Area Forecast Discussion
Issued by NWS Los Angeles, CA

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349
FXUS66 KLOX 201845
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1145 AM PDT Sun Aug 20 2017

.SYNOPSIS...
The weak low pressure system off the coast will remain through
Wednesday. A persistent and deep marine layer will continue to
keep a cooler air mass in place through midweek. There is outside
chance of afternoon and evening showers and thunderstorms across
the mountains and desert through Tuesday.

&&

.SHORT TERM...(TDY-TUE)

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
Wednesday.

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.

.LONG TERM...(WED-SAT)

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
now.

&&

.AVIATION...

20/1800Z

At 0930Z at KLAX, the marine layer depth was near 2400 feet. The
top of the inversion was 2900 feet with a temperature of 14 degrees
Celsius.

Good confidence in TAFs through 01Z. Less confidence after 01Z
with return of low clouds. Low will return to coastal sites in the
01Z-06Z window while vly TAFs will see clouds return between 06Z
and 09Z.

KLAX...Good confidence in TAF through 01Z. Then less confidence
with return of MVFR cigs. There is a 20 percent chc that low
clouds will arrive at 01Z...a 30 percent chc of 03Z and a 10
percent chc of between 03Z and 06Z. Good confidence in TAF from
06Z-18Z

KBUR...Good confidence in TAF through 07Z. Then less confidence
with return of MVFR cigs. There is a 20 percent chc that low
clouds will arrive at 08Z.

&&

.MARINE...

20/830 AM

Outer Waters...high confidence in forecast. Winds and seas will
remain below Small Craft Advisory (SCA) levels through Tuesday. On
Wednesday and Thursday, northwest winds will increase with SCA
level winds likely.

Inner Waters...high confidence in forecast. For the waters north
of Point Sal, winds and seas will remain below SCA levels through
Tuesday. On Wednesday and Thursday, SCA level winds are likely
each afternoon and evening. For the waters south of Point
Conception, winds and seas are expected to remain below SCA levels
through Thursday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect through this evening for
      zones 40-41. (See LAXCFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Isolated showers and thunderstorms for the mountains and desert
cannot be ruled out for Tuesday.

&&

$$

PUBLIC...Hall
AVIATION...Rorke
MARINE...Rorke
SYNOPSIS...jld

weather.gov/losangeles



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