Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
843 PM PDT sun may 19 2013
Synopsis...
Breezy to locally gusty Santa Ana winds and warming temperatures
are expected through Monday evening as weak offshore flow
develops. A cooling trend will begin on Tuesday along with an
increase in night through morning low clouds and fog. Cool weather
and partly to mostly cloudy conditions will likely continue into
next weekend.
&&
Update...
A ridge of high pressure aloft continues to nose into the region
this evening from the west. A cold air mass...in the wake of a
trough moving through the intermountain west...will settle into
the Great Basin and allow surface high pressure to establish. An
offshore flow pattern will develop across the area as surface
pressure high pressure builds into the Great Basin. The air mass
will warm and dry...especially across the coastal and valley areas
due to compressional heating on Monday morning. The latest models
are suggesting a marginal advisory level burst of winds
developing on Monday morning with the highest confidence placed
in the Los Angeles and Ventura County mountains and the Santa
Clarita valley. Temperatures have been warmed slightly for the
foothills and mountains tonight as the offshore winds develop and
increase mixing...and cooled slightly for the interior sections
on Monday as a cooler air mass filters into the desert.
&&
Previous discussion...
Short term...a deep upper low will begin to move south along the
eastern Gulf of Alaska Monday into Tuesday. For southwest
California...winds will shift back to onshore allowing a more
widespread marine layer to form Monday night. The nam12 shows a
coastal eddy forming near Catalina Island but heights aloft will
still be high enough to keep the marine layer shallow...so that
low clouds and fog will likely be limited to near-coastal areas.
The upper closed low will continue to sink south along the West
Coast Tuesday into Wednesday...resulting in a cooling trend and a
deepening marine layer for SW California. By Tuesday night marine
layer clouds should advance inland through all but the most interior
valleys...and wednesday's highs will be capped near 80 degrees.
Long term...the medium range models agree fairly well in placing
the closed upper low over Oregon early Thursday...where it is
forecast to remain nearly stationary into Friday. The proximity of
the upper closed low will produce cyclonic curvature aloft and
onshore flow over the California coast. This scenario will create
a deep marine layer over southwest California...with low clouds
moving well inland to the interior valleys Thursday and Friday
nights. The upper low is forecast to lift northeast out of the
area late Friday into Saturday as an upper trough forms along the
California coast. This scenario will likely produce little change
through late in the extended...with a deep marine layer and
extensive low clouds forming each night to morning period...along
with lower than normal daytime high temperatures.
&&
Aviation...19/2350z.
VFR conditions are expected throughout the period except for a
chance of LIFR to IFR conditions at Los Angeles County coast and
central coast terminals between 08z and 16z. Period of moderate to
strong wind shear and turbulence are possible at ksba through 08z
and Ventura County terminals between 10z and 18z.
Klax...VFR conditions are expected throughout the period except for
a 30 percent chance of LIFR to IFR conditions.
Kbur...VFR conditions are expected throughout the period with a 20
percent chance of moderate low-level wind shear and turbulence
between 10z and 16z.
&&
Fire weather...
Gusty northwest to north winds will continue to bring lowering
humidities and elevated fire danger through this evening across the
mountains...Antelope Valley...Santa Clarita valley...and sba South
Coast. These areas will generally see gusts in the 30 to 40 miles per hour
range. Already seeing humidities lowering to around 10 percent this
afternoon across the Santa Clarita valley...along with temperatures
climbing into the lower 90s.
By late tonight and Monday...surface high pressure builds into the
Great Basin...setting the stage for a brief offshore wind event for
the region. The projected lax-Daggett gradient is only expected to be
around -1.5 mb...with some upper level wind support. Offshore
winds are expected to develop across the mountains late
tonight...then descend into the valleys on Monday morning. At this
time...wind gusts in the 35 to 40 miles per hour range can be expected in the
mountains on Monday morning...with gusts in the 25 to 35 miles per hour range
across the valleys (mainly Santa Clarita and eastern Ventura
valleys). There will likely be widespread humidities between 10 and
15 percent on Monday across the mountains and valleys of Los Angeles
and Ventura counties...with isolated single digit readings likely.
While this offshore wind event does not appear to be strong enough or
have sufficient duration to warrant a red flag warning...there will
be elevated fire danger across the mountains and valleys on
Monday...especially considering the very dry fuels for this time of
year. The offshore winds are expected to diminish fairly rapidly on
Monday afternoon...with onshore winds gradually spreading across
coastal and valley areas through the afternoon and evening hours.
&&
Marine...a thermal trough of low pressure will intensify near shore
while high pressure persists over the eastern Pacific through early
Monday and a Gale Warning is in effect over the outer waters from
San Clemente Island to Point Piedras Blancas and including the inner
waters from Point Piedras Blancas to Point Sal through late tonight.
The ridge of high pressure over the eastern Pacific will weaken
Monday. A trough of low pressure is then expected to move southeast
over the offshore and coastal waters early Tuesday through early
Wednesday while a ridge of high pressure strengthens over the
eastern Pacific. Northwest winds will likely increase near shore
north of Point Conception and over the outer waters south of Point
Conception Tuesday evening through Wednesday. A gale watch will be
issued when the risk of gale force winds has significantly
increased.
&&
Lox watches/warnings/advisories...
Gale Warning (see laxmwwlox).
Small Craft Advisory (see laxmwwlox).
Wind Advisory (see laxnpwlox).
High rip current risk (see laxsrflox).
Marine weather statement (see laxmwslox).
&&
$$
Public...Hall
aviation...Hall
marine...30
fire weather...gomberg
synopsis...kj/Hall
Www.Weather.Gov/losangeles
