area Saturday through Sunday, causing 500 mb heights
to fall, and bringing broad sw flow aloft to the forecast area.
Onshore flow will also increase on Sat. Expect several degrees of
cooling in all areas on Saturday, down by 10 degrees or possibly
more across portions of the coastal plain.
Expect stratus to be more widespread across the coastal plain
Sat night and Sunday morning. By Sun morning, mid level moisture
is forecast to move up from the s into L.A. and VTU Counties, so
skies should become mostly cloudy Sunday, even in places that do
not receive stratus.
The low level flow is forecast to turn se to sw by Sun, with gusty
winds expected mainly over the foothills, mtns and deserts by Sun
afternoon and lingering into Sun evening, with another round of
gusty winds in these areas Mon afternoon.
A weak short wave embedded in the southwesterly flow will
cause mid level clouds to overspread all areas Sun night and Mon.
The WRF and the EC show more in the way of moisture than does the
GFS, and suggest there is a chance for showers Sunday night into
Monday morning, mainly from sern SBA County through L.A. County.
Pops may have to be expanded and increased a bit to account for
the latest slightly wetter model run of the WRF.
***from previous discussion***
Temps will continue to lower through the weekend. For Sat, highs
will be near normal to slightly below normal along the coast, but
still be 4 to 8 deg above normal inland. For Sun and Mon, temps
are expected to be several degrees below normal along the coast,
and near normal to slightly above normal inland. The warmest vlys
and foothills should reach the lower to mid 80s Sat, and upper 70s
to low 80s Sun and Mon.
The GFS and EC are in decent agreement overall with the synoptic
scale features Tue thru Fri, with just some minor timing differences
showing up by late in the week. Weak upper level ridging is forecast
over srn ca for Tue and Wed. A deep upper level trof is expected to
develop over the ern Pac well off the CA coast by Wed. Plenty of
energy will drop into the upper trof and develop an upper level and
surface low about 600 miles sw of CA by early Thu. This system will
move e with a surface cold front Thu and Thu night. The frontal
system should move thru the forecast area late Thu night and Fri.
The EC is slightly faster with this system by late in the week.
Dry weather with some mid and hi clouds at times can be expected Tue
thru Wed. As the e Pac wx system approaches on Thu with a warm
front, clouds will overspread the area with a chance of rain
developing in nearly all areas. The chance of rain will continue
across the region Thu night and Fri as the cold front moves thru.
The medium range models have been pretty consistent with this
weather system, and there is increasing confidence swrn CA could be
affected by a rather strong Pacific storm, so current POPs may be
underdone. The models are showing the potential for significant
amounts of rain and wind to move in with this storm. Swrn CA will
even potentially be under a diffluent flow aloft ahead of the upper
low Thu night into Fri morning, which could lead to the possibility
of enhanced rainfall due to thunderstorm development and increased
orographic flow into the mtns. This system also has the potential
for flash flooding and debris flows in recent burn areas. Snow
levels will be very high due to the broad southerly flow aloft ahead
of the system. If later runs of the medium range models continue to
be in good agreement with this Pacific storm, later shifts will need
to increase POPs significantly across the forecast area.
Temps will continue to be a few degrees below normal along the
coast, and near normal to several degrees above normal inland Tue
thru Thu, then cool to several degrees below normal across the
region on Fri. The warmest vlys and foothills will be in the mid 70s
to low 80s Tue thru Thu, then drop into the lower 70s for Fri.
At 23Z, there was a shallow marine layer depth to 300 feet at
KLAX. The top of the inversion was near 2000 feet with a
temperature of 26.5 degrees Celcius.
High confidence in VFR flight conditions for all TAFs through 10Z
tonight with weak S-SE winds. Stratus and fog was near the San
Luis Obispo County coast and offshore late this afternoon and
should expand southward overnight, nearing some central coast
terminals toward Saturday morning. More widespread coastal stratus
is anticipated Saturday night into Sunday. There is a 40 percent
chance for lifr conditions at KSMX 10-16Z Saturday...and a 20
percent chance at KSBP and less than 10 percent chance at KLAX and
KLAX...High confidence in VFR conditions through 10Z. Less than 10
percent chance of LIFR fog conditions 10-16Z Saturday morning.
Light SE winds likely on Saturday.
KBUR...Very high confidence in VFR conditions through Saturday.
The Small Craft Advisories (SCA) for zones PZZ670 and PZZ673 will
likely continue through late Saturday night as planned. The SCA
through late tonight will be for winds and seas with gusts of
25-30 kt. Winds will subside by Saturday morning, but a moderately
large, long period NW swell will enter the coastal waters N of
Pt. Conception by Saturday afternoon. SCA conditions Saturday
afternoon through late Saturday night will be for hazardous seas,
with combined seas of around 10 ft. Combined seas of 10 ft will
also be possible for PZZ645 Saturday afternoon and evening, and a
SCA for hazardous seas may need to be considered.
After late tonight, winds will generally remain light for all
areas through Tuesday or Wednesday. No SCAs are anticipated after
Areas of dense fog are expected tonight through noon on Saturday
for the nearshore waters along the Central Coast, along with the
outer waters. Dense fog is also possible for the inner waters
south of Pt. Conception by late tonight or early Saturday morning.
Red Flag Warnings have expired, although a few locations
still had single digit humidities. While winds have
decreased, humidities, especially in the mountains, are expected
to remain very low through the weekend, with poor overnight
recoveries. In addition, increasing south to southeast winds
Saturday into Sunday will present some elevated fire weather
conditions in the mountains, though the duration is not expected
to meet Red Flag criteria.
At lower elevations, a return to a weak onshore flow will lead to
higher humidities, an earlier sea breeze, and much cooler
weather over the weekend.
CA...Dense Fog Advisory in effect until 9 AM PDT Saturday For
zones 34-35. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Sunday For
zones 670-673. (See LAXMWWLOX).