Flash Flood Watch 2, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
216 PM PST Wed Feb 28 2018

.SYNOPSIS...28/118 PM.

Breezy winds should occur in the mountains and deserts from Thursday
to Friday. The front with precipitation will move over the Central
Coast by Thursday evening, then over the South Coast after midnight,
and into Ventura county by sunrise Friday, and on to Los Angeles
county on Friday. On Saturday, a low will arrive and bring
precipitation, then the low and precipitation should exit on
Sunday. Temperatures should warm to normal by Monday and Tuesday
as a high builds in.

&&

.SHORT TERM (TDY-SAT)...28/208 PM.

An upper low just off the British Columbia coast is expected to
drop south tonight and Thursday and sit off the Oregon coast
through early Saturday before moving inland. Although most of the
dynamics remain north of the area, precipitable waters get close
to 1 inch and there is enough south to southwest flow level flow
to interact with that moisture to generate periods of moderate to
locally heavy rain when the front comes through. Without the
dynamics or convection, rainfall amounts with this system will
likely have a big coast/mountain differential so the higher
elevations of the south facing slopes will have the best chance of
getting those higher rainfall rates. The last couple model runs,
including the 18z NAM, have delayed the peak period of rain by a
few hours so the exact timing is still somewhat uncertain, but in
general Thursday night through noon Friday continue to be the
focus period. The 18z NAM also lowered amounts slightly but based
on the amount of lifting expected in the moist layer rainfall
rates, especially in elevations above 2000-3000 feet, still have
the potential to reach those low end USGS criteria for mud and
debris flows. So a flash flood watch will be in effect for the
most recent burn areas, Thomas, Whittier, Creek, and La Tuna.

Rainfall forecast still about the same, generally around an inch
for coastal areas and 2-3" foothills/mtns. Snow levels will be
higher with this event than the last one and should start out
between 6000 and 7000 ft before falling to around 5000 ft late Fri
and Sat. Higher elevations should see around a foot of snow with
this event along with some gusty south to southwest winds up to 50
mph.

Behind the main front Friday morning a couple of additional
impulses will come through and bring additional showers to the
area through Saturday morning. The trough will be dropping further
south during that time and that will help bring the snow levels
down to around 5000'. Still no convection expected and with less
moisture to work with additional precip amounts should be fairly
light. Could be some brief moderate showers but not heavy enough
to generate any significant flooding issues.

By Saturday afternoon we should see some sunshine with showers
tapering off.

.LONG TERM (SUN-WED)...28/213 PM.

Dry conditions expected Sunday as the trough slides to the east.
Still fairly cool with some breezy west to northwest winds. A
little ridge pops up Mon/Tue along with some light offshore flow for
dry and warmer conditions. The GFS/EC diverge quite a bit now for
the next system with the EC now keeping the ridge in place through
at least Thu while the GFS pushes the ridge east and allows a weak
system to come through Tuesday night into Wed. Will stick with
some low pops during that time for now and see how the models
handle this over the next few days. Either way it would just be a
light precip event at best.

&&

.AVIATION...28/1741Z.

At 1701Z at KLAX, There was no marine layer nor a SFC based
inversion.

Hi confidence in VFR TAFs. There is a 20% chance that VCSH could
start a few hours earlier for KPRB, KSBP and KSMX.

KLAX...High confidence in VFR TAF. There is a 30 percent chc of
calm winds 18Z to 21Z.

KBUR...High confidence in VFR TAF.

&&

.MARINE...28/935 AM.

For the Outer Waters, high confidence in the forecast. A Gale
Watch remains in effect for PZZ670 and PZZ673 starting Thursday
morning through early Friday morning. The Gale Watch might be a
tad early for the southern portion of PZZ673 but will strengthen
quickly. Latest model runs were indicating winds not as strong
for areas S of Point Conception therefore have taken down the Gale
Watch for zone PZZ676 for areas NW of San Nicolas Island. There
is a 30% chance for gusts over 35 knots NW of San Nicolas Island
from Thursday evening through early Friday morning. 50% chance for
gusts to 35 knots for the northern portion of the coastal waters.

For the Inner Waters north of Point Sal, a Gale Watch remains in
effect starting Thursday morning through early Friday morning. The
start of the Gale Watch might be a few hours early but should
strengthen quickly when it does.

NW swell and combined seas will subside today. The next system
will increase SW winds over all waters on Thursday with a 30%
chance for gales followed by building NW swell Friday into
Saturday morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch in effect from Thursday evening through
      Friday morning for zones 39-40-44-52-53. (See LAXFFALOX).
     Winter Storm Watch in effect from Thursday afternoon through
      Saturday morning for zone 53. (See LAXWSWLOX).
     Winter Storm Watch in effect from Thursday evening through
      Saturday afternoon for zone 54. (See LAXWSWLOX).
     Flash Flood Watch in effect from late Thursday night through
      Friday afternoon for zones 54-88-547. (See LAXFFALOX).
PZ...Gale Watch in effect from Thursday morning through late
      Thursday night for zones 645-670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SAT-WED).

Snow in the mountains will create hazardous traveling conditions
Saturday.

&&

PUBLIC...MW
AVIATION...Sweet
MARINE...Sweet
SYNOPSIS...STu

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