Scientific Forecaster Discussion
Southwest California area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
1050 am PDT sun Apr 26 2015
The gusty winds will shift more northerly today...and then northeast
by Monday as high pressure moves in. This high and offshore winds
will bring fair skies and above normal temperatures into midweek.
But the last half of the week will have a cooling trend with an
increasing marine layer as a low approaches.
Short term (today-tue)...
upper level ridge of high pressure building across the eastern
Pacific will bring warming and drying conditions to Southern
California today. Satellite imagery showing some residual low level
moisture generating some clouds on the north slopes near the
Grapevine as well as the San Gabriel valley. Some gusty northerly
winds linger this morning across the district...but the trends are
down and current advisories and warnings will likely be allowed to
expire at 10 am. Temperatures will climb to around 80 degrees today
in warmest valley locations. Another surge of northerly winds are
expected to pick up this evening. Already have Wind Advisory posted
for tonight for sba South Coast and Santa Ynez Mountains...with
Montecito hills likely having the strongest winds with gusts up to
55 miles per hour. Will also need to consider issuing a Wind Advisory for the
northerly wind push this evening for the I-5 corridor extending from
the la County mountains into the Santa Clarita valley.
The northerly winds will shift to northeast after midnight across the
mountains...with the Santa Ana winds then descending into the valleys
and coastal areas late tonight into Monday morning. The projected
offshore pressure gradient between lax-Daggett is not all that
impressive...forecasted to be around -3.5 to -4 mb by Monday
morning. However...models continue to ramp up the upper level wind
support...with 850 mb winds now projected to be as high as 50 to 55
knots across typical wind corridors in la County. With this in
mind...should be a widespread Wind Advisory event for Los Angeles
and Ventura counties. The focus of the strongest winds will be the
Los Angeles County mountains...where a High Wind Warning will need
to be considered due to the potential for a brief surge of winds
gusting as high as 60 miles per hour early Monday morning. Significant warming
and drying will take place on Monday with the gusty Santa Ana
winds...bringing elevated fire danger to the region. Warmest
coastal/valley locations expected to climb to around 90 degrees on
*** From previous discussion ***
Just a little bit weaker in all regards Tuesday. Less offshore
flow...less ridging. This will make for a cooler but still above
normal day under mostly sunny skies.
Long term (wed-sat)...
excitement levels low for the extended forecast. The upper ridge
will be replaced by a slowly developing trough and the offshore flow
will turn onshore and slowly strengthen during the period. There
will be a little marine layer stratus Wednesday probably confined to
the Long Beach Torrance area...the a little more Thursday. It will
will take to Friday for the marine layer stratus pattern to fully
develop and overspread the coasts and into most of the valleys. This
cloudy pattern will persist into Saturday. Maximum temperatures will fall some
each day reaching normal values Friday.
Middle/upper level ridge of high pressure upstream will shift east and
build over the area. Upper level moderate north-northwest winds will
become light north-northwest after 27/19z while middle level moderate
north winds become light to moderate northeast after 27/19z. Mixed
weak to moderate northerly and onshore pressure gradient through
27/04z then weak to moderate northeasterly gradient between
27/04-27/19z and weak to moderate onshore gradient after 27/19z.
Marine layer at lax at 1605z is 219 feet deep and the inversion top
is at 915 feet with a temperature of 16 degrees c.
Klax...current sky/visibility conditions likely through the period. Chance
low level winds 04014kt approximatly 1.5kft between 27/10-27/15z.
Kbur...virtually certain current sky/visibility conditions will presist
through the period. Chance low level winds 03025kt approximately
1.9kt after 27/08z.
Virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less
It is likely northwest winds will increase and gale force gusts will
exist from Piedras Blancas to San Clemente Island through late
tonight. Small Craft Advisory conditions will likely exist 0-10 nm slo County as well
as in Santa Barbara basin to San Pedro basin this afternoon through
late tonight. Steep combined seas will likely build to 9 to
13 feet in the outer waters and 6 to 9 feet in the near shore basins
this afternoon and subside Monday morning. A low pressure system
moving east over the South Pacific and was southeast of New Zealand
this morning will continue to generate storm force winds through
Monday morning. The highest seas will propagate toward South America
as the wind fetch area is oriented west-east but a south swell
originating from this storm are expected to arrive Saturday.
Fire weather...26/1015 am
northerly winds will be on the increase once again this evening for
the sba South Coast and Santa Ynez range...as well as the Interstate
5 corridor extending from the la County mountains southward into the
Santa Clarita valley. Humidity values will generally remain above 30
percent during the evening hours. Winds will shift to the northeast
and increase in speed overnight into Monday morning. The strongest
surge of winds in the la/Ventura County mountains is expected to
occur between 2 am and 10 am on Monday...when gusts between 50 and
60 miles per hour will likely occur. For the valleys and coastal areas...the
strongest winds will likely occur between 4 am and noon...when gusts
between 30 and 50 miles per hour are expected. Humidity values are expected to
lower into the teens by middle to late morning on Monday...with some
single digit reading by afternoon. Temperatures will climb into the
80s across much of the district...with warmest coastal/valley
locations around 90 degrees. The combination of gusty Santa Ana
winds...low humidities...and warm temperatures...and drying fuels
will bring elevated fire danger to Los Angeles and Ventura counties
on Monday. There will also likely be a brief period of critical fire
weather conditions from middle morning Monday through early
afternoon...but durations are not sufficiently long to warrant a red
Wind Advisory (see laxnpwlox).
Gale Warning (see laxmwwlox).
Small Craft Advisory (see laxmwwlox).