Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
330 am PDT Sat Sep 20 2014
Upper level low pressure will bring cool weather through the weekend
and night through morning low clouds and fog for many coastal and
valley areas today. Isolated thunderstorms are possible over the
mountains this afternoon. Temperatures will warm slightly above
normal early next week as the upper low moves east and high pressure
briefly returns. A trough of low pressure will bring another cool
down late next week.
Short term (today-mon)...latest fog product imagery indicated
widespread stratus across much of the outer waters and central coast
as well as portions of the Santa Ynez Valley. Skies were mostly
clear across Los Angeles...Ventura and southern Santa Barbara
counties. Latest acars sounding showed a very weak inversion near
lax. So although skies were mostly clear across this area...random
low clouds will likely develop near sunrise along coastal areas and
possibly in the coastal valleys and linger through middle morning.
Models continued to show some low level moisture lingering around
the mountains and foothills...therefore have kept partly cloudy
wording for the mountains and valleys for this afternoon while
coastal areas should be mostly sunny by this afternoon.
Synoptically...an upper level low that was located off the central
coast yesterday will slowly meander south over the socal bight
today...then push inland over the forecast this afternoon. The upper
low will then exit to the northeast over Nevada by Sunday morning
leaving a weak troughing pattern over the region through Sunday.
As for today...models were advertising plenty of instability over
the mountains...but moisture is lacking with this system. A vorticity maximum
is expected to rotate into la/vtu counties later this morning
allowing for additional cooling aloft to reinforce the instability
over the mountains. A slight chance of thunderstorms remain in
effect for the Ventura and sba County mountains...and have added slight
chance for thunderstorms across the la County mountains and the
eastern Antelope Valley for this afternoon through this evening. That
being said...based on the lack of any convection yesterday and not
much of a diffluent area associated with this upper low...would look
at the glass half full and figure there will be an 80 percent chance
that there will not be any convection this afternoon. If isolated
thunderstorms to develop...there will be little chance of flash
flooding due to the lack of moisture to work with and the relatively
strong steering flow aloft for storms.
With the upper low overhead and slightly stronger onshore
flow...expect a degree or two of cooling across the forecast area
today..with warmest valley locations reaching the middle 80s...with a
few degrees warmer across the Antelope Valley. Expect a return of
night through low clouds tonight into Sunday. As the upper low kicks
out over northern Nevada...thickness levels and boundary layer temperatures will
increase a bit. Expect a more typical burnoff time of low clouds
across coast and valley locations with mostly clear skies for
Sunday. There will be a few stratocu over the local mountains Sunday
afternoon. By Monday...a weak upper level ridge will build in from the
south allowing for a modest warm up.
Long term (tue-fri)...
both GFS and European model (ecmwf) remain in pretty good agreement through late
next week with minor variations of the upper trough's position late
next week. Overall...weak ridging is expected to continue through
Tuesday before a broad upper trough begins to swing down from the
eastern Pacific by Wednesday and persist over the forecast through
the remainder of the week. This will translate into additional
warming for Tuesday with highs reaching the lower to middle 90s across
the warmest valleys and interior locations. By Wednesday...a few
degrees of cooling and an earlier afternoon seabreeze can be
expected with similar high temperatures for interior areas.
However...Thu-Fri...a cooling trend will occur as the upper trough
digs south over the forecast area. There will be increasing low
clouds and fog likely into the coastal valleys Thu/Fri. There could
be a few showers as a cold front could reach the Bay area on
Friday...but slo County should remain dry. High temperatures will be a tad
below normal by Friday next week.
Aviation...20/1100z...widespread ceilings with low MVFR to high IFR
conds in coastal sections this morning...with only patchy IFR conds
expected in the valleys. Skies should clear in most areas by late
morning or early afternoon. Expect some buildups in the mountains...and
there is a slight chance of showers and thunderstorms in the mountains of
sba...vtu and l.A. County and the Antelope Valley this afternoon and
evening. Expect widespread stratus in coastal and valley areas
tonight/Sun morning...with widespread high IFR to low MVFR ceilings once
Klax...moderate to high confidence in the 12z taf. There is a 20
percent chance that skies will not scatter out until as late as 22z.
Kbur...moderate to high confidence in the 12z taf. There is a 20
percent chance of IFR to low MVFR ceilings between 13z and 17z this
morning. There is a 20 percent chance that skies will remain clear
Moderate confidence in forecast. There is a 20 percent chance of Small Craft Advisory
level winds across the southern two thirds of the outer waters late
this afternoon through late tonight. There is a somewhat better
chance of a period of Small Craft Advisory level winds across much of the outer
waters Sunday night into Monday.