Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
1040 am PDT Wednesday Sep 17 2014
..aviation discussion updated...
it still will be hot today away from the coast. Lingering middle level
moisture will allow for a chance of showers and thunderstorms over
eastern Los Angeles County late this morning through this evening.
An approaching trough of low pressure will bring significant
cooling trend to the area Thursday through Friday...with increasing
night through morning low clouds and fog...and possibly some local
drizzle. Upper level high pressure may bring some warming to the
region late in the weekend into early next week.
Short term (today-fri)...temperatures off to the races again today after a
very warm start as overnight temperatures didn't drop below 80 in some
places. Gradients, though still 1.5mb offshore, are trending ever so
slightly onshore and even the trend of the trend is positive meaning
the onshore flow should continue to strengthen through the Day.
Basin profilers, while warmer in the lowest 500' or so, are showing
a cooling trend aloft which is a good sign. So when all is said and
done this afternoon most areas should be on the order of 3-6 degrees
cooler than they were yesterday. Still hot, but not quite as hot,
and probably no threat to records. The one exception to that may be
Santa Barbara Airport which is expected to see another spike in
afternoon/evening temperatures with a repeat of sundowners. Models have
been underdoing the gradient and northerly flow each of the last few
nights and probably is again for this evening. So will likely be
putting out another Wind Advisory for southern sba County this afternoon
and evening with temperatures climbing into high 80s and possibly low to
middle 90s in the warmest spots.
Thunderstorm threat looks to be less than yesterday. Models are
noticably less impressive with stability parameters today than
yesterday, and while 850 dewpoints are still in that 10-12 range
across eastern la County, the atmospheric conditions just aren't
ideal. Model soundings show a decent inversion right around 15000'
which will cut down on the available energy and likely stall
development. Lifted indice's stay above zero through the day. K indices are
only in the middle 30s after being in the low 40s yesterday. And
steering flow today has shifted to the west which means we're
pulling in drier air from the trough to the west. So while there is
still a chance of convective development and even an isolated storm
or two, I think the chances for anything significant is very small
and confined to eastern la County. For now will leave up the Flash
Flood Watch but will monitor development over the next couple hours
and evaluate whether one is still needed.
***From previous discussion***
An upper low will approach the northern California coast tonight and Thursday...with
a trough extending southward to west of the forecast area. Southwesterly flow
aloft will spread across the region this evening...shunting any middle
level moisture and convection east of the area. Low level north-S
gradients will be strong enough for some gusty north winds through and
below passes and canyons of the Santa Ynez range and the adjacent S
coast of sba County...and winds could get close to advisory levels
once again. Night through morning low clouds and fog will become
widespread in coastal areas tonight...with the exception of the S
coast of sba County...where northerly flow will likely keep skies mostly
clear. Some stratus could even push into the Lower Valley locations
for a few hours Thursday morning. With good onshore gradients...skies may
have a tough time clearing near the coast on Thursday...especially north of
pt Conception...where there will also be some middle level clouds
associated with a weakening surface front. At this point...do not expect
anything more than a few sprinkles with the dying front across
northern sections. With considerable height falls and increasing
onshore flow...expect significant cooling across the entire area on
Thursday...especially in the valleys...where maximum temperatures may be down more
than 15 degrees from those reached today in some areas.
All models show the upper low dropping down the coast of California Thursday
night and Friday morning...reaching a position near Point Conception
Friday afternoon. Expect a rather deep marine layer Thursday night/Friday
morning...with clouds possibly up to the coastal slopes. There
could be some drizzle or even a little light rain Friday morning...
especially across the foothills of l.A. County as some low level
lift develops in the increasingly cyclonic flow. There may be only
partial clearing on Friday. Maximum temperatures will be down a few more
degrees in most areas...down to below normal levels in most areas
for the first time since the 4th of the month.
Long term (sat-tue)...the upper low will remain nearly stationary
near Point Conception Friday night and Sat. Expect the deep marine
layer to continue...with plenty of night through morning low clouds in
coastal and valley areas...possibly up to the coastal slopes once
again. Some drizzle or light showers will remain possible. It will
be cool again on Saturday across the region. The upper low will lift
into central California Sat night and sun. Expect better clearing on
Sunday...and maximum temperatures will likely be up a few degrees in most
areas. An upper ridge will build into the region on Monday...with
additional warming likely...and a reduction in night through morning
stratus. Some additional warming is likely on Tuesday as the ridge
amplifies a bit...and if low offshore flow develops...warming may be
moderate confidence in 18z tafs. Isolated thunderstorms this
afternoon and evening in the Antelope Valley. Locally gusty winds
through and below passes and canyons of southern sba County late tonight
with local low level wind shear near ksba. Ninety percent chance of overnight
vlifr-LIFR conditions north of Point Conception... sixty percent
chance of early morning LIFR-MVFR conditions along Ventura and Los
Angeles County coast... sixty percent chance of early morning
IFR-MVFR conditions in the adjacent inland valleys and at kprb.
Otherwise and elsewhere VFR conditions will prevail.
Klax...moderate confidence in 18z taf. Sixty percent chance of early
morning IFR-MVFR conditions. Otherwise VFR conditions will prevail.
Kbur...moderate confidence in 18z taf. Sixty percent chance of early
morning IFR-MVFR conditions. Otherwise VFR conditions will prevail.
high confidence in current marine forecast. Advisory level winds
across much of the outer waters through late in the week. The
strongest winds will be near Point Conception during the middle
afternoon to early evening period.
The western half of the Santa Barbara Channel will have local
northwest wind gusts to 25 to 35 knots during the next few late
afternoons and evenings related to sundowner winds off the Santa
Barbara County South Coast. May need an advisory again this
afternoon and evening if it develops beyond a local event.
the long period southern hemisphere swell has been diminishing. It
will continue to bring elevated surf along south facing beaches and
a significant amount of rip current activity through at least
Thursday though these effects will diminish over time.
Flash Flood Watch (see laxffalox)
Small Craft Advisory (see laxmwwlox).
High rip current risk (see laxsrflox).