Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
922 am PDT Sat Oct 22 2016
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.
A surface-based low and strong inversion was noted at kvbg this
morning, with the inversion also surface-based but not as strong at
klax. Marine layer clouds formed last night over the socal bight but
with weak offshore flow the low clouds remained just off the coast
this morning, with mostly sunny skies noted in all areas. A few hi
clouds should move overhead thru this afternoon, otherwise the
mostly sunny skies will persist thru the day. Breezy onshore winds
can also be expected this afternoon mainly over the foothills, mtns
and deserts. Temps today are forecast to be much cooler than
yesterday for most areas but remain several degrees above normal.
Highs in the warmest vlys and foothills this afternoon should reach
the mid to upper 80s.
Weakening upper level ridging will persist over srn California 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 California on Mon.
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 by late 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. Patchy dense fog cannot be ruled
out along the central coast later 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 as well. 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 central coast and sba County S coast 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 along coastal areas of sba/slo counties late
Sun night and Mon morning. Any rainfall that does occur will be on
the light side and generally less than 0.05 inch.
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.
Temps will continue to have a cooling trend sun through Mon. 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. The warmest vlys and foothills should reach the upper 70s to
around 80 on sun, and mid to upper 70s for Mon.
***From previous discussion***
The GFS and ec which had been in good agreement through next
Friday have diverged late this week on the latest 00z runs.
Overall, through Wednesday, both remain in agreement with only
minor large scale differences. However, the European model
continues to swing an upper trough that becomes a closed low
several hundred miles off the Monterey coast. The 549 dm low
takes on a negative tilt and brings a diffluent area that moves
over much of the forecast area. This feature will help to enhance
rainfall across slo/sba counties initially late Thursday before
moving across vtu/la counties Thursday night into Friday morning.
Also, a 990 mb surface low will be a few hundred miles west of
Monterey Thursday afternoon and should help enhance southerly
winds ahead of the front which would also help to enhance rainfall
totals across south facing mtns.
The GFS which had this same feature now keeps a positive tilted
open trough across the forecast area which would be quicker
to move through. Since this is the first change with the GFS, will
not be making any changes, especially with better confidence with
ec model. Nevertheless, there is a good chance of rain beginning
Thursday afternoon and continuing into Friday. Rainfall totals
will depend on how dynamic this system will be. Still too early
to give more details, except with the system tapping into sub-
tropical moisture, snow levels will be quite high and there will
be a chance of thunderstorms embedded with this system if it remains
For Tue/Wed expect mostly sunny to partly cloudy conditions
overall with temps around normal for this time of year. High temps
will cool off as this system approaches the area by Thursday and
continue cool into the weekend as another possible storm system
moves in late Saturday night or Sunday.
At 1600z, there was a surface-based inversion at klax. The top of
the inversion was 2000 feet with a temperature of 22 degrees
Overall, low to moderate confidence in 18z taf package. With a
shallow marine inversion and weak onshore flow, stratus and fog
will likely impact coastal taf sites this afternoon and tonight.
However confidence in timing of impacts, extent of impact and
flight category is low. IFR/LIFR conditions could impact coastal
sites south of Point Conception this afternoon then all coastal
Klax...low confidence in 18z taf. Moderate confidence in VFR
conditions persisting through this evening. For tonight, moderate
confidence in return of stratus/fog, but low confidence in timing
and flight category.
Kbur...high confidence in 18z taf. VFR conditions are anticipated
through the taf period.
For the outer waters, moderate confidence in current forecast. Low
end Small Craft Advisory (sca) level winds will likely continue
across western sections of the waters through this evening while
seas build to near 10 feet (possibly reaching Small Craft Advisory levels). For
Sunday through Wednesday, winds and seas are expected to remain
below Small Craft Advisory levels.
For the inner waters, moderate to high confidence in current
forecast. For the waters north of Point Sal, winds and seas are
expected to remain below Small Craft Advisory levels through Wednesday (although
there is a 30% chance of seas reaching Small Craft Advisory levels this afternoon
and evening). For the waters south of Point Conception, winds and
seas are expected to remain below Small Craft Advisory levels through Wednesday.
Some dense fog, with visibility one nautical mile or less, will
continue to impact the coastal waters this morning and could
linger into the afternoon.
An approaching upper level trough of low pressure will bring
increasing southeast to southwest flow across the region this
weekend. The strongest winds will be focused across the
mountains, Santa Clarita valley, and Antelope Valley where gusts
between 25 and 35 mph will be common. These areas will also
generally see humidities in the 8 to 15 percent range during
the day with poor recoveries at night. However, very dry air
humidities in the 2 to 5 percent range is being observed this
morning across the higher elevations of the Los Angeles County
mountains above 6000 feet. 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. At lower elevations, the increasing onshore flow will
bring a slight cooling trend with increasing humidities through
Pz...Small Craft Advisory in effect until 3 am PDT Sunday for
zones 670-673. (See laxmwwlox).