Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 092346
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
346 PM PST Sun Dec 9 2018

Updated Aviation Discussion

.SYNOPSIS...09/236 PM.

There will be elevated winds, cooler temperatures, and a slight
chance of precipitation on the north slopes of the Ventura and
Los Angeles County mountains Monday night and Tuesday. Another
slight chance of precipitation will develop across the region next
weekend. Otherwise dry and slightly warmer conditions will prevail.

&&

.SHORT TERM (TDY-WED)...09/241 PM.

A 581-583 dm H5 dirty ridge prevailed over srn CA today. Plenty of
hi clouds moved over the area from the W with filtered sunshine at
times. The LAX-DAG gradient at 21Z was still -3.7 mb which helped
to keep some offshore flow going with with local gusts up to 30
mph noted in the L.A. County mtns and up to 20 mph for the wind-
prone vlys and foothills of VTU/L.A. Counties. Mild temps
prevailed despite the hi clouds thanks to the offshore flow. Highs
in many of the inland coastal areas and vlys reached the upper
60s to lower 70s, which is about 2-8 deg above normal for this
time of year.

The upper level ridge is forecast to move E of the region tonight
with a broad positively-tilted upper level trough approaching
from the ern Pac. This generally weak upper level trof is expected
to move into central CA on Mon then thru srn CA Mon night into
early Tue. There are some model differences with the evolution of
the trof, with the NAM cutting it off into an upper level low
near the srn NV/CA border by late tonight, while the EC cuts off
the upper low over swrn CA late tonight into Tue morning then
moves the upper low S of the region thru the day on Tue.
Meanwhile, the GFS remains progressive with the upper trof thru
Tue. Even with the cutoff over swrn CA in the EC, the lack of
moisture precludes any pcpn from developing over the area. The GFS
likewise forecasts no pcpn over the area. The NAM does predict
north slope showers later Mon night and early Tue. All the models
show a northerly gradient tightening up Mon night into Tue, with
gusty N winds likely from the SBA County S coast and mtns thru the
L.A. County mtns and some of the adjacent vlys. Upper level
ridging will then move into srn CA Tue night and Wed with offshore
flow persisting.

Some offshore flow will continue tonight into Mon morning with
partly to mostly cloudy skies expected thru the day Mon as
additional hi clouds push into the area. Partly cloudy skies as a
whole should prevail Mon night with clearing expected Tue, except
the N mtn slopes will have mostly cloudy skies and a slight chance
to chance of rain and snow showers late Mon night into Tue
morning. Skies are then expected to be mostly clear across the
region Tue night and Wed.

In addition, gusty N winds will develop Mon night and persist
into Tue, with the strongest winds likely from the SBA County S
coast and mtns to the I-5 corridor in the L.A./VTU County mtns. The
winds should also spread to the rest of the mtns and into some of
the adjacent vlys later Mon night thru Tue morning. It looks like
winds will be near advisory levels at times with wind gusts up to
35 to 45 mph. Wind advisories may be needed for this event
especially for the SBA County S coast and mtns. Altho winds should
weaken some Tue night into Wed morning, there should still be
local N to NE gusts in the usual wind-prone areas.

Temps across the region are expected to have a cooling trend for Mon
and Tue, with highs by Tue falling to a couple of degrees below
normal for many areas, then turn milder for Wed. Temps in the
warmest inland coastal areas and vlys are forecast to be in the
upper 60s to low 70s on Mon, fall into the mid 60s to around 70 on
Tue, then warm back to the upper 60s to lower 70s for Wed.

.LONG TERM (THU-SUN)...09/250 PM.

The EC and GFS are in good agreement thru Thu, then differences show
up Fri thru Sun. Upper level ridging will continue on Thu. An upper
level trof moving thru the E Pac will approach the CA coast on Fri,
with the GFS weaker and EC stronger with this system, altho dry
weather should prevail thru Fri across SW CA.

The EC brings a front and fairly decent rain into the area Fri
night and Sat morning while the GFS dissipates the front with dry
weather during the period except for a slight chance of showers
over far NW SLO County Fri night. Decided to lean more toward the
GFS for this time frame.

The EC and GFS both agree to some extent on pushing another upper
level trof and surface frontal system into the area on Sun, with
the EC faster than the GFS with this system. Also leaned more
toward the GFS with a slight chance to chance of rain moving
mainly into SLO/SBA Counties on Sun.

There is really low to moderate confidence in the forecast for
the Fri night thru Sun timeframe given all the model differences
in speed and strength of the two weather systems. Additional model
runs will be needed to fine tune the pcpn fcst during the period.

The EC and GFS agree that there will be good offshore gradients
Wed night into Thu with the potential for advisory level winds in
the usual offshore wind-prone areas of VTU/L.A. Counties. Gusts up
to 35 to 45 mph will be possible.

Temps will be several degrees above normal for many of the coast and
vlys Thu, then cool to slightly above normal Fri. It looks like
temps will cool to near normal for the most part Sat and Sun.

&&

.AVIATION...09/2342Z.

At 23Z over KLAX, there was no marine layer or inversion.

At KPRB, there is a 90 percent chance of conditions similar to
the last 24 hours, with MVFR conditions starting 06-09Z, lowering
to LIFR starting 11-13Z, improving to VFR by 16-20Z. At KSMX,
there is a 30 percent chance of LIFR FG 09-17Z. All other TAF
sites, high confidence in VFR conditions and SCT-BKN250. NE winds
could affect KSBP 16-24Z Monday.

KLAX...High confidence in VFR conditions through Monday, and any
east winds staying under 6 KT.

KBUR...High confidence in VFR conditions through Monday with
light winds.

&&

.MARINE...09/343 PM.

High confidence in winds staying below Small Craft Advisory
levels through tonight, except for a 20 percent chance of Small
Craft Advisory level winds from Point Conception to San Nicolas
Island. High confidence in increasing NW flow Monday through
Wednesday, along with short period seas on top of a building long
period NW swell. Hazardous marine conditions are likely by Monday
across the coastal waters with Small Craft Advisory conditions
in the outer waters near certain from the Central Coast to San
Nicolas Island. There is a 60 percent chance of Small Craft
Advisory winds and seas for the western portions of the Santa
Barbara Channel and Santa Monica Basin in the afternoon and
evening hours.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect from 10 AM Monday to 4 AM PST
      Tuesday for zones 34-35. (See LAXCFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Gusty advisory level north to northeast winds are expected
Wednesday night into Thursday. High surf and dangerous rip
currents are expected on the Central Coast Thursday through
Saturday, possibly expanding to most beaches on Saturday.

&&

$$

PUBLIC...Sirard
AVIATION...Kittell
MARINE...Sweet
SYNOPSIS...Kj

weather.gov/losangeles


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