Scientific Forecaster Discussion
Southwest California area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
1007 am PST sun Dec 8 2013
..updated aviation discussion...
overnight frost and hard freeze conditions in wind sheltered
locations will continue through Tuesday across the region.
Afternoon temperatures will be below normal throughout the week.
Gusty offshore winds will develop across much of Los Angeles and
Ventura counties Monday through Tuesday.
Short term (today-tue)...
middle-morning discussion update...
very cold temperatures across the service area this morning. Winds
across the coastal waters have diminished and surf is subsiding.
No significant changes are anticipated to the forecast package.
Below normal temperatures are expected throughout the week and
overnight temperatures still look to be especially chilly Monday and
Tuesday mornings. Two flies in the ointment are an incoming pulse of
energy and high clouds just off the South Coast this morning and the
offshore winds expected to develop early on Monday. The incoming
weather will spread a blanket over the region during the afternoon
heating period and may keep temperatures from rebounding as much...
but if it lingers may also help to keep temperatures from dropping
as much overnight. The developing offshore winds will help to scour
the cold air mass out of much of Ventura and Los Angeles counties...
but timing is bit untypical and this could make a difference in how
and where cold temperatures reform on Tuesday morning.
Tweaks will be made for as these conditions develop and also to
upgrade the high wind watch to either an advisory or warning.
However these should remain tweaks and not widescale changes.
Early morning discussion...
some patchy stratocu has developed in the valleys of l.A. County early
this morning...but do not expect these clouds to last too long.
Otherwise...skies were mostly clear across the region.
However...some high clouds were pushing toward the central coast
early this morning...and these should overspread the region this
morning. Will go with a partly cloudy forecast for today. It was
very chilly across the region this morning... especially in wind
sheltered locations. Frost advisories are in place through middle
morning for most coastal and valley areas...with the exception of
the l.A. County coast and the San Gabriel valley. While many
locations remained above freezing due to a bit of lingering
wind...once winds drop off...expect temperatures to drop in most areas by
daybreak...so will leave frost advisories in place.
A short wave trough zipping southeastward through the Pacific
is forecast to pass just to the S and west of the forecast area this
afternoon and early this evening. Thicker low and middle level
cloudiness associated with this system may push into portions of slo
and sba counties this afternoon...and skies may actually become
mostly cloudy for a while. All models keep any rain off the
coast...but it will get very close to portions of western slo and sba
counties. Would not be surprised if a few showers pushed in into
some coastal sections of slo and sba counties this afternoon.
It will be another very chilly day across the region...with maximum
temperatures 8 to 15 degrees below normal in most areas.
With skies becoming clear...and the atmosphere drying out
tonight...expect minimum temperatures to be lower in most areas. In
fact...it may be the coldest night of the season in many locations.
In wind sheltered locations of the central coast...the Santa Ynez
Valley...across the S coast of sba County and in the Ojai
Valley...hard freezes are likely...so freeze warning have been
Elsewhere...the low temperature forecast will be very tricky...since
a decent Santa Ana wind event is expected to unfold. Right now...the
current thinking is that temperatures will drop quickly through midnight
into the early morning hours...with temperatures dropping to near or
slightly below freezing in the valleys of l.A. County...the Santa
Monica Mountains...the coastal valleys of vtu County and the Ventura
County coastal plain. Frost advisories have been issued for those
areas for tonight.
With steepening offshore gradients and decent upper support in the
form of strong winds aloft...good subsidence and cold
advection...expect a rapid increase in NE winds in the mountains of l.A.
And vtu counties late tonight...with gusty winds spreading into most
valley areas...the Ventura County coastal plain and below passes and
canyons from Malibu to Hollywood Monday morning. Where gusty winds
develop...temperatures should rise quickly above freezing Monday morning.
There is the potential for damaging wind gusts to 60 miles per hour or greater
in the mountains of l.A. And vtu counties and in the Santa Clarita
valley...but with still some uncertainty have issued high wind
watches rather than warnings for those areas. Across most of the
rest of l.A. And vtu counties...wind advisories have been issued. It
appears as though winds will stay strong for the entire day
Monday...then possibly drop off briefly Monday evening before
increasing again late Monday night and early Tuesday.
Therefore...have kept advisories and watches going right through
noon on Tuesday.
It will be another chilly day on Monday...then maximum temperatures should edge
upward a few degrees on Tuesday...though they still will be well below
normal. Hard freezes are possible once again Monday night and early Tuesday
on the central coast...in the Santa Ynez Valley...on the S coast of
sba County and in the Ojai Valley. Elsewhere...there should be
enough wind to keeps temperatures above freezing Monday night/Tuesday morning.
In the mountains...the combination of low temperatures and strong
winds will create very low wind chill values at times through
Tuesday...down to between zero and 5 or 10 degrees below zero in
some locations during the late night and early morning hours.
Frostbite can occur rather quickly on exposed skin with wind chill
values this low.
Long term (wed-sat)...upper ridge lingers over the region on Wednesday.
Low level flow will remain offshore...but quite a bit weaker than on
Tuesday...so expect only locally gusty winds in the mountains and valleys Tuesday
night and Wednesday. It will still be cold Tuesday night...but probably a few
degrees warmer in most areas. Maximum temperatures should edge upward a bit
more on Wednesday. A weakening trough will move into the West Coast on Thursday.
Any rain should remain north of the region...but there will likely be
some cloudiness pushing into the forecast area. An upper ridge will
begin to build into the region Friday and Sat...and if the 06z run of
the GFS is correct...temperatures may actually rise to slightly above
normal levels on Sat...and maximum temperatures currently in the forecast
for Sat will have to be trended upward.
Aviation...08/1800z...VFR conditions expected across the forecast
area through the next 24 hours...except there is a 10 percent chance
of MVFR ceilings and -shra for coastal sites kspb...ksmx and ksba
between 20z and 00z. An offshore flow pattern will bring low level wind shear to
portions of the San Fernando Valley and kbur by 12z. 20% chance low level wind shear
affects the Los Angeles coastal plain after 12z.
Klax...high confidence in 18z taf with VFR conditions through the
period. There is a 20 percent chance that east winds will increase
to 10 miles per hour or higher between 14z and 18z Monday.
Kbur...high confidence in 18z taf with VFR conditions through the
period...and low level wind shear/turbulence due to offshore winds by 12z.
Fire weather...gusty winds and very dry conditions are expected
Monday through early Tuesday across the mountains and valleys of Los
Angeles and Ventura counties. Despite the cold conditions...fire
danger may approach critical levels...especially since the fuels
are still rather dry.
Marine...gusty NE winds will affect the inner waters Monday through
early Tuesday. Small craft advisories will likely be needed. Gusty
NE winds and short period NE winds waves of 3 to 5 feet are expected
to affect Avalon Harbor and Smugglers Cove at times Monday through
early Tuesday...creating hazardous conditions for boaters in The
Harbor...and for improperly moored vessels.
freeze warning (see laxnpwlox).
High wind watch (see laxnpwlox).
Wind Advisory (see laxnpwlox).
Frost advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).