Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
830 PM PDT Monday may 20 2013
Synopsis...
A cooling trend will begin on Tuesday along with an increase in
night through morning low clouds and fog. Cooler than normal weather
with a late clearing marine layer is expected going into the weekend.
&&
Short term (tonight-thu)...an upper level ridge of high pressure
combined with offshore winds brought very warm and dry conditions to
inland areas today. Several valley stations soared into the 90s...with
Saugus (in the Santa Clarita valley) topping out at 98 degrees. Pressure
gradients have now switched back to weakly onshore this evening...allowing
for a return of onshore winds across coastal and valley areas. Upper level
ridge of high pressure will begin to weaken and shift eastward overnight
into Tuesday as an upstream trough moves into northern and central
California. This will cause increasing onshore pressure gradients...especially
Tuesday afternoon. In fact...latest 00z NAM model has lax-Daggett gradient
peaking at +8.5 onshore by late Tuesday afternoon.
The increasing onshore flow combined with a weak eddy circulation is expected
to spin up some low clouds and fog across coastal areas of Los Angeles...Ventura
and Santa Barbara counties...as well as the San Gabriel valley. Latest
acars data has marine layer depth around 1000 feet across the la basin...with
model cross sections showing a slight deepening of marine layer by Tuesday
morning. In addition...there will be some higher level clouds that will pass
through the district during the next 24 hours...mainly north of Point
Conception. The onshore trends and increased marine influence will result in
a cooling trend on Tuesday...with most coastal and valley areas seeing 5 to
10 degrees of cooling. There will also be gusty onshore winds across the
mountains...Antelope Valley...and Santa Clarita valley. These areas will
generally see gusts in the 30 to 35 miles per hour range...except isolated gusts to 45 miles per hour
in the foothills of the Antelope Valley. The gusty onshore winds combined
with lingering low humidities will bring elevated fire danger to these areas
on Tuesday afternoon and evening.
Upper trough combined with fairly strong onshore flow will continue to
cool temperatures Wednesday through Thursday. There will be a widespread
gale force wind event across the outer coastal waters during this time.
This increased northwest flow pattern will also generate some gusty sundowner
winds each night across southern sba County.
&&
Long term (fri-mon)...troffing to continue through the period,
though models indicate some rising of heights/thickesses Friday
before the trough deepens again over the weekend, especially Sunday.
GFS continues to be deeper and cooler but still dry. Could see some
night/morning drizzle over the weekend but otherwise remaining
cooler than normal with slow marine layer clearing. Continued breezy
in the Antelope Valley and central coast.
&&
Aviation...21/0030z.
Satellite imagery showing mostly clear skies with low clouds pushed
all the way southward to the coast of San Diego County. Weak eddy
circulation is expected to bring some of the low clouds and fog northward
along the coast to la and Ventura counties later tonight. With acars
sounding showing marine layer depth around 800 feet across la
basin...any ceilings that do form along the coast should be in IFR
category...with a 20 percent chance of LIFR conditions. Otherwise
increasing onshore flow on Tuesday will bring gusty winds to
kpmd and kwjf in the afternoon.
Klax...high confidence in VFR conditions this evening. IFR ceilings
expected to develop later tonight...within +/- 2 hours of 11z.
Kbur...mostly VFR conditions through period...with the exception
of a chance of visibilities being reduced in haze on Tuesday morning. A
20 percent chance of a brief LIFR/IFR ceilings early Tuesday morning.
&&
Lox watches/warnings/advisories...
Gale Warning (see laxmwwlox).
Small Craft Advisory (see laxmwwlox).
&&
$$
Public...gomberg/mw
aviation...gomberg
synopsis...Sweet
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