Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 220337
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
837 PM PDT Sun Oct 21 2018

.SYNOPSIS...21/518 PM.

Temperatures will be cooler today through midweek then turn
warmer again late in the week. Patchy overnight and morning
coastal low clouds return this week.

&&

.SHORT TERM (SUN-WED)...21/720 PM.

A couple of weak upper level troughs will move through the region
during the next 24 hours, which has resulted in pressure gradients
turning onshore. As expected, temperatures cooled today across
coastal and valley areas, but still had many readings well into
the 80s. Some higher level cumulus clouds developed this afternoon
across the mountains due to some mid level moisture and slight
instability, but clouds dissipating now after sunset. Satellite
imagery showing a much more extensive area of low clouds and fog
off the coast this evening as compared to yesterday evening. As of
8 pm, already seeing low clouds moving ashore across the Central
Coast and along the immediate LA county coast.

The combination of weak upper level troughing, a return of onshore
flow, and a developing eddy circulation will lead to a deeper
marine layer tonight into Monday. ACARS data showing a marine
layer depth around 1300 feet across the LA basin this evening,
and is expected to deepen to between 1500 and 1800 feet overnight
into Monday morning. As a result, expecting low clouds and fog to
fill in across the coastal areas of LA/Ventura counties, Central
Coast, and San Gabriel Valley overnight. Skies will be mostly
clear by Monday afternoon except some patchy low clouds likely to
linger across some of the coastal areas, and some cumulus cloud
development once again across the local mountains. Temperatures
will continue to trend down a few degrees on Monday across most
coastal and valley areas due to the increase marine influence.

*** From previous discussion ***

Not too much excitement in the short term. Synoptically there will
be weak trofing over the area Monday followed by weak ridging on
Tuesday and then dry NW flow on Wednesday. At the SFC...On Monday
and Tuesday there will be a diurnal pattern of weak offshore flow
in the morning and weak onshore flow in the afternoon. On
Wednesday sfc high pressure strengthens over the great basin and
offshore flow will increase over Srn Ca.

Tuesday will be similar to Monday. The eddy will be weaker in the
morning so low clouds will likely not make it into VTA county. The
weaker eddy and slightly higher hgts will lead to a degree or two
of warming across the board.

.LONG TERM (THU-SUN)...21/233 PM.

Both the EC and GFS agree in principle but not exactly through
the xtnd period and there is decent confidence that the forecast
will go as planned.

Both EC and GFS show a ridge starting to really ramp up on
Thursday. The offshore flow will also increase esp from the north.
This will eliminate the stratus and will bring another 2 to 4
degrees of warming to the coasts and vlys with lesser amounts
inland.

The heat will peak on Friday with 588 DM hgts and a -2 MB push
from the east and a -4 MB push from the north. There will be some
morning canyon winds but with no upper support they will be sub
advisory. Another couple of degrees of warming will bring max
temps up into the upper 80s and lower 90s across much of the
coastal and vly area. Most max temps will be 8 to 12 degrees above
normal.

Not much change on Saturday. Grads will be a little less offshore
so there will be weaker morning canyon winds. Max temps will
likely be unaffected and will be very close to Friday`s warm
values.

The ridge should begin to weaken on Sunday and there will be good
onshore trends (much like today) This will lead to 3 to 6 degrees
of cooling across the coasts and vlys.

&&

.AVIATION...21/2344Z.

At 2340Z at KLAX, the marine layer depth is around 1000 feet.
The top of the inversion was at 2200 feet with a temp of 20C.

Moderate confidence in the 00z TAF package. Onshore trends in
pressure gradients and developing eddy circulation are creating
an increase in the presence of marine cklouds late this afternoon
off the coast. Low clouds already along the immediate coast of
LA county, likely spreading into KLAX, KSMO, and KLGB this
evening, pusing up the coast to KOXR and KCMA overnight, with
a 30 percent chance of cigs reaching KSBA by morning. Fairly
high confidence in cigs returning to KSMX and KSBP.

KLAX...Moderate confidence in the 00Z TAF. Low clouds already
reaching the immediate coastal areas of LA county late this
afternoon, so anticipating a fairly quick arrival of IFR cigs
this evening, then likely deepening to MVFR cigs by Monday
morning. There is a 30 percent chance of east winds of 7 kt or
more Monday morning.

KBUR...Moderate confidence in the 00Z TAF with VFR conditions
generally prevailing across KBUR through the period, except
some 5sm hz on Monday morning due to approaching marine influence.
There is a 20 percent chance of IFR ceilings moving into the
airfield near sunrise.

&&

.MARINE...21/808 PM.

For the outer waters, moderate confidence in the forecast. A Small
Craft Advisory (SCA) has been issued for the waters near Point
Conception with gusts between 25 to 30 knots. Occasional gusts to
25 kt will extend southward into PZZ676 as well overnight. There
is a 40-50% that a SCA will be needed Monday in all the outer
zones, with SCA conds expected Monday night through Thursday.

Across the inner waters N of Point Conception, winds should remain
below SCA levels through Monday. There is a 30%-40% chance of SCA
level NW winds during the evening hours Tuesday, then SCA conds
are likely during the afternoon/evening hours Wednesday and
Thursday.

For the inner waters S of Point Conception, there is a 20% chance
of SCA level gusts across western portions of the SBA Channel
Monday and Tuesday evenings.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for zone
      673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

A significant warming trend is possible late next week.

&&

$$

PUBLIC...Gomberg/Rorke
AVIATION...Gomberg
MARINE...Smith
SYNOPSIS...Kj

weather.gov/losangeles


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