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Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
430 am PDT Sat Jul 12 2014

..updated aviation discussion...

Synopsis...

High pressure will expand west over the weekend...resulting in a
significant warming trend...especially for inland areas. Some
monsoonal moisture will return by Sunday...bringing a slight chance
to chance of thunderstorms for the mountains and deserts for much of
next week.

&&

Short term (today-mon)...latest fog product imagery indicated
widespread stratus across the central coast and Santa Ynez
Valley...while low clouds have had a tough time becoming organized
across the so cal bight and adjacent coast and valleys across la/vtu
counties this morning. There were some high clouds moving in from
the south over la/vtu counties which could have helped disrupt the
marine layer stratus. There should be some low clouds across the
Salinas River Valley as well before dawn. Latest acars sounding near
lax was around 1200 feet deep with a relatively strong inversion in
place. Once high clouds race through...low clouds should become
better organized...at least across coastal areas including the Santa
Barbara South Coast.

For today...low clouds that do develop should burnoff by middle to late
morning for most coastal areas...with quicker burnoff inland. High
temperatures will be on the increase...in fact Paso Robles should be
around 15 degrees warmer today with highs reaching just under 100
degrees from the 84 recorded yesterday. Most coastal valleys will be
in the upper 80s to lower 90s today...with the Antelope Valley
reaching around 100 this afternoon.

Synoptically...the broad upper trough that was over the region will
begin to get squeezed to the north as the upper level ridge over the
southwest begins to expand westward. This will cause boundary layer
temperatures to bump up significantly across the forecast area...which will
translate to triple digit temperatures for inland areas not affected by the
marine layer influence. The Antelope Valley and Paso Robles are
expected to reach 105 degrees...with a few la County valley areas
reaching 100 Sunday afternoon. The marine layer will be more shallow
due to the stronger ridge aloft. Low clouds will be limited to
coastal areas of la/vtu counties as well as the central coast and
Santa Ynez Valley...and possibly the San Gabriel valley...but should
burnoff quickly.

By late tonight into Sunday...the upper pattern becomes more
favorable for a return of the monsoon flow over Southern California.
An upper high Will Park itself over Nevada and western Utah on
Sunday/Tue. There will be plenty of easterly to southeasterly flow
to draw in middle level moisture from northern Mexico and Arizona into
southeastern California. There should be enough middle level moisture
to bring increasing clouds over the eastern San Gabriel mountain
range with a slight chance of and isolated elevated thunderstorm by
late Monday afternoon and evening. The air mass will remain quite
dry at the lower levels and if a thunderstorm does develop...the
main impact would be dry lightning and gusty outflow winds. Models
are indicating deeper moisture to push into la County mountains and
Antelope Valley by Monday...and push farther west into the Ventura
mountains and interior sba County by Tuesday.

For Monday...there will be increasing high clouds pushing over much
of the forecast area...likely from debris clouds from thunderstorms
that occurred over the deserts the evening before. One area of
concern that was mentioned in the earlier discussion was the NAM-WRF
model picking up on a strong vorticity maximum moving up from the San Diego
County into la/vtu counties late Sunday night into Monday morning.
If this does occur...there could be a few elevated nocturnal
thunderstorms that could make it to the coast and adjacent coastal
waters early Monday morning. Once again...main threat would be dry
lightning and gusty outflow winds. For now have left probability of precipitation around 10
percent for la/vtu counties from late Sunday night into Monday
morning.

Most of Southern California will begin to feel increased humidity in
the air as a good surge of monsoonal moisture moves across the
region. The monsoonal front (850mb dewpoints over 8 degrees c) will
push into l.A./Ventura counties. At this time it looks like there
will be a chance of thunderstorms across the San Gabriel mountain
range and the adjacent Antelope Valley Monday afternoon...with a
slight chance for the mountains of Ventura County. Precipitable
water increase significantly to over 1.5 inches on Monday over la
County. The potential for flash flooding across the la County
mountains and the arroyos flowing into the Antelope Valley will be
of some concern. One factor that will minimize this is that steering
winds will continue to be relatively strong. If storms begin to
train and backbuild...some flash flooding could be a concern. High
temperatures will be a bit cooler on Monday with the combination of
increasing clouds and humidity over the region.

Long term (tue-fri)...
monsoonal moisture will continue to affect the local mountains and
interior regions from l.A. To the sba County mountains and interior areas
on Tuesday. There will continue to be plenty of middle level moisture
to tap into to drive thunderstorm activity. Steering winds continue
to be fairly strong which should help to alleviate flash flooding to
occur...but precipitable water continues to be over 1.5 inches. If
steering winds become weaker with later model runs...there could be
greater concern for flash flooding across the la/vtu County mountains and
adjacent Antelope Valley once again.

By Wednesday...the upper high becomes more elongated from north to south
over California/Nevada. There will continue to be lingering middle
level moisture over the region which could bring a slight chance of
thunderstorms across the la/vtu County mountains and Antelope Valley once
again both Wed/Thu...but not as widespread as Tuesday. High temperatures
will remain quite warm through the extended period...but cooler than
the prior weekend. Onshore flow should strengthen slightly by
Wednesday allowing for the marine layer stratus to return the
coastal areas.

&&

Aviation...12/1130z...

Overall...moderate confidence in 12z taf package. Latest amdar
soundings indicate marine inversion based around 1200 feet south of
Point Conception...and likely a bit deeper north of Point Conception
(based on surface obs). Stratus behaving decently north of Point
Conception...but having a hard time becoming established south of
Point Conception. Whatever stratus does develop should dissipate
pretty quickly this morning. For tonight...expect any stratus to
remain confined to the coastal tafs...and likely to only impact
ksmx/ksbp/klax/klgb.

Klax...moderate confidence in 12z taf. High confidence in MVFR ceilings
this morning...and VFR conds this afternoon. Low confidence in
return/timing of IFR ceilings tonight.

Kbur...high confidence in 12z taf. There is a 20% chance of IFR
ceilings this morning between 12z and 16z.

&&

Marine...12/200 am...

Overall...moderate to high confidence in development of Small Craft
Advisory level winds across the outer waters (pzz670...673..676) as
well as pzz645 today. Expect the small craft conditions to continue
through Sunday night.

For the inner waters...do not anticipate anything more than local
Small Craft Advisory gusts across far western sections of the inner
waters through Saturday evening.

&&

Lox watches/warnings/advisories...
Small Craft Advisory (see laxmwwlox).
Beach hazards statement (see laxcfwlox).

&&

$$

Public...Kaplan
aviation/marine...rat
synopsis...sukup

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