Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 180512
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
912 PM PST Sun Dec 17 2017

.SYNOPSIS...

Dry and more tranquil weather is expected Monday and Tuesday with
patchy morning frost in the coldest locations. Another round of
strong offshore winds will affect the region beginning on Wednesday
and likely continuing through the end of the week. There is a slight
chance of light rain along the Central Coast Wednesday morning.

&&

.SHORT TERM...(SUN-WED)

High pressure aloft is building into the area from the north as a
trough of low pressure continues to exit the region. Offshore flow
remains in place across the area this evening. Model solutions
suggest the surface pressure gradient tightening again later
tonight with enough upper-level support to warrant the extension
of the Wind Advisory. Winds are expected to restrengthen between
10 pm tonight and 4 am Monday morning, then diminish through
Monday morning. Winds have fallen below criteria currently but
will push back into Wind Advisory criteria late tonight.

A dry air mass in place with the offshore flow pattern and
clearing skies will allow for overnight temperatures to fall near
or below freezing across the interior valleys and wind-sheltered
areas. Radiative cooling effects will be more efficient overnight
the next several nights. For late tonight and early Monday
morning, a hard freeze warning was added for the Cuyama Valley, a
freeze warning for the Santa Ynez Valley, and a frost advisory for
the Central Coast.

*** FROM PREVIOUS DISCUSSION ***

The upper level ridge will then persist over the area Mon night
and Tue. A broad upper level trof is forecast to move into nrn CA
Tue night then slide SE thru Central CA for Wed, with swrn CA on
the srn part of the upper trof. A weakening cold front associated
with this upper level trof will move into the forecast area Wed
afternoon and early evening.

Gusty mainly sub-advisory NE winds will prevail across VTU/L.A.
Counties tonight into Mon morning, then weak to increasing
onshore flow can be expected Mon afternoon thru Tue. Skies will be
mostly clear across the region thru Tue, except for areas of
smoke persisting in the vicinity of the Thomas Fire.

It looks like marine layer clouds will redevelop and expand over the
coastal waters into the coastal plain Tue night into Wed morning.
The weak cold front is forecast to bring cloudiness to much of the
forecast area on Wed, along with a slight chance of showers over
portions of SLO/SBA Counties Wed afternoon. The passage of the
front will also increase a northerly flow over for region by late
Wed into Wed evening, with gusty winds possible in the SBA County
mtns and S coast, possibly to advisory levels. Gusty NW winds should
also affect the L.A./VTU County mtns by Wed evening, especially
along the I-5 corridor. These winds should turn more N to NE later
Wed night with gusty winds possible over the vlys and possibly
coastal areas of VTU/L.A. Counties. Winds in these areas could
approach or reach advisory levels at times. The gusty offshore
flow will clear out any marine layer clouds Wed night.

Temps are expected to be slightly above normal for many areas on Mon
and Tue, altho Tue will be the cooler of the two days. Temps will
drop considerably on Wed with temps expected to be near normal to
slightly below normal across the region.

.LONG TERM...(THU-SUN)

The EC and GFS still are showing some run to run inconsistencies,
but on today`s 12Z run the large scale features are not too
different until Sun. The models agree that the upper trof from Wed
will be progressive and move E of srn CA on late Thu and Thu night.
Weak, flat upper ridging will move into the area late Thu night and
Fri. A large upper level trof is forecast to approach the area from
the nrn Great Basin on Sat. For Sun, the GFS stalls a large
positively tilted upper trof over the central Great Basin thru
central CA, while the EC continues to gradually deepen the upper
trof into srn CA. The models do bring some clouds and possible
showers to the area on Sat night into early Sun, but confidence in
any pcpn is low this far out, so kept POPs out of the forecast.

Clear to partly cloudy skies will prevail across the region Thu thru
Sun. There will be gusty N to NE winds especially over VTU/L.A.
Counties Thu morning, otherwise winds will turn lighter with some
onshore trends during the period. Temps are forecast to have a
slight warming trend during the period, with Thu being the coolest
day. Highs should be slightly below normal to near normal
overall on Thu, then warm to slightly above normal across the
region next weekend. It also looks like a dry air mass and clear
skies will promote cold overnight lows Wed night and Thu night,
where several sheltered valleys and coastal areas could have
frosts or freezes.

&&

.AVIATION...17/1800Z.

At 1622Z at KLAX, there was no marine layer. There was an
inversion with a base near 2500 feet and a top near 4000 feet and
a max temperature of 16 degrees Celsius.

VFR conditions are expected throughout the period, except for a chance
of MVFR conditions in smoke at Santa Barbara and Ventura county
terminals. Periods light to moderate wind shear and turbulence are
possible at all terminals through 20Z.

KLAX...Good confidence in Cig/Vis portions of the TAF. Lower
confidence in the wind forecast. But only a 20 percent chc of east
winds greater than 8 kt. Lgt turbc at times DURGD.

KBUR...Moderate confidence in Cig/Vis portions of the TAF. Lower
confidence in the wind forecast. There is a 60 percent chc of
north to northeast winds at times through 20Z. Turbc at times
DURGD.

&&

.MARINE...17/900 PM.

Across the outer waters...conditions will remain below Small
Craft Advisory (SCA) criteria through Wednesday morning. Wednesday
afternoon through Thursday, north winds up to 25 knots are
expected. Fairly good confidence in this forecast. Low confidence
in offshore winds developing Friday into next weekend.

Across the inner waters south of Point Conception...northeast
winds will likely continue with gusts to 25 knots nearshore from
Ventura to Santa Monica and across the San Pedro Channel, through
early Monday morning. Choppy seas are also expected during this
time. Otherwise, conditions will remain below SCA levels through
Friday.

There will continue to be areas of smoke from the Thomas Fire
over portions of the Coastal Waters, mainly through the Santa
Barbara Channel and over portions of the outer waters. Visibilities
could locally drop to 2 NM or lower at times. Starting Monday,
weakening offshore flow early with onshore flow in the afternoon
will push smoke toward Ventura and far western L.A. County.

&&

.FIRE WEATHER...17/300 PM.

A strong Santa Ana wind event impacted Ventura and Los Angeles
counties today, with alignment of the offshore pressure gradient,
upper level wind support, and cold air advection. The LAX-Daggett
gradient peaked at -5.8 mb this morning. Observations this morning
showed damaging gusts of 60 to 70 mph in the mountains, and 50 to
60 mph across portions of the coast and valleys. Critical Red
Flag conditions can be expected for much of Los Angeles and
Ventura counties to continue through 8 pm this evening. Any new
fire ignitions could bring very rapid fire spread, long range
spotting, and extreme fire behavior into the evening hours.

There will be a return of more extensive onshore flow on Tuesday with
moderate humidity recovery across coastal and valley areas. Another upper
level trough will dig through California on Wednesday, with a weak cold
front bringing a slight chance of rain to areas north of Point Conception.
For areas south of Point Conception, including the Thomas Fire area,
there will be cooler temperatures, higher humidities, and increasing
west to northwest winds through the day on Wednesday. By late Wednesday
afternoon into Wednesday night, northwest to north are expected to peak
as upper level wind support, cold air advection, and north-south pressure
gradients align. Wind speeds across the Santa Barbara county mountains
and south coast will have the potential to be fairly similar to
Saturday morning`s event, with sustained winds in the 20 to 35 mph range,
and gusts generally ranging between 40 and 55 mph, except isolated gusts
to 60 mph across favored ridgetops. Gusty north winds are expected to
continue Thursday into Thursday night across much of Santa Barbara
county, and possibly continuing into  Friday. Gusty northwest to north
winds will also impact other mountain areas, including the Highway 33
and Interstate 5 corridors. While humidity levels will likely remain
above 15 percent with this event, there is still the potential for
critical fire weather conditions across the Santa Barbara county
mountains and south coast considering the potential for strong winds
and continued very dry fuels.

Later in the week into next weekend, confidence remains low for the
potential Santa Ana wind pattern as computer models continue to widely
fluctuate on the timing, duration, and strength of the Santa Ana
winds.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory in effect until 9 AM PST Monday for zones
      34-35. (See LAXNPWLOX).
     Freeze Warning in effect until 9 AM PST Monday for zone 36. (See LAXNPWLOX).
     Hard Freeze Warning in effect until 9 AM PST Monday for zone
      38. (See LAXNPWLOX).
     Wind Advisory in effect until 9 AM PST Monday for zones
      44>46-53-54-88. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Monday for
      zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(WED-SUN)

Critical fire weather conditions are possible with gusty
Sundowner winds Wednesday night and gusty offshore winds
Thursday and Friday. Frosts and freezes will be possible in
sheltered areas Wednesday night and Thursday night.

&&

$$

PUBLIC...Sirard
AVIATION...Sweet
MARINE...Smith
FIRE WEATHER...Gomberg
SYNOPSIS...Kj

weather.gov/losangeles



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