Scientific Forecaster Discussion
Southwest California area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
230 am PDT Tuesday Sep 16 2014
New marine section
A ridge of high pressure and weak onshore flow will result in very
hot weather through middle week...especially for areas away from the
immediate coast. There is a slight chance of thunderstorms for middle
week due to remnant moisture from Hurricane Odile...mainly for the
mountains and desert. Temperatures will return to near normal late
in the week as a trough of low pressure approaches from the west.
in the immediate short term...main issue is low end advisory
sundowner event across southern Santa Barbara County. Sba-smx
gradient was -4.7 mb last hour...and will likely increase a bit in
the next couple of hours. Due to this gradient...northerly winds
gusting to between 35 and 45 miles per hour have developed in the usual north
wind-prone spots. Expect these winds to continue through this
evening before slowly diminishing tonight. Have issued a Wind
Advisory until 400 am PDT Tuesday for zones 39/52.
Otherwise...no major concerns in the immediate short term. Latest
satellite shows clear skies everywhere as afternoon cumulus has
dissipated over the mountains after sunset. Overnight...expect some
stratus/dense fog to return to the central coast. Otherwise...skies
should remain clear overnight.
Short term (tonight-thu)...
high pressure was centered over the Desert Southwest states with
another day of well above normal temperatures throughout southwest
calif. Many valley areas started the day about 2-5 degrees cooler
than yesterday and have maintained that slight cooling effect this
afternoon...however heat index values are still reaching 105 degree
criteria across the Ventura and Los Angeles County valleys.
Meanwhile...the mountains were a few degrees warmer and some coastal
areas were also warmer than yesterday. Cumulus clouds formed over
the mountains again this afternoon but a cap near 18k feet should
limit the vertical development and clouds will evaporate by sunset.
Expect readings to be slow to cool down tonight with many foothill
communities only getting into the upper 70s for minimum
temperatures. Patchy low clouds/fog will be limited to the central
coast areas late tonight.
Tuesday looks about the same overall as today with potentially
another 1-2 degrees of cooling. The excessive heat warning will run
until 7pm Tuesday evening and that still looks on-track for the
interior valleys of Ventura and Los Angeles County...as well as the
Santa Monica Mountains. Pressure gradients are rather neutral
tomorrow with little change overall. Expect additional mountain
cumulus Tuesday afternoon and local sundowner winds along the south
Santa Barbara coast Tuesday evening.
Things get a little more interesting by Wednesday. Moisture
associated with tropical cyclone Odile begins to move into Los
Angeles County with 850mb dewpoints going above 12 degrees celsius
and precipitable water over 1.5 inches. There is enough instability
to continue the possibility of isolated thunderstorms over the
Ventura and Los Angeles County mountains and the Antelope Valley
Wednesday afternoon/evening. Steering winds look fairly light so
there could be heavy downpours in storms where they materialize on
Wednesday. Cloud cover will increase across many areas on Wednesday
which will slow the heating from direct sunshine...although
increasing dewpoints will make it feel fairly uncomfortable but not
quite reach excessive heat warning criteria.
Model data shows that a low pressure system will move down the
northern calif coast on Thursday toward the Bay area increasing
southwest flow aloft over our region. This will push the moisture
eastward during the day but we will continue a slight chance of
thunderstorms for the mountains and desert until this moisture
leaves the area. Due to more cloud cover and falling heights...we
should see 4-8 degrees of cooling on Thursday compared to Wednesday.
Long term (fri-mon)...
model solutions are getting closer with the movement of the upper
low along the calif coast. Both the GFS and 12z ecm now have the low
near The Channel islands by Saturday morning. In this scenario
expect temperatures to moderate Friday-Sat with more potential for
onshore flow and coastal low clouds in the mornings. By Saturday
afternoon some monsoon moisture could be pulled westward from the
High Desert back into our mountains and Antelope Valley. Have bumped
up clouds and precipitation chances just below mention in the
forecast for these areas in case the GFS works out. The ecm keeps a
cut-off low pressure system west of calif through Monday while the
GFS begins to rapidly build a strong high pressure ridge across the
Great Basin. There is plenty of uncertainty for next week...however
the 00z ecm and latest 5-wave GFS models are indicating a giant high
pressure system across the entire western U.S. A significant heat
wave could be in the works next week...but confidence is low at this
point due to conflicting solutions (12z ecm and 00z gem) of a large
fall-like low pressure trough approaching the Pacific northwest
states by midweek.
Overall...high confidence in 06z taf package. Except for some
possible LIFR/vlifr across portions of the central coast late
tonight through middle morning Tuesday...expect VFR conds through the period. Locally
gusty winds through and below passes and canyons of southern sba County
through late tonight...and again Tuesday evening with local low level wind shear and MDT
uddf near ksba for the area through Tuesday.
Klax...high confidence in 06z taf with VFR conds through the period.
Kbur...very high confidence in 06z taf with VFR conds through the period.
High confidence in current coastal forecast. The strongest winds
will occur around Point Conception from middle-afternoon through early
The western half of the Santa Barbara Channel will see local
northwest gusts to 25 to 30 knots during the next few late afternoons
A long period (19-20+ second) southern hemisphere swell will move
through the waters through today. It will bring elevated surf
along south facing beaches and a significant amount of rip current
There is no longer a threat of swells from Hurricane Odile.
excessive heat warning (see laxnpwlox).
Red flag warning (see laxrfwlox).
Wind Advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).
High rip current risk (see laxsrflox).