Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 181055
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
355 AM PDT Wed Oct 18 2017

...RECORD HEAT POSSIBLE EARLY NEXT WEEK...

.SYNOPSIS...

A cooling trend will continue through Friday as a series of weak
troughs moves over the region. A warming trend will develop over
the weekend as offshore flow develops across the region. Record
heat is forecast for early next week when offshore flow combines
with strong ridging aloft to bring a hot air mass to the area.
Cooling is possible for the latter part of next week.

&&

.SHORT TERM...(TDY-FRI)

The latest satellite imagery shows a shortwave trough located over
the Central Coast this morning. This trough will slowly move east
over the forecast area today and into the Mojave Desert this
evening. As most of the moisture is at the middle levels of the
atmosphere, there is outside chance of a shower or some sprinkles.
The main impact will be cooler temperatures and partly cloudy
skies continuing throughout today.

Weak high pressure aloft will move over the region for Wednesday,
the a weak disturbance near 27N and 138W moving northeast toward
Southern California will phase with a trough of low pressure in
the Gulf of Alaska. The disturbance will move over the region
between Thursday and Friday. Ample 850mb-700 mb mixing ratios
values are present with this system, but the system is lacking
good instability for widespread rain chances. With flow pattern
currently more favorable for the Central Coast, showers cannot be
completely ruled out for the Central Coast. PoPs have been nudged
higher for Thursday night and Friday but still remain below
mentionable levels.

More confidence is placed in a return of the marine layer stratus
through Friday. Some patches of stratus are developing this
morning near the Channel Islands. NAM-WRF solutions suggest the
marine layer deepening to around 1200 feet late tonight through
Thursday morning, then up to around 2500 feet late Thursday night
and Friday morning. With the weak instability aloft, drizzle has
been added to the forecast for late Thursday night and Friday. In
the meantime until the marine layer deepens enough on Thursday
morning, any stratus that develops will likely be in the form of
dense fog. Fog issues will likely plague the forecast for the
coastal areas for tonight into Thursday morning and again for
Thursday night.

With a northerly surface gradient remaining across the area,
Sundowner winds could develop again tonight, although the winds
should be weaker relative to last night. Local gusts to 40 mph
are in the forecast for now and an advisory may be issued later
today once a bit more confidence arrives. Local 2-km and 3-km WRF
solutions have near advisory level winds developing again tonight.
A weak boundary moving over the area on Friday will likely
tighten the surface pressure gradient between Thursday night and
Friday night, with Friday night likely being a stronger wind event
for Southern Santa Barbara.

The weak boundary could bring some piling of cold air and clouds
along the northern slopes of the mountains on Friday morning.
PoPs remain in the forecast for the north slopes for Friday
morning. Advisory level winds could also develop through the
Interstate 5 Corridor late Thursday night and into Friday morning,
though the pattern is marginal at this time.

.LONG TERM...(SAT-TUE)

The flow will turn offshore on Saturday as a cold air mass
settles into the Great Basin behind the trough currently in the
Gulf of Alaska. Surface high pressure will build into the Great
Basin through the weekend. A warming and drying trend will take
shape through the weekend and into early next week. Monday and
Tuesday will likely be the warmest days of the period when
offshore flow teams up the ridge axis aloft to warm the air mass
additionally. Temperatures have been nudged higher across the area
for Monday and Tuesday with values about 3-5 degrees above MOS
guidance for Monday and Tuesday. Record heat is forecast for
Monday and Tuesday as even MOS guidance values are near or above
record levels for Monday and Tuesday.

Gusty offshore winds cannot ruled out during this period either.
ECMWF solutions are stronger with the surface pressure pattern
relative to the GFS. GFS winds suggest 20-25 knots at 850 mb,
strong enough this far out. Confidence continues to grow for
gusty Santa Ana Winds this weekend.

&&

.AVIATION...18/1025Z...

At 1015Z, the marine inversion at KLAX was based at around 800
feet. The top of the inversion was 2300 feet with a temperature
of 28 degrees Celsius.

Overall, moderate to high confidence in 12Z TAF package. High
confidence in valley/desert TAFs as VFR conditions will prevail
through the period. For coastal TAFs, high confidence in VFR
conditions through this evening then moderate confidence in return
of CIG/VSBY restrictions tonight.

KLAX...moderate confidence in 12Z TAF. High confidence in VFR
conditions through this evening. For tonight, moderate confidence
in return of CIG/VSBY restrictions, but low confidence in flight
category (could range from IFR to VLIFR) and timing (arrival time
could be +/- 3 hours of current 07Z forecast).

KBUR...high confidence in 12Z TAF as VFR conditions will prevail
through the period.

&&

.MARINE...18/200 AM...

For the Outer Waters, high confidence in current forecast. Small
Craft Advisory (SCA) level winds are expected to continue today
through Sunday. There is a 40% of Gale force winds Friday and
Saturday.

For the Inner Waters, moderate confidence in current forecast. For
the waters north of Point Sal, there is a 30% chance of SCA level
winds this afternoon/evening then a 60% chance of SCA level winds
each afternoon/evening Thursday through Saturday. For the waters
south of Point Conception, there is a 20% chance of SCA level
winds across western portions today through Thursday then a 50%
chance of SCA levels winds Friday and Friday night.

A large storm off western Canada will generate 40 foot seas in
that area. This will produce a large long-period NW swell, moving
into our waters by Friday. Seas up to 15 feet are likely, with 20
foot seas possible across the outer and northern waters. There
will likely be dangerous breaking waves on the Central Coast
through the weekend with dangerous conditions in/near harbors.

&&

.FIRE WEATHER...17/800 PM.

Elevated fire danger will continue across much of Southwest
California through this evening as weak offshore flow continues to
bring very warm and dry conditions...with widespread single digit
humidities today across inland areas.

Gusty sundowner winds are expected to impact southern Santa
Barbara county much of this week. This evening, wind gusts
between 35 and 45 mph with humidities falling into the teens will
bring elevated to brief critical fire weather conditions across
the Santa Ynez Range and Santa Barbara south coast. In the wake of
a passing upper level trough, there is the potential for stronger
sundowner winds Friday night into Saturday evening.

From Sunday through next Tuesday, there is the potential for a
prolonged duration of gusty Santa Ana winds with hot and very dry
conditions. While there is still some uncertainty with the
strength of this upcoming Santa Ana event (which will depend on
the upper level wind support), there will be the potential for at
least moderate Santa Ana winds, triple digit heat, and humidities
falling into the single digits and teens which could bring a
return of critical fire weather conditions to portions of
Southwest California.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from Friday morning through
      Friday evening for zones 34-35. (See LAXCFWLOX).
     Beach Hazards Statement in effect through Thursday afternoon
      for zones 40-41. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX).

&&

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

Hot temperatures and elevated fire weather conditions are
expected Sunday into early next week as another Santa Ana wind
event develops. Record heat is possible for Monday and Tuesday.
Large surf and strong rip currents is expected to develop at
Central Coast beaches between Friday and Sunday.

&&

$$

PUBLIC...Hall
AVIATION...RAT
MARINE...RAT
FIRE WEATHER...Gomberg/Sirard
SYNOPSIS...Hall

weather.gov/losangeles



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