Area Forecast Discussion
Issued by NWS Los Angeles, CA

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455
FXUS66 KLOX 261108
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
408 AM PDT FRI AUG 26 2016

.SYNOPSIS...

A weak upper-level trough moving into the region will bring cooler
temperatures through Saturday, then high pressure will aloft will
build into the area Sunday with above normal temperatures
forecast for many areas early next week. A cooling trend should
occur for the latter half of next week as trough of low pressure
approaches the West Coast.

&&

.SHORT TERM...(TDY-SUN)

Onshore flow continues to strengthen early this morning ahead of
a trough entering the Great Basin today. This trough will continue
to flatten a ridge of high pressure near 22N and 123W, or about
800 miles south-southwest of Los Angeles. As a result, a cooling
trend will occur with a deepening marine layer today. Currently,
the marine layer has deepened to near 2300 feet deep at KLAX.
Additional deepening should take place later this morning to near
2700 feet. The latest fog product shows a well-entrenched marine
layer stratus deck pushing well into the valleys this morning.
With the deep marine layer, some coastal areas may struggle to
clear today.

The trough will likely remain over the region through Saturday.
The main concern is the short-term timing of the trough passage.
GFS/ECMWF solutions are less progressive with the trough and hold
a bit more energy west of the area, while 00Z NAM-WRF solution are
more progressive. With a majority of the model solutions being
less progressive and 06Z NAM-WRF trending towards these solutions,
forecast timing holds back on the timing of the trough passage
until Saturday afternoon/evening. With this in mind, a few clouds
and an isolated mountain shower or two cannot be ruled out for
Saturday. Despite the trough being moisture-starved, model
solutions have a thin layer of mid-level moisture streaming over
the area and unstable soundings on Saturday afternoon and evening.
K index values approach 30 and lifted index values -1 in the 00Z
GFS solutions over the San Gabriel Mountains. Some cloud cover
was reintroduced into the forecast with non-zero PoPs over the
mountains. Future shifts will need to pay close attention to how
this situation evolves.

The trough should exit the region on Sunday and allow for high
pressure to start to build into the area for early next week.
Model solutions weaken onshore flow across the area, possibly
turning offshore over the interior areas. A warming trend will
trend sharply upward on Sunday for a very warm weather pattern to
setup for early next week.

.LONG TERM...(MON-THU)

500 mb heights and 1000-500 mb thickness values continue to trend
higher into Monday. Onshore flow continues to weaken through
Monday, possibly turning neutral or briefly offshore on Monday
and Tuesday mornings. Monday will likely be the hottest day of
the week, with high temperatures exceeding 100 degrees in the
warmest valley areas.

Southwest flow will start to increase for the remainder of the
week. A trough of low pressure spinning near 35N and 140W this
morning will phase with a developing trough in the Gulf of Alaska
and break down the ridge through late next week. With southwest
flow aloft strengthening into Friday, a cooling trend with a
deepening marine layer should develop. Model solutions are moving
a series of weak cool fronts south, washing them out over Central
California. GFS solutions are a tad more aggressive relative to
the ECMWF solutions with the trough moving over the area, but
moderately high confidence exists in a cooling trend for late
next week, possibly cooler than currently advertised.

&&

.AVIATION...26/1105Z...

At 1100Z, the marine inversion at KLAX was based at 2500 feet. The
top of the inversion was 3300 feet with a temperature of 22
degrees Celsius.

Overall, low to moderate confidence in 12Z TAF package. Current
IFR conditions (north of Point Conception) and MVFR conditions
(south of Point Conception) will continue through the morning and
into the early afternoon along the coast. There is a chance that
KLAX...KOXR and KSBA may remain cloudy all afternoon. For tonight
expect a near repeat of stratus coverage with similar flight
categories developing.

KLAX...moderate confidence in 12Z TAF. Timing of dissipation of
MVFR ceilings could be +/- 2 hours of current 20Z forecast. There
is 20% chance that MVFR ceilings will not dissipate at all this
afternoon.

KBUR...moderate confidence in 12Z TAF. There is a 20% chance that
MVFR ceilings may clear 1-2 hours later than current 17Z forecast.
For tonight, high confidence in the return of MVFR conditions, but
only moderate confidence in timing.
&&

.MARINE...26/200 AM...

For the Outer Waters, moderate confidence in current forecast.
High confidence in winds remaining below Small Craft Advisory
levels through Saturday. From Sunday through Tuesday, northwest
wind will increase with a good chance of Small Craft Advisory
conditions with even a slight chance of Gale Force gust on
Tuesday.

For the Inner Waters, moderate confidence in current forecast. For
the waters north of Point Sal, moderate confidence in current
forecast with a good chance of Small Craft Advisory level winds
developing Monday and Tuesday. For the waters south of Point
Conception, high confidence in current forecast as winds are
expected to remain below Small Craft Advisory thresholds through
Tuesday (although there is a slight chance of gusts reaching
advisory levels across the western half of the Santa Barbara
Channel on Tuesday).

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC...Hall
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Hall

weather.gov/losangeles



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