Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
210 PM PDT Tuesday Jul 28 2015
High pressure Parks itself in The Four Corners region by Friday.
This pattern favors a return of the monsoon. Thunderstorms are
possible south of Point Conception Wednesday through Friday.
Thunderstorm chances shift inland and continue through Tuesday.
Short term (today-fri)... a large upper level high pressure
currently centered over Texas will expand westward tonight and
Wednesday. The increasing heights will lower the marine layer and
reduce the morning low cloud coverage tomorrow. By Wednesday
afternoon the increasing heights will cause maximum temperatures to increase a
few degrees and all stations away from the immediate coast will be a
couple of degrees above normal. Southwest flow around the upper high
will begin a monsoonal moisture transport into the area and by later
afternoon there will be enough moisture to create a slight chance of
thunderstorms over the eastern San Gabriel and the Antelope Valley.
By Thursday the upper high will move over the 4 corners area and
becomes stationary. This placement is ideal for delivering monsoonal
flow into the state. The naefs has precipitable water in the 99th
percentile starting late Wednesday but models are still pretty dry
with respect to actual precipitation. The temperatures will
continue to raise a few more degrees to their peak on Thursday.
The synoptic picture remains essentially unchanged on Friday with
heights remaining around 591 dm. Thursday and Friday will be the two warmest
days with maximum temperatures 4 to 8 degrees above normal everywhere except
the immediate coast. There will be enough additional humidity to
raise the heat index values close to advisory levels although right
now it only looks like a few isolated spots will actually meet or
exceed these thresholds.
Long term (sat-tue)... very little change over the extended period.
Long range models and ensembles show the 4 corners high remaining
nearly stationary. It expands a little which turns the flow over our
area to a more southerly direction which is not quite as conducive
to thunderstorm formation. Onshore flow increases a little by the
weekend which should bring some cooling but mostly near the coasts
and coastal valley locations.
Middle/upper level trough of low pressure over the area will shift
west while high pressure over Texas expands west. Upper level
moderate southwest winds will become moderate south after 29/17z
while middle level light east winds become light to moderate southeast
after 29/10z. Middle/upper level moisture will increase after 29/20z.
Capping marine inversion was relatively weak and the
base over lax was approximately 2kft this morning with an organized
cloud field. The inversion is expected to strengthen and the
base will differ by minus 1kft with a disorganized cloud field
Wednesday morning. Weak onshore pressure gradient through 29/19z
becoming weak to moderate after 29/21z.
Marine layer at lax at 1600z is 2094 feet deep and the inversion top
is at 4276 feet with a temperature of 21.5 degrees c.
Klax...chance ceilings 003 between 29/07-29/16z then ceilings 006 between
Kbur...chance current sky/visibility conditions will persist through
29/13z then low chance visibility 5sm haze after 29/13z.
Virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less
Small Craft Advisory conditions are not expected through Friday.
There is a chance of Small Craft Advisory conditions with west-northwest winds in the
vicinity of Point Conception to san nicloas island Saturday
Special Weather Statement (see laxspslox).