Area Forecast Discussion
Issued by NWS Los Angeles, CA

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994
FXUS66 KLOX 171030
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
330 AM PDT Tue Oct 17 2017

.SYNOPSIS...
Offshore winds will bring above normal temperatures through
Tuesday. Patchy fog and low clouds return to the coast Thursday
and Friday as temperatures cool to near normal late in the week.
Temperatures increase again this weekend and into the following
week as another offshore wind event develops over the region.

&&

.SHORT TERM...(TDY-THU)

About three MB of offshore flow both from the east and north. Hgts
remain high at about 589 DM. There will be a little more cloud
cover today than yesterday and this well help knock a few degrees
off of temps compared to ydy esp over SLO and SBA counties. All
eyes on a nice little vort max 350 miles to the WSW of LAX. This
is a rather unstable system and is currently putting out quite a
bit of lightning. This system will bring scattered TSTMs to the
nrn waters later this morning. It is forecast to weaken somewhat
as it moves to the NE but it will still bring a slight chc of
TSTMs to SLO and SBA counties as well as the VTA mtns this
afternoon and evening. The cloud bases could be so high that most
of the precip will fall as virga. Dry lightning could be a
problem with this system.

The little system will knock the ridge down for Wednesday and
bring WNW flow to the area. There will be some mid and high level
clouds embedded in the flow so skies will be partly cloudy. There
will be decent onshore trends and the offshore flow will be very
weak and will likely trend to neutral during the day. Hgts also
fall about 5 DM so all in all look for a good cooling trend of 4
to 8 degrees everywhere.

A little eddy spins up Wednesday night and the W/E gradient will
turn onshore this should be enough to bring some low clouds to the
coastal areas expect for the SBA south coast where there will be
some light north north winds.

A weak trof will approach the west coast. It will keep the partly
cloudy skies going. At one point it looked like this system could
bring rain to the area but now the energy/moisture will keep to
the north of SLO county. Hgts fall to 578 DM and there will be a
moderate 5mb push to the east at the sfc...both of which will
combine to lower max temps 3 to 6 degrees. Most areas will be
right at seasonal normals.

.LONG TERM...(FRI-MON)

The trough that at one time looked like it would possibly bring us
some rain Friday is now weaker and farther north on the models and
at least for now isn`t posing a threat for precipitation in our
area. So rain chances have been dropped below 15. Even a lot of
the moisture that models had been entraining into the cold front
is gone so we`re looking at partly cloudy skies at worst. Will
probably see a decent low cloud layer developing Friday morning as
the trough gets closer and possibly limited/slow clearing during
the day. Temps will dip below normal for one day before ridging
quickly returns for the weekend and temps climb. Expecting a peak
in the northwest flow Friday night into Saturday with some
impressive (-5 to -7mb) SBA-SMX and SBA/LAX-BFL gradients on the
GFS. If this holds we can expect to at least see advisory level
Sundowners and I5 corridor winds and possibly touching warning
level.

The warming trend will start quickly Saturday then really ramp up
Sunday/Monday as offshore gradients return. We`re not seeing quite
the good upper level support we saw a couple days ago as now both
the GFS and ECMWF show a more progressive trough pattern rather
than an upper low dropping south through the Great Basin. And in
fact both now show 591-594dm heights Sunday-Wednesday with
moderate offshore gradients and 950mb temps in the low 30s
celcius. That would likely yield highs in the 90s for coastal
areas, lower 100s for Downtown LA and a few degrees warmer for the
valleys. Current forecast is somewhat conservative given the
pattern but quite a bit above MOS guidance so confidence not
particularly high just yet.

&&

.AVIATION...17/0056Z.

At 00Z, there was a surface based inversion up to around 1500 feet
at KLAX.

Moderate-to-high confidence in the current forecast. VFR
conditions are expected throughout the period with no significant
wind concerns. There is a very slight chance of thunderstorms
north of KSBA after 19Z Tuesday.

KLAX...VFR conditions are expected throughout the period with no
wind concerns.

KBUR...VFR conditions are expected throughout the period with no
wind concerns.

&&

.MARINE...16/900 PM.

High confidence in winds below Small Craft Advisory through this
evening. From the Central Coast to San Nicolas Island, northwest
winds will increase on Tuesday from the Central Coast to San
Nicolas Island. Advisory level winds with gusts to 30 kt and
widespread choppy seas will develop, including near shore waters
where it will not be as windy). These winds and choppy seas look
to continue through Saturday, and could be strongest Friday. The
inner waters portion of the Santa Barbara Channel will flirt with
advisory conditions. There is a 30 percent chance of an advisory
Tuesday or Wednesday evenings, increasing to 60 percent chance
Thursday and Friday.

Dense fog is possible Tuesday night into Wednesday.

A large storm will form off British Columbia Tuesday with 40+
feet seas expected near the storm. This will produce a large long-
period NW swell reaching our waters by Friday with seas of 15-20
feet possible. There is a possibility for dangerous breaking
waves near shore Friday and Saturday, including Morro Bay Harbor.

&&

.FIRE WEATHER...16/900 PM.

Elevated fire danger will continue across much of Southwest California
through Tuesday as weak offshore flow continues to bring very warm and
dry conditions. Widespread single digit humidities will continue across
the region through Tuesday, with very poor humidity recoveries in the
mountains and foothills tonight.

From Tuesday into early Wednesday, there will be an increase in
mid level subtropical moisture from the southwest with a shortwave
trough that could bring isolated dry thunderstorm to areas mainly
north of Point Conception with a 10 percent chance of light
showers or elevated thunderstorms elsewhere. Any storms that
develop over the area, the main threats would be isolated dry
lightning strikes and gusty erratic downdraft winds due to the
very dry lower levels of the atmosphere.

Gusty sundowner winds are expected to impact southern Santa
Barbara county much of this week, during the late afternoon and
nighttime hours. Initially, on Tuesday and Wednesday evenings,
wind gusts between 30 and 40 mph with humidities falling into the
teens will bring elevated to brief critical fire weather
conditions. In the wake of a passing upper level trough, there is
the potential for stronger sundowner winds Friday night into
Saturday evening.

From late Saturday night 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 lower
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 Wednesday evening
      for zones 40-41. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Wednesday for zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM this morning to 6 AM
      PDT Wednesday for zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(THU-MON)
Hot temperatures and elevated fire weather conditions are
expected over the weekend into early next week as another Santa
Ana wind event develops. Large surf is possible at Central Coast
Beaches between Friday and Sunday. Strong rip currents are
possible.

&&

$$

PUBLIC...Rorke/MW
AVIATION...Hall
MARINE...Hall
FIRE...Gomberg/Hall
SYNOPSIS...Kj

weather.gov/losangeles



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