High Wind Watch 6, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
415 AM PDT Sun Apr 15 2018

.SYNOPSIS...14/732 PM.

High pressure over the region will weaken as an upper trough
approaches the coast, bringing a sharp cooling trend to the area
through early this coming week. There will be a chance of rain
Monday and Thursday, mainly north of Point Conception and over the
interior mountains. Gusty west to northwest winds are expected as
the storms move east of the area, Monday afternoon and night and
again Thursday night.

&&

.SHORT TERM (TDY-TUE)...15/401 AM.

Latest satellite imagery showed mid to high clouds streaming into
the forecast area from the southwest ahead of an approaching
trough of low pressure off the Pacific Northwest coast. These high
clouds will continue to push south across the area today bringing
partly to mostly clouds skies to the area. These high clouds have
largely inhibited low clouds or fog from forming across the
Central Coast, although patchy fog has been observed at a few
locations. Strong onshore trends this morning will continue
through the day as the low pressure system pushes into Norcal with
the LAX-DAG gradient likely peaking between +6 and +7. The onshore
trends, lowering H5 heights, and high clouds will support 5 to 10
degrees of cooling for most areas.

A weakening cold front is expected to bring generally light rain
focused to the north of Point Conception later tonight into
Monday. Although brief heavier shower activity is possible behind
the front Monday, mainly across SLO County. A marine layer with
low clouds expected to develop tonight with patchy fog possible
across LA and possibly VTA counties. The marine layer and
associated low clouds are expected to deepen rapidly Monday,
possibly bringing patchy drizzle to portions of eastern LA County.
Latest model guidance now brings the low pressure system closer
to SLO county. Marginal instability and cold temperatures aloft
(-28 C at 500 mb) is enough to introduce the slight chance of a
thunderstorm or two for interior SLO county including Paso Robles
Monday afternoon. Brief heavier rain, gusty winds, and small hail
is possible if a thunderstorm does form. Storm movement would
likely be to the east or southeast. The cold air filtering in
behind the front Monday evening will lower snow levels below 4000
feet with very light snow accumulations possible above this level,
mainly across northern slopes of VTA/LA counties. Rainfall
amounts will generally be less than a tenth of an inch except
possibly high SLO foothills and in a thunderstorm.

The strong onshore push today will likely support advisory level
west to southwest winds for interior areas late this afternoon
through at least this evening, longer for interior LA County. Even
strong winds are expected Monday with mountain wave activity
likely enhancing winds across the Antelope Valley. As a result, a
High Wind Warning has been issued, starting mid morning Monday
through Monday night for interior LA mountains into the Antelope
Valley with damaging southwest winds likely through Monday
evening, before turning northwesterly and continuing into Monday
night (see NPWLOX to be issued shortly for the latest details).
Gusty advisory level winds are also likely for SBA/VTA mountains
and possibly SBA South Coast Monday night.

Weak ridging aloft, offshore flow, and clearing skies (save for
linger clouds banking up against the north slopes) will bring
around 5 degrees of warming with mid 60s to mid 70s common,
warmest west of the mountains. Lingering gusty northerly winds in
the morning will likely weaken by the afternoon as low pressure
system and associated upper level support for the winds shifts out
of the are to the east.

.LONG TERM (WED-SAT)...15/414 AM.

Above average confidence in the extended period due to model
guidance coming into better agreement. Models have trended a bit
quicker and now agree on a more inland trajectory of the next
trough of low pressure poised to push through the region late
Wednesday through Thursday. As a result, temperatures will likely
only warm a few degrees over Tuesday with increasing high clouds
late in the day. A weak eddy with increasing onshore flow ahead of
the next system may be enough for low clouds to form across
southern LA County. Low clouds across this area become more likely
Thursday with increasing cyclonic flow aloft and a slightly
stronger eddy in place. Have trended precipitation chances down
for most areas, save for north facing slopes as the more inland
trajectory won't have as much moisture to work with.

Winds might be the biggest impact with this system with gusty
onshore winds likely as early as Wednesday afternoon or evening
for interior areas, transitioning more from the north and
affecting typical mountain and foothill locations by later
Thursday and possibly continuing into Friday.

Mixed signals exist for next weekend. At one time models were
showing a fairly healthy ridge building into the region. They now
suggest weak troughing persisting which would be a much cooler
pattern.

&&

.AVIATION...15/1114Z.

At 08Z, the marine layer depth was around 125 feet deep at KLAX.
The top of the marine inversion was around 400 feet, with another
inversion up to around 1500 feet.

Low confidence in the current forecast. There is a chance of VLIFR
to LIFR conditions at coastal terminals through 16Z. LIFR to IFR
conditions have a better chance of formation tonight at coastal
terminals, possibly being one category higher than forecast. Some
intrusion into the valley terminals is possible after 08Z tonight.

KLAX...There is a 40 percent chance of VLIFR to LIFR conditions
through 16Z. LIFR to IFR conditions are possible as soon as 01Z,
but more likely after 08Z tonight. There is a chance that
conditions could be one category higher tonight than forecast.

KBUR...VFR conditions are expected throughout the period, except
for a chance of MVFR conditions after 10Z tonight.

&&

.MARINE...15/414 AM.

Moderate to high confidence in the current forecast.

For the Outer Waters, lingering hazardous seas and increasing
northwest winds will make for marginal advisory level conditions
today and this evening. Higher confidence for widespread Small
Craft Advisory level winds and seas late tonight through Monday
night. There is 30-50 percent chance of gales from late Monday
afternoon through Monday evening, highest for the southern
portion. Another period of gusty winds and moderately large seas
looks possible between Wednesday night and Friday.

For the inner waters north of Point Conception, there is a 60%
chance of Small Craft Advisory winds Monday and Monday night, and
again at times Tuesday night through Thursday.

For the inner waters south of Point Conception, winds increase
between this evening and tonight. Moderately high confidence in
Small Craft Advisory level winds this evening. Higher confidence
in winds increasing and seas building late tonight into Monday.
There is 40 percent chance of gales from late Monday afternoon
through Monday evening. Another period of gusty winds and
steep short period seas looks possible between Thursday and
Friday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 3 PM this afternoon to 9 PM PDT
      this evening for zones 37-51. (See LAXNPWLOX).
     Wind Advisory in effect from 3 PM this afternoon to 10 AM PDT
      Monday for zones 54-59. (See LAXNPWLOX).
     High Wind Watch in effect from Monday morning through late
      Monday night for zones 54-59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 9 PM this evening to 3 AM
      PDT Tuesday for zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

Light snow is possible down to 3,500 foot elevations on the north
slopes Monday night into Tuesday, possibly affecting the I-5
Tejon Pass. Widespread gusty west to northwest winds will create
driving and boating hazards on Thursday into Friday.

&&

PUBLIC...Munroe
AVIATION...Hall
MARINE...Hall
SYNOPSIS...Smith

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