Scientific Forecaster Discussion
fxus66 klox 201031
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
331 am PDT sun Aug 20 2017
A weak trough of low pressure off the coast will remain over the
region through Wednesday. A persistent and deep marine layer will
continue to keep a cooler air mass in place through midweek.
Through Tuesday, there is outside chance of afternoon and
evening showers and thunderstorms across the mountains and
desert, and night through morning drizzle for the coast, valleys
and foothills. A warming trend should develop for late week as
high pressure builds in.
The latest satellite imagery shows a trough of low pressure
centered near 33n and 121w, or about 125 miles west of Los Angeles
this morning. This trough will remain over the region, wobbling
around off the coast through Wednesday night. A deepening marine
layer and a strengthening onshore flow pattern will bring a
cooling trend across the area into at least Monday, and possibly
Tuesday or Wednesday. Model solutions hold the trough near the
bight through Monday, but slightly retrograde the trough west
between Monday and Tuesday, then eject it across the area on
The latest amdar soundings from klax indicate a marine layer
depth currently near 2200 feet this morning. The marine layer
depth should continue to deepen up to around 2500-2700 feet later
today. If NAM-WRF solutions play out, a marine layer depth closer
to around 3000-3500 feet on Monday morning could develop. With
the instability from trough aloft and the deepening marine layer
taking place, drizzle cannot be ruled out across the area as the
stratus deck lifts and squeezes out some precipitation. Highest
confidence for drizzle remains for areas south of Point
Conception on the front side of the trough axis and where
orographic effects may aid in development through Monday. Less
confidence exists for Tuesday and Wednesday as models dampen the
marine layer quite a bit. With the trough in place, the marine
layer could remain equally as deep or just slightly less deep.
Pops remain in the forecast for the next several days as isolated
showers and thunderstorms cannot be ruled out across the mountains
and desert. Some negative lifted index values and positive cape
values remain through Tuesday, but the main question will be the
amount of moisture. Some model solutions start dry slotting the
area which could cut-off the moisture supply. For the current
time, a consistent forecast remains as confidence is lower in the
trough's exact movement over the coming days, but future shifts
may need to address this development.
Clouds will likely struggle to clear the next couple of days,
reminiscent of a common may or June weather pattern for Southern
California. The best clearing will likely take place at Los
Angeles County beaches each afternoon and evening where ocean
temperatures are warmest, with areas north and west of Point Dume
or mugu struggling to clear. With that being said, an earlier
arrival of the stratus clouds should expected the next several
afternoons and evenings.
The trough should exit the region on Wednesday night. 500 mb
heights start to rise in the wake of the trough as high pressure
builds. Drier southwest flow aloft should keep a persistent marine
layer in place, but the areal extent may keep in confined mainly
to the coast. A warming trend should develop for late this week
with a thinning marine layer depth in weakening onshore flow.
Model solutions are subtle hinting at the possibility of a return
of monsoonal moisture for late in the week or over the weekend.
Given the pattern setting up, it cannot be ruled out. With much
focus given to the short-term, the forecast keeps low pops for
At 0930z, the marine inversion at klax was based around 2200 feet.
The top of the inversion was 4800 feet with a temperature of 21
Overall, moderate confidence in 12z taf package. MVFR conditions
impacting all coastal/valley sites this morning. With deepening
marine inversion, there is a 30% chance of drizzle with IFR cigs
12z-18z from ksba southward. Stratus will dissipate a couple hours
slower than Saturday with the possibility of ksba/koxr/klax
remaining MVFR through the afternoon. For tonight, all coastal and
valley sites should experience MVFR conditions.
Klax...moderate confidence in 12z taf. There is a 40% chance that
MVFR cigs will persist through the afternoon. There is a 20%
chance of drizzle 12z-18z with IFR cigs.
Kbur...moderate confidence in 12z taf. Dissipation of MVFR cigs
could be as late as 20z. There is a 30% chance of drizzle 12z-18z
with IFR cigs. For tonight, arrival of MVFR cigs could be 1-2
hours earlier than current 06z forecast.
For the outer waters, high confidence in current forecast. Winds
and seas will remain below Small Craft Advisory (sca) levels
through Tuesday. On Wednesday and Thursday, northwest winds will
increase with Small Craft Advisory level winds likely.
For the inner waters, high confidence in current forecast. For the
waters north of Point Sal, winds and seas will remain below Small Craft Advisory
levels through Tuesday. On Wednesday and Thursday, Small Craft Advisory level winds
are likely each afternoon and evening. For the waters south of
Point Conception, winds and seas are expected to remain below Small Craft Advisory
levels through Thursday.
California...beach hazards statement in effect through this evening for
zones 40-41. (See laxcfwlox).
Hazard potential outlook...(tue-sat)
isolated showers and thunderstorms for the mountains and desert
cannot be ruled out for Tuesday.