Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 292054

154 PM PDT SUN MAY 29 2016

The cut-off low pressure system over southern California will
support the slight chance of thunderstorms this afternoon and
early evening across the mountains and Antelope Valley. High
pressure aloft will build into the area by the middle of next
week, supporting much warmer conditions that may persist into next



The main short term weather concern of the day will be threat of
afternoon/early evening thunderstorms across interior sections of
the forecast area. As of 2 pm, already seeing some convective
showers across the mountains and Antelope Valley, with a couple of
thunderstorms across the western portion of the Antelope Valley.
Upper level low currently centered near Orange county is expected
to track slowly eastward through this afternoon. Satellite
estimates and GPS sensors showing observed precipitable water
values between .75 and 1.00 inches this afternoon. The associated
cold pool aloft (with 500 mb temperatures of -16 degrees celsius)
combined with daytime heating will provide increased instability
across the mountains and deserts through the afternoon.

Slight chance of thunderstorms in the forecast through early
evening across the mountains, Antelope Valley, and interior
valleys of SLO/SBA counties. Best convective parameters this
afternoon focused across mountains of LA/Ventura counties as
well as the Antelope Valley, with lifted index values ranging
between -3 and -6, K-index values up to 39, and CAPE values
between 1000 and 1500 J/kg. Mid and upper level flow is fairly
weak and from the northeast. As a result, any storms that develop
this afternoon/early evening will potentially drift into adjacent
foothill/valley areas, including the Colby Fire burn area. The
only concern today with the developing convection will be the
deep marine layer which may inhibit some daytime heating across
the foothills and coastal slopes. Due to the fairly weak steering
flow, any storms that develop today will be slow moving, capable
of producing brief heavy rain. As a result, there is a threat of
localized flash flooding with thunderstorms, along with the
potential for gusty downdraft winds, small hail, and dangerous
cloud to ground lightning.

Satellite imagery showing low clouds hanging on across portions
of the forecast area this afternoon. Pilot reports indicate a
marine layer depth around 3500 feet across the LA Basin. Low
clouds and fog expected to fill in across most coastal/valley
areas tonight into Monday morning. By Monday, upper low is
expected to shift eastward into Western Arizona, with an upper
level ridge of high pressure building off Northern California. As
a result, look for better clearing of marine layer clouds (except
along the immediate coast), along with some warming across
interior sections on Monday. Tuesday through Wednesday, the upper
level ridge of high pressure will continue to slowly slide
eastward and become more centered across California, resulting in
a continued warming trend, along with a shrinking marine layer.


Thursday looks to be the warmest day of the week for many areas as
thickness values peak. Warmest valleys expected to climb to around
90 degrees, with Antelope Valley climbing to around 100 degrees.
A shallow marine layer will continue to keep coastal low clouds
and fog. Friday through Sunday, 12z GFS and ECMWF models showing
another cutoff low developing off the Southern California coast,
with cross sections showing a good surge of mid and upper level
clouds. With this type of pattern, not out of the question that
we could see some showers or mountain tstms develop across the
forecast area sometime during this period, but probability at this
time is still around 10 percent.



Upper level low over the southeastern portion of the area will shift
east and a ridge of high pressure upstream will build over the
area while a mid level low centered south of the area shifts
east. Upper level moderate north-northwest wind will become
moderate north-northeast after 30/19z while mid level light north
winds prevail over the area. Isolated thunderstorms are possible
over the Santa barbara...Ventura and Los Angeles county mountains
between 29/21-30/03z. Thunderstorm tops approximately 30kft and
will move south less than 10 kt. Weak to moderate onshore pressure
gradient through 30/03z and after 30/20z otherwise Catalina eddy
will develop. Capping inversion was 2.2kft at VBG and 2.8kft at
LAX this morning and is expected to differ by minus 1kft alng the
Central Coast and differ little over LAX Monday morning and with
an organized low cloud field.

Marine layer at LAX at 1600Z is 3096 feet deep and the inversion
top is at 4650 feet with a temp of 15.4 degrees C.

KLAX...Chance cigs 035 through 29/23z then chance cigs 012-017
between 29/23-30/17z. Chance cigs 020-025 after 30/17z.

KBUR...Chance cigs 016-023 through 29/22z then chance cigs
014-020 thereafter.

virtually certain - 95-100%
very likely       - 80-95%
likely            - 60-80%
chance            - 30-60%
very unlikely     - 20% OR LESS


.MARINE...29/200 pM...

Small craft advisory conditions are not expected through
Friday. Otherwise a Catalina eddy is expected to develop each
night over the bight through Thursday and northwest winds are
expected elsewhere and will fill in the bight each afternoon.
Complex storm systems over the Southern Ocean last Monday and
Tuesday with a fetch oriented 190-210 degrees from Ventura
generated swells that will arrive Monday and build through
Wednesday. The swells will remain below small craft advisory for
hazardous sea conditions but there will be extra surging and
currents along exposed south facing shores. In the meantime a
mixed small south swell and locally generated northwest swell





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