Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 180836

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
136 AM PDT Sun Mar 18 2018

.SYNOPSIS...17/311 PM.

Below normal temperatures will continue into Sunday, then the air
mass should warm to normal through Tuesday. Showers are possible
through tonight, mainly north of Santa Barbara. A drying trend is
expected Sunday through Monday. A significant storm system could
bring widespread moderate to heavy rainfall to the region between
Tuesday and Thursday night, with a potential debris flow threat.


.SHORT TERM (TDY-TUE)...17/908 PM.

A cold trough of low pressure over Central California will
continue to move east into the Desert Southwest tonight, while
ridging aloft builds in across the area from the west. Shower
activity lingers across the area this evening. A few strikes of
lightning occurred earlier over the Ventura County Mountains as
the colder air aloft with the storm system spread in. Snow levels
are sitting near 3700-4200 feet elevation currently and the
colder air should continue to filter in later tonight. Snow levels
should drop the snow levels closer to what is forecast. A Winter
Weather Advisory remains in effect through 5 AM PDT Sunday.

A warming and drying trend will establish across the area into
Sunday and Sunday night as the ridge builds in, but temperatures
will still remain on the cool side of normal for this time of
year. Temperatures look agreeable for Sunday, but they could be a
tad too cool with March sunshine.


Upper level ridging will linger over srn CA Mon night and Tue.
However, a moisture plume associated with a large upper level low
pressure system 1000 miles W of srn CA will start to encroach upon
the forecast area by Tue afternoon with increasing chances of
rain expected especially for SLO/SBA Counties. There are still
some model differences with this first influx of moisture as the
NAM keeps most of this moisture to the N of the forecast area,
while the EC and GFS push it into SLO/SBA Counties by late Tue
afternoon. Have opted to lean more toward the EC/GFS solution. The
GFS solution does bring the possibility of up to 0.50 inch of
rain to the Central Coast Tue afternoon, while the EC is drier
initially with up to 0.15 inch. So some uncertainty remains on the
potential rainfall for Tue afternoon for this area.

Temps on Tue will turn cooler especially for SLO/SBA Counties. Highs
for the Central Coast should reach the mid 60s, while the coast and
vlys of VTU/L.A. Counties are expected to top out in the upper 60s
to low 70s.

.LONG TERM (WED-SAT)...17/309 PM.



The focus on the medium range will be the increasing confidence of a
significant hydrologic event for swrn CA. The large upper level low
well off the coast will fill and merge with a broad upper level trof
over the E Pac Tue night and Wed, then the large upper trof will
move E toward the CA coast Wed night before moving inland late Thu
into Thu night. A broad SW flow of moist subtropical air will push
into srn CA during the period, with high snow levels at least thru
Wed night before dropping Thu and Thu night.

The EC and GFS both agree that there will likely be plenty of rain
over swrn CA between Tue night and late Thu night. However, the
timing of the heaviest rain is problematic as the GFS brings in the
heaviest rain potential late Tue night thru Wed evening, while the
EC has the heaviest rain potential late Wed thru Thu evening. Both
models do have rain persisting thru the period. There will be some
orographic enhancement of rainfall on the south-facing slopes at
times with this system as well. At this time, it appears there will
not be enough instability for thunderstorm development, but we will
have a better idea on this as well as timing of the heaviest rains
as we draw closer to the event.

It looks like there is the potential for significant rain in all
areas, with preliminary rainfall estimates from late Tue thru Thu
night of 1.50 to 4.00 inches for the coast and valleys, and 3-6
inches in the foothills and mtns. Peak rainfall rates have the
potential to be 0.50-0.75 inch per hour, with local rates up to
1.00 inch per hour, especially on S facing slopes. During the
heaviest rains, it will be possible to see 3-hour rainfall up to 1
to 2 inches in some areas.

This storm will bring the potential for major impacts on the recent
burn areas, including the Thomas, Whittier, La Tuna and Creek burn
areas. With the expected rainfall rates exceeding USGS thresholds,
significant flash flooding and debris flows will be possible in and
around these burn areas. There will also be the potential for
widespread urban and small stream flooding, and rockslides in in the
susceptible areas.

All interests are urged to pay close attention to later forecasts,
discussions, statements and possible watches and warnings as we draw
closer to this potential major rain event.

As for Friday and Saturday, it looks like weak upper troffiness will
linger over swrn CA, with generally fair but cool conditions



At 05Z at KLAX, there was no marine layer or a sfc based

Good confidence in TAFs except at KPRB where there is low
confidence through 16Z due to variable cigs and vis in low clouds
and fog.

KLAX...Good confidence in TAF. Any east winds should remain less
than 7 knots.

KBUR...Good confidence in TAF.


.MARINE...18/121 AM.

For the Outer Waters, good confidence in current forecast. Today
through Monday night, high confidence in winds and seas remaining
below Small Craft Advisory (SCA) levels. For Tuesday and
Wednesday, 60% chance of SCA level southeast to south winds
across PZZ670/673 and a 30% chance across PZZ676. On Thursday,
60% chance of SCA level west to northwest winds across the entire
Outer Waters.

For the Inner Waters, good confidence in current forecast. For the
waters north of Point Sal, there is a 60% chance of SCA level
southeast to south winds Tuesday and Wednesday then a 50% chance
of SCA level west winds on Thursday. For the waters south of Point
Conception, there is a 50% chance of SCA level southerly winds on
Wednesday and westerly winds on Thursday.


CA...Winter Weather Advisory in effect until 5 AM PDT early this
      morning for zones 52>54. (See LAXWSWLOX).



Periods of moderate to heavy rain are possible Tuesday night
through Thursday night. Rainfall rates during this time will
likely exceed USGS thresholds for debris flows for recent burn



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