Area Forecast Discussion
Issued by NWS Los Angeles, CA

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720
FXUS66 KLOX 280405
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
905 PM PDT TUE SEP 27 2016

.SYNOPSIS...

A weak area of low pressure will bring a slight chance of
afternoon showers and thunderstorms to the mountains and adjacent
valleys of Ventura and Los Angeles Counties on Wednesday. By
Friday, a broad upper level trough of low pressure will arrive to
increase the marine layer and cool temperatures to near normal
through the weekend and into early next week.

&&

.SHORT TERM...(TONIGHT-FRI)

The marine inversion was around 1200 ft deep at KVBG this evening.
Low clouds were along and off the central CA coast including
Monterey County this evening and were moving down the coast into SLO
County. These low clouds will continue to move down the coast to the
SBA County Central Coast overnight, with low clouds moving inland
some over the coastal plain. Patchy dense fog will also be possible.
Further south, varying amounts of mid level clouds are expected
mainly over VTU/L.A. Counties overnight. Otherwise, clear to partly
cloudy skies will prevail across the forecast area thru the night,
along with generally light winds.

An upper level low over nwrn Mexico this evening will move into far
sern CA late tonight and Wed. The upper low will weaken Wed night,
with weak upper ridging expected over srn CA on Thu. A broad upper
level trof will slowly approach the area from the nw Thu night and
Fri.

Some mid level moisture will continue to move into the area thru Wed
then linger over the area thru Thu for partly cloudy skies overall
mainly over VTU/L.A. Counties, except some areas could see mostly
cloudy skies at times on Wed. With some instability expected (LI`s
down to -4 at 21Z Wed), there will also be a chance of showers and a
slight chance of thunderstorms Wed afternoon and early evening for
the mountains of VTU/L.A. Counties. Some of this activity could
drift off the mountains into nearby valley areas due to light
northeasterly steering currents at mid levels. A slight chance of
showers and thunderstorms cannot be ruled out in the southern and
western Antelope Vly as well. The lingering moisture and slight
instability Thu afternoon could lead to an isolated thunderstorm
mainly in the eastern San Gabriels, but any chance of these on Thu
should be 10 percent or less. By Fri, most of the mid level moisture
will be pushed east of the area, with mostly sunny skies expected
across the region.

Night and morning marine layer clouds will continue to affect the
Central Coast thru Fri morning. South of Point Conception, there
will be the lack of an eddy circulation, lingering dry air in the
boundary layer at least thru Thu, and a very shallow marine
inversion. This will lead to minimal low cloud formation over the
coastal waters. The 00z NAM was forecasting some increase in low
level moisture by Fri morning over the SoCal bight, but any low
clouds that develop should remain off the coast. This will keep
coastal areas free of the marine layer low clouds thru Fri.

There will be a gradual increase in the onshore flow over swrn CA
Wed thru Fri. This will help to bring a slow cooling trend to the
region during the period altho highs will remain several degrees
above normal during the period. Highs in the warmest vlys and
foothills Wed and Thu should reach the upper 80s to lower 90s, then
drop into the mid 80s to around 90 on Fri.

***FROM PREVIOUS DISCUSSION***

.LONG TERM...(SAT-TUE)

After the trough passage Friday night we`ll be under a very
persistent northwest flow, at least through early next week.
Sundowner winds expected each evening, probably strongest Saturday
evening, and these will at least limit, if not prevent low clouds
from reaching srn SB County. Otherwise, a fairly solid marine lyr
during the night and morning hours into most of the coast and
valleys. Temps dropping several degrees most areas Saturday and
Sunday behind the trough. Some differing solutions for early next
week so confidence lower than usual. Last night the ECMWF started
advertising an early cold frontal passage Monday or early Tuesday
with some decent moisture with it. The 12z run today scaled back
considerably for our area but still hinted at some rain chances
north of Pt Conception. The GFS has been consistent keeping any
real chances well north of SLO county. Leaving the forecast
precip-free for now but adding some clouds for northern areas.
Though it`s worth noting the EC performed much better with the
upper low that generated some rain across far southern California
last week so don`t want to discount the wetter solution too much
yet.

&&

.AVIATION...27/2350Z...

At 2304Z...the marine inversion at KLAX was 600 FT deep. The top of
the inversion was at 1400 ft with a temp of 27 deg C.

Generally hi confidence in the 00Z TAFs. There is a 60-70 percent
chance of low clouds and LIFR conditions at KSBP and KSMX from about
10Z-18Z, altho the timing of the onset of the low clouds may be off
+/- an hour or so. Otherwise, VFR conditions can be expected at all
airfields thru Wed afternoon.

KLAX...Hi confidence in the 00Z TAF with VFR conditions expected
thru Wed evening.

KBUR...Hi confidence in the 00Z TAF with VFR conditions expected
thru Wed afternoon.

&&

.MARINE...27/835 PM...

Mostly light winds are expected across the coastal waters through
Thu afternoon with SCA conds not expected. Northwest winds are
expected to increase across the outer waters and northern inner
waters late Thu, with SCA conditions likely in those areas
by Fri evening and continuing through at least Sun. There is a 30
percent chance that SCA spread into the SBA Channel by Sat with a
50 percent chance Sun, with a 20 percent chance of SCA conds
across the southern inner waters by Sun. There is a 20 percent
chance of Gale Warning conditions sometime this weekend mainly
across the outer waters from Point Conception to San Nicolas
Island.

Extra surging and currents are possible near the coast due to a
long period south swell of 2 to possibly 4 feet by Thursday
morning, peaking late in the day into Friday before subsiding this
weekend.

&&

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

&&

$$

PUBLIC...Sirard
AVIATION...Sirard
MARINE...Munroe
SYNOPSIS...Seto/Sirard

weather.gov/losangeles



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