Flash Flood in California, February 11, 2017

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
251 PM PST Sat Feb 11 2017

.SYNOPSIS...
A cold low will move across the region into tonight with possible
showers and widely scattered thunderstorms. Then gusty offshore
winds are forecast into early next week and should decrease clouds
and warm temperatures to above normal through midweek. Another
storm will approach from the north by late Thursday, and reach
Los Angeles by Friday.

&&

.SHORT TERM...(TDY-TUE)

Interesting wx early this afternoon as a cold upper level low moved
down the CA coast from near Monterey to the SLO County coast. A
decent diffluent flow pattern out ahead of the upper low combined
with some instability and afternoon sunshine helped to build showers
and thunderstorms over parts of the forecast area. The main areas
where the activity developed were over southern SLO County, northern
SBA County, the mtns and vlys of VTU County, Santa Clarita Vly and
western San Fernando VLY. Thunderstorms have been producing small
hail and heavy rain. There will continue to be a chance of showers
and a slight chance of thunderstorms across much of the forecast
area thru the rest of the afternoon as the upper low moves to near
Point Conception by late in the day. Sunset today is around 530 PM
to 540 PM, and with the loss of daytime heating, the threat of
thunderstorms is expected to end by 6 PM.

There will continue to be a chance of showers over portions of
VTU/L.A. Counties thru the evening hours, and in the mtns overnight.
Otherwise, dry conditions with decreasing clouds can be expected
thru tonight as the upper low quickly moves away and northerly winds
increase.

A decent northerly gradient and upper support will help to bring
strong and gusty n to ne winds to much of the forecast area from the
SBA County mtns thru L.A. County mainly after midnight through early
Sun afternoon. The strongest winds will be in the L.A./VTU County
mtns where damaging wind gusts up to 60 mph are expected in some
areas. This is supported by 850 mb winds topping out at 50 kt on the
12Z NAM. Gusty advisory level winds will also affect the VTU County
vlys, Santa Monica mtns, Santa Clarita Vly, western San Fernando
Vly, and the L.A. County coast from Malibu thru the Hollywood Hills.
Wind gusts in these areas will range from 40 to 50 mph at times
later tonight thru early Sun afternoon. A high wind warning is in
effect for the L.A./VTU county mtns during the period, with wind
advisories in effect elsewhere. Please see the latest non-
precipitation weather message (LAXNPWLOX) for further details.

Upper level ridging is forecast to build into central CA on Sun,
then into swrn CA for the most part Mon thru Tue. Offshore flow
mainly night and morning hours will bring mostly clear skies to the
forecast area Sun thru Tue, except some hi level clouds may affect
the region on Tue with partly cloudy skies at times. Northeast winds
will continue to be gusty Sun night into Mon morning, but they
should be generally below advisory levels. Some locally gusty ne
winds cannot be ruled out Mon night into Tue morning in the favored
areas as well.

Temps across the region will warm to near normal to several degrees
above normal Sun, and to about 3 to 10 deg above normal on Mon and
Tue. Highs in the warmest coast and vlys should reach the lower
to mid 70s on Sun, and generally in the 70s Mon and Tue.

.LONG TERM...(WED-SAT)

The EC and GFS are in decent agreement in the synoptic scale
features Wed thru Thu. An upper level ridge over srn CA Wed and Wed
night will be followed by the upper ridge breaking down some and
moving e while a broad e Pac upper trof moves toward the w coast on
Thu. Some differences show up in the models Fri thru Sat. The GFS
forecasts generally broad upper level troffiness over the e Pac into
the western U.S. The EC drives more shortwave energy s just off the
w coast with a sharper upper trof moving to the CA late Fri and Fri
night then move slowly e over the state thru Sat.

Dry and relatively warm conditions can be expected Wed into Thu,
altho by later in the day on Thu an approaching cold front should
bring an increasing chance of rain to the SLO County central coast.
It looks like the front will move thru the forecast area Thu night
into Fri morning with the potential for fairly significant rain.
Will lean toward the wetter EC and broad-brush the forecast with
lingering chances of rain Fri afternoon thru Sat over the area along
with plenty of cloudiness.

Temps will be about 4 to 14 deg above normal on Wed, then cool to
near normal to several degrees above normal Thu. It will turn much
cooler on Fri and Sat with temps about 4 to 8 degrees below normal.
Highs in the warmest vlys and coastal areas should reach the mid 70s
to around 80 on Wed, lower to mid 70s Thu, and lower to mid 60s on
Fri and Sat.

&&

.AVIATION...11/1620Z...

At 1615Z, there was no marine inversion at KLAX.

Overall, moderate confidence in 18Z TAF package. Deep moist layer
still in place across the area and SCT-BKN cloud decks will
prevail through early this evening. Generally, conditions are
expected to be VFR with the possibility of some periods of MVFR
CIGs. Also, there is the possibility of an isolated light showers
this afternoon/evening. On Sunday morning, offshore winds will
develop with the potential for LLWS/turbulence issues across
Ventura and Los Angeles counties.

KLAX...moderate confidence in 18Z TAF. Current MVFR CIGs could
linger an hour or two beyond current 19Z dissipation forecast. For
tonight, high confidence in VFR conditions. After 15Z, light
northeast winds are expected, ranging at or below 6 knots.

KBUR...moderate confidence in 18Z TAF. There is a 20% chance of
MVFR CIGs 18Z-00Z today. On Sunday morning, there is a chance of
some LLWS/turbulence as offshore winds develop.

&&

.MARINE...11/130 PM...

There is a slight chance of thunderstorms this afternoon over
the coastal waters. Any thunderstorms that develop may produce
small hail, gusty and erratic winds, locally rough seas and brief
heavy rain.

For the Outer Waters, moderate confidence in current forecast.
Through tonight, high confidence in Small Craft Advisory (SCA)
level northwest winds continuing. On Sunday and Sunday night,
there is a 50% chance of SCA level winds continuing. For Monday
through Wednesday, winds and seas are expected to remain below
SCA levels. On Thursday, there is a chance of SCA level winds and
seas developing as winds increase and swells approach 10 feet.

For the Inner Waters, moderate confidence in current forecast. For
the waters north of Point Sal, northwest winds are expected to
increase to SCA levels this afternoon through tonight then drop
below SCA levels Sunday through Wednesday. On Thursday, there is
a chance of SCA level winds and seas developing as winds increase
and swells approach 10 feet. For the waters south of Point
Conception, moderate confidence in northwest winds increasing to
SCA levels across western sections through this evening. For late
tonight and Sunday, there is a 60% chance of SCA level northeast
winds developing from Ventura south to Santa Monica and across the
San Pedro Channel. For Monday through Thursday, winds and seas
are expected to remain below SCA levels.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 2 AM to 2 PM PST Sunday for
      zones 41-44>46-88-547. (See LAXNPWLOX).
     High Wind Warning in effect from 2 AM to 2 PM PST Sunday for
      zones 53-54. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 4 AM PST Sunday for
      zones 645-670-673-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PST this evening
      for zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Rain is possible later Thursday and Friday. At this time it does
not appear rainfall will be heavy enough for flash flooding and
debris flows in recent burn areas. However, localized street
flooding will be possible.

&&

PUBLIC...Sirard
AVIATION...RAT
MARINE...RAT
SYNOPSIS...STu

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