Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 230004
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
504 PM PDT Sat Oct 22 2016
Updated Aviation and Marine Discussions
A trough of low pressure will persist off the West Coast through
next week, supporting high clouds at times with showers possible
early in the week and a more significant rain event possible later
in the week. Onshore flow will bring near normal temperatures with
night to morning clouds and patchy fog to coastal areas.
The marine inversion at KLAX early this afternoon was very shallow
at less than 500 ft deep. Low clouds have lingered over the SoCal
Bight w and nw of Santa Catalina Island to around Point Conception.
A light southerly low level flow has caused the low clouds to edge
northward to the SBA County Central Coast early this afternoon, with
low clouds also along some of the beaches from the southern SBA
County coast to near Zuma Beach. Patchy dense fog has been observed
with these low clouds as well. Otherwise, fair skies with some hi
clouds have persisted over the forecast area today. Breezy s to w
onshore winds will prevail this afternoon mainly over the foothills,
mtns and deserts. Temps much cooler for the coast and vlys today
with highs ranging from the 70s to low 80s along the coast to the
80s to around 90 in the vlys.
Weakening upper level ridging will persist over srn CA thru today. A
broad upper level trof over the e Pac will gradually move into the
area tonight thru Sun, with a broad sw flow aloft and gradually
lowering 500 mb heights. A shortwave trof embedded in the broad
upper level trof is forecast to approach the area Sun night, then
move across srn CA on Mon. Behind this system, a flat upper ridge is
expected to develop over srn CA for Mon night and Tue, with a broad
wsw flow aloft.
Marine layer clouds are forecast to become more organized over the
SoCal bight tonight, with the low level flow turning se to s,
including along and just off the Central Coast tonight and
persisting into Sun. This flow will help to push low clouds into the
SBA s coast and into the central Coast later tonight and Sun
morning, with some low clouds possibly lingering at the Central
Coast beaches thru Sun afternoon. Low clouds will also be possible
along the immediate coast of VTU/L.A. Counties overnight. Patchy
dense fog cannot be ruled out along the Central Coast and SBA County
s coast tonight into Sun morning as well thanks to a lingering
shallow marine inversion. Otherwise, mostly clear skies can be
expected across much of the forecast area thru Sun morning. However,
by Sun morning, mid level moisture is forecast to move up from the s
and into L.A. County, with mostly cloudy skies developing.
Additional mid level clouds moving in will bring partly to mostly
cloudy skies to VTU County by Sun afternoon. The upper level
disturbance will help to spread this cloudiness to all areas Sun
night and Mon. Marine layer clouds are also forecast to persist
along all coastal areas Sun night into Mon morning.
There should be enough moisture with the upper level disturbance to
bring a slight chance of showers to L.A. County and srn VTU County
Sun night into Mon, and to parts of SBA and SLO Counties late Sun
night and Mon morning. Any rainfall that does occur will be on the
light side and generally less than 0.10 inch, altho the EC is
predicting quite a bit more rainfall but is the outlier and will
lean toward the GFS/NAM solutions for this event.
Se to sw low level flow is forecast over the region on Sun, with
gusty winds expected mainly over the foothills, mtns and deserts by
Sun afternoon and lingering into Sun evening. Locally gusty s to sw
winds will persist in these areas Mon afternoon as well as the upper
disturbance moves through the area.
Dry weather is forecast across the region Mon night and Tue. Marine
layer low clouds and fog should persist along the coastal areas
night and morning hours as well. Otherwise partly cloudy skies can
be expected for the most part.
Temps will continue to have a cooling trend Sun through Mon, then
turn slightly warmer on Tue. Highs on Sun are expected to be several
degrees below normal along the coast, and near normal to slightly
above normal inland, then for Mon highs will be several degrees
below normal for the coast and vlys, and near normal to slightly
above normal for the mtns and Antelope Vly. For Tue, temps are
expected to be several degrees below normal along the coast and
near normal to slightly above normal for the vlys, mtns and deserts.
The warmest vlys and foothills should reach the upper 70s to around
80 on Sun, mid to upper 70s for Mon, and mid 70s to low 80s for
GFS and EC causing headaches today with regard to agreement with the
synoptic scale feature`s and timing especially Thu thru Sat. As a
result of the uncertainties and lower confidence, decided to broad-
brush the forecast with a loose model blend.
Fair weather with near to slightly above normal temps can be
expected for Wed. A large upper level trof is still forecast to
develop over the e Pac about 800 miles w of CA by Wed, then move e
toward the CA coast Thu. The EC is much quicker with the upper trof
and associated surface low pressure system and spreads rain into the
Central Coast as early as Thu morning, then pushes rain thru the
rest of the forecast area Thu afternoon and Thu night. The GFS
spreads rain into the area Thu night and Fri morning before tapering
off Fri afternoon. Went with a chance of rain across the region Thu
thru Fri, but again confidence is low on the timing of the weather
system. A moist onshore flow with a residual slight chance of
showers should linger across the region Fri night into Sat morning
as well. There is still the chance of significant rain of an inch or
more across parts of the forecast area from the upcoming weather
system, with the potential for flash flooding and debris flows in
recent burn areas. We will monitor this situation very closely in
the coming days. Further model runs thru early next week should help
to resolve timing and strength issues along with the finer details
of this late-week Pacific storm.
At 2300Z, the marine layer was 400 feet deep with a weak inversion
extending to 4500 feet and 20C at the top.
Moderate confidence in winds. Low confidence in timing and
prevalence of stratus and fog. 60 percent chance of mostly IFR
cats at KSMX KSBA KOXR, 40 percent chance at KLGB KLAX KSMO KCMA
KSBP. Delayed onset from previous TAFs at most airports based on
currently minimal stratus coverage. Mid-level moisture will
stream in Sunday through Monday, and a few showers are possible
pretty much everywhere mainly Sunday night into Monday. Isolated
thunderstorms cannot be ruled out and may be added to the
KLAX...Low confidence in timing and prevalence of stratus. 40
percent chance of IFR CIGS with best chance 10-16Z. 20 percent
chance of arrival before 06Z. Southeast winds will increase Sunday
morning. 70 percent chance of SE winds 8 KT or greater 15-18Z.
Winds should shift to southwesterly by 21Z.
KBUR...High confidence VFR conditions through Sunday. Southeast
winds a little stronger than normal are expected on Sunday.
High confidence that dense fog with visibility below 1 mile will
be prevalent over much of the coastal waters tonight into
Sunday...though visibilities might be slightly above 1 mile south
of Point Mugu Sunday morning. Not sure how close to shore the fog
will be however.
Low end Small Craft Advisory (SCA) winds and seas likely through
this evening beyond 30 miles from the Central Coast shore.
Widespread southeast flow will form tonight into Sunday nearly
everywhere, strongest through the San Pedro Channel, Anacapa
Passage, and western Santa Barbara Channel where gusts of 15-20 KT
are possible. Otherwise, weaker than normal winds are expected
through Wednesday with no SCA expected.
.FIRE WEATHER...22/400 PM.
An approaching upper level trough of low pressure will bring
increasing southeast to southwest flow across the region this
weekend. The strongest winds will continue to be focused across
the mountains, santa clarita valley, and Antelope Valley where
gusts between 25 and 40 mph will be common. These areas will also
generally see humidities in the 7 to 15 percent range during the
day with poor recoveries at night. As a result, elevated fire
danger will continue across the above mentioned areas through the
weekend, with brief critical fire weather conditions in the LA
county mountains. Already seeing a few stations in the LA county
mountains reaching brief critical fire conditions this afternoon
as single digit humidities have combined with southerly winds
gusting up to 35 mph. At lower elevations, the increasing onshore
flow will bring a slight cooling trend with increasing humidities
through the weekend.
Mid level subtropical moisture will bring a slight chance of
showers to areas south of Point Conception late Sunday into Monday
morning. Any rainfall that does fall is expected to be light,
generally less than a tenth of an inch. More significant
precipitation is possible later next week, sometime in the
Thursday to Friday time frame but there is considerable
uncertainty in timing and strength with this system.
CA...Beach Hazards Statement in effect through Sunday evening For
zones 34-35. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Sunday For
zones 670-673. (See LAXMWWLOX).