Gale Watch 2, NWS Los Angeles, CA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
836 PM PST Tue Jan 17 2017

.SYNOPSIS...

A series of storms will bring rain and mountain snow, heavy at
times, and strong, gusty southerly winds to southwestern
California through early next week. The first storm will arrive
Wednesday night into Thursday with a chance of thunderstorms. The
second weather system can be expected Thursday night into Friday.
The third storm is expected Sunday and Monday, which should bring
the heaviest precipitation amounts. Temperatures will be below
normal across the area into next week.

&&

.SHORT TERM...(TUE-FRI)

The latest satellite imagery indicates a dirty ridge of high
pressure in place across the region this evening. An upstream
trough near 40N and 140W is poised to move over the area between
late Wednesday through Thursday. High level moisture associated
with this trough is starting to stream over the area this evening.
Models are in good agreement with the trough and the forecast for
widespread rain on Wednesday night and Thursday remains in place
across the area.

Models do indicate the possibility of a weak warm frontal boundary
possibly developing developing rainfall a little sooner on
Wednesday afternoon for the Central Coast and Wednesday evening
south of Point Conception. PoPs have been nudged higher in the
latest forecast update. Warm air advection ahead of the cold
frontal boundary should aid orographic effects with this system,
especially with the strong south to southwesterly flow aloft. Wind
advisories and winter weather advisories are being discussed
currently internally and with surrounding offices. Strong winds will
likely develop across the mountains, desert, and interior portions
of the area. Strong southerly winds could also impact areas north
of Point Conception as early as Wednesday afternoon. A slight
chance of thunderstorms remains in the forecast for late Wednesday
night and Thursday morning. Current rainfall amounts with the
first storm seem reasonable with between 0.50 to 1.00 inch for the
coast and valleys with up to two inches possible along the coastal
slopes of the mountains and adjacent foothills.

*** FROM PREVIOUS DISCUSSION ***

By late Thursday night and early Friday morning the jet will have
shifted into a more perpendicular flow with most of the mountains
and this will result in a stronger and wetter second storm.
Precipitation amounts will be about double those of the first
storm including snowfall amounts. The airmass will be colder and
snow levels will drop to near 3500 feet with flurries possible
down to near 3000 feet. Northwesterly winds will be stronger and
high wind warnings are possible... particularly in the Los Angeles
County mountains. The heaviest precipitation from this storm will
come throughout the day on Friday and transportation passes
through the mountains... including the Grapevine... will likely be
impacted by snow.

Rainfall intensities will both of these storms could be one
quarter to one half inch per hour at times and higher in favored
locations. This would be enhanced further by any thunderstorm
activity. These rates could generate some mud and debris flows
around recent burn areas. Winter Weather Advisories or Winter
Storm Warnings may be needed with this event.

.LONG TERM...(SAT-TUE)

Effects from the second storm will linger into Saturday but
there should be a brief respite from the precipitation and
strongest winds. By early Sunday morning the third storms will
move into the region. This last storm will be a warmer storm coming
more from the west and southwest. Snow levels will therefore be
higher but it will also be the wettest of the three storms and
with the strongest winds. Estimated rainfall amounts of one and a
half to three inches of are possible with this storm and amounts
in favored locations across coastal slopes and foothills could be
as much as five inches. The warmer airmass will raise snow levels
up to near 7500 feet Sunday night so most roadways will avoid
additional snowfall. Strong and potentially damaging south to
southwest winds will help to enhance the rainfall but also will
bring the greater potential for downed trees and power lines.
Rainfall rates could be four tenths of an inch per hour at times
and this would again bring the possibility of mud and debris flow
problems for recent burn areas. There is also the threat of mud
and rock slides along canyon roadways and urban and small stream
flooding due to the higher rainfall intensities.

This last system shows some signs of slowing as compared to
earlier model runs and this could keep the heavier precipitation
falling into midday Monday in Los Angeles County and lingering
precipitation could remain in the area into Tuesday. A colder
airmass drops in behind the front and roadways above 3000 feet
could become slick as residual water on the roadways freeze Monday
night.

Skies should be clear by late Tuesday.

&&

.AVIATION...18/0025Z.

At 00Z, the marine layer depth was around 500 feet deep at KLAX.
The top of the inversion was near 1100 feet with a temperature
around 14 degree Celsius.

Moderate confidence in the current forecast. IFR to MVFR
conditions will spread into most coastal and valley terminals
through 13Z. Conditions should become predominantly MVFR after
13Z. There is a chance that MVFR conditions could linger
throughout the day on Wednesday.

KLAX...MVFR conditions will spread into KLAX between 10Z and 13Z.
There is a 30 percent chance of IFR conditions between 06Z and
10Z. There is a 50 percent chance that MVFR conditions could
linger throughout the day on Wednesday. Any east winds should be
less than 5 knots.

KBUR...IFR to MVFR conditions will spread into KBUR between 10Z
and 13Z. There is a 50 percent chance that MVFR conditions could
linger throughout the day on Wednesday.

&&

.MARINE...17/800 PM.

No SCA level wind gusts expected through Wed morning. The first
of several storms will approach the coast by Wed night. Ahead of
the front, southerly winds will increase through the afternoon
where SCA to near Gale Force level winds will be likely for all
coastal waters north of Pt. Conception. A Gale Watch may
eventually be needed for the northern outer waters for Wed night.
Otherwise, SCA level winds will then spread across the entire
outer waters Wed night. Rain and a slight chance of thunderstorms
will accompany the front. South winds will increase to SCA levels
again late Thursday night into Friday morning along with another
frontal passage, with gale force gusts possible over the northern
waters. Moderate to strong northwest winds will follow behind the
front through Saturday.

For the Inner Waters south of Pt Conception, light winds are
expected through Wednesday afternoon then southerly winds will
increase on Wednesday ahead of a frontal system. There is a 50%
chance that a SCA will be needed for Wednesday night into
Thursday. Slightly stronger southerly winds for Thursday night
into early Friday ahead of another front. Increasing northwest
winds behind the front Friday afternoon into Saturday will likely
reach SCA levels. Slight chance of thunderstorms expected for
Thursday morning.

A large long period swell is expected to arrive Thursday, then a
very large swell is forecast to arrive Friday night into Saturday.
This third swell will be the largest, potentially reaching
heights over 20 feet over the northern outer waters late Friday
night into early Saturday. Breaking waves will become possible
across west facing harbors on Saturday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from late Wednesday night
      through Monday evening for zones 34-35. (See LAXCFWLOX).
     Beach Hazards Statement in effect from Thursday morning
      through Monday evening for zones 39>41-87. (See LAXCFWLOX).
     Beach Hazards Statement in effect until 10 PM PST this
      evening for zones 40-41. (See LAXCFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK...(FRI-TUE)

A wet and unsettled weather pattern is likely Wednesday night
through Monday. The heaviest rain during this period will probably
fall on Friday and again later Sunday and Monday. There is a chance
of damaging warning level surf and coastal flooding issues starting
Friday and continuing through Saturday, especially along the
Central Coast.

&&

PUBLIC...Hall/Kj
AVIATION...Hall
MARINE...Sweet/Sirard
SYNOPSIS...Sirard

weather.gov/losangeles