Area Forecast Discussion
Issued by NWS Los Angeles, CA

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411
FXUS66 KLOX 182127
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
227 PM PDT Sun Apr 18 2021

.SYNOPSIS...18/816 AM.

A warming trend will continue with very warm temperatures
expected through Monday thanks to weak to moderate offshore flow.
A cool down with a return of night through morning low clouds and
fog is expected starting Monday night and continuing through late
in the week. Drizzle or light rain will be possible Wednesday
night into Thursday morning as well for parts of Los Angeles
County.

&&

.SHORT TERM (TDY-WED)...18/221 PM.

There were low clouds along the beaches of the Central Coast
early this afternoon, otherwise sunny skies prevailed across the
forecast area, with little change expected the rest of the day.
Temps this afternoon will be about 5-15 deg above normal. Highs in
the warmest inland coastal areas and vlys will top out in the 80s
to low 90s.

Offshore pressure gradients and trends to the N and E continued
early this afternoon, with the 20Z LAX-DAG gradient at -2.6 mb
(-2.9 mb trend in 24-hours) and the LAX-BFL gradient was at -2.4
mb (-2.7 mb trend in 24-hours). Gusty N-NE winds to Advisory
levels prevailed early this afternoon over the L.A. County mtns
including the Santa Monica Mtns, Santa Clarita Vly, western San
Fernando Vly and parts of the VTU County vlys. The highest wind
gusts as of 1 PM were 56 mph at SCE Magic Mtn Truck Trail and 51
mph at the Malibu Hills RAWS. However, most gusts have been in the
30-40 mph range. The Wind Advisory was in effect until 2 PM for
these areas, but with winds expected to quickly diminish into mid
afternoon, the Wind Advisory was allowed to expire.

An upper level low was over AZ this afternoon, with an upper
level ridge building along and off the central and nrn CA coast.
H5 heights over swrn CA will increase to about 573-576 dm this
afternoon, with a broad N-NE flow aloft. The upper level ridging
is forecast build into the state tonight into Mon, with H5 heights
around 578 dm across the region by Mon afternoon.

Weaker offshore flow is expected tonight into Mon morning, with
increased onshore flow for the most part Mon afternoon as pressure
gradients trend strongly onshore. Skies are expected to remain
mostly clear across the region thru Mon, altho some low clouds and
fog may affect the immediate Central Coast tonight. Temps will warm
further on Mon to about 8-16 deg above normal for inland areas, with
a few degrees of cooling along the coast but still be near normal to
several degrees above normal overall. Highs on Mon for inland
coastal areas, vlys and lower mtns should be in the 80s to low 90s.

Flat upper level ridging with H5 heights around 575-576 dm should
prevail over the fcst area Mon night, then an upper level low will
move into nrn CA on Tue, allowing H5 heights over the forecast area
to lower to around 571-573 dm Tue afternoon. A broad westerly flow
aloft can be expected on Tue.

A decent eddy should develop over the SoCal Bight Mon night, with
low clouds expected along the L.A./VTU County coast, spreading to
the SBA County S coast and some adjacent vlys overnight. There
should also be low clouds along the Central Coast. The low clouds
should linger along the immediate coast thru Tue afternoon,
otherwise mostly clear skies will continue across the region. Temps
on Tue will turn about 7-15 deg cooler for the coast and vlys and a
few degrees cooler for interior areas thanks to increased onshore
flow. However, it should still be several degrees above normal for
the inland vlys, mtns and deserts. The warmest inland coast and vlys
on Tue should top out in the mid 70s to around 80. However, much of
the lower mtns and Antelope Vly should reach the low to mid 80s.

There should be strong onshore pressure gradient trends to the E
and N on Tue. By 00Z Wed the LAX-DAG gradient is fcst by the NAM
to be +8.6 mb, with LAX-BFL at +5.6 mb. These gradients will
result in gusty SW-NW winds across the region Tue afternoon,
strongest along the Central Coast, and in the mtns and deserts.

The upper level low over nrn CA is forecast to move SE as an inside
slider thru far eastern CA Tue night and into NV on Wed, with its
associated upper level trof moving over the forecast area during
the period. The cyclonic flow aloft and developing eddy will help
to expand low clouds quickly inland Tue evening and to the coastal
slopes overnight into Wed morning. Patchy drizzle will also be
possible over VTU/L.A. Counties from the coast to coastal slopes.
There should be some clearing toward the coast by midday Wed, but
lingering clouds will be possible along the coast and into some
vlys. It may wind up being a reverse clearing pattern on Wed with
the coast scattering out and the vlys remaining mostly cloudy.
There will continue to be good onshore flow on Wed, with breezy to
gusty S-W winds for many areas, strongest over the L.A. County
mtns and Antelope vly. Temps will turn 4-8 deg below normal for
many areas on Wed, with highs for the coast and vlys only in the
60s to low 70s.

.LONG TERM (THU-SUN)...18/225 PM.

The deterministic GFS and EC continue to have differences in the
timing and location of upper level trofs and ridges over the
eastern Pac into CA thru the extended period, altho the mean
ensembles are in somewhat better agreement and will focus on this
for the synoptic discussion. Overall, it looks like another upper
level trof, and possibly and upper level low, should strengthen
over nrn and central CA Wed night then move thru srn CA on Thu.
This system will move S and E of the region Thu night, with upper
level ridging building into CA on Fri. By Sat, the upper level
ridge will begin to move E as a large upper level trof moves into
the E Pac. This upper level trof is expected to move closer to the
CA coast by Sun, with the EC mean ensembles a bit stronger and
faster with this system, and even brings in a Pacific front with
rain to the Central Coast by by late Sun.

Good onshore flow and and deep marine layer is expected Wed night
and Thu. Low clouds will again cover much of the area from the
coast to coastal slopes into Thu morning. There should also be
some drizzle and a slight chance (15-20 percent) of some light
rain over the L.A. County coast, vlys and coastal slopes by later
Wed night into Thu morning. The marine layer should become a bit
shallower Thu night into Sat morning with night and morning low
clouds along the coast and some adjacent vlys. Otherwise and
elsewhere, mostly clear skies should prevail thru Sat. Partly to
mostly cloudy skies overall are expected Sat night and Sun, and
will hold off on any POPs for Sun over northern areas pending
additional model runs and any better agreement. Temps are forecast
to be about 5-10 deg below normal for many areas on Thu, then a
gradual warming trend is expected into the weekend, with highs
4-8 deg above normal away from the coastal plain by Sat. It should
then turn slightly cooler across the region for Sun thanks to more
cloudiness and good onshore flow.

&&

.AVIATION...18/2049Z.

At 1715z at KLAX... there was no inversion.

Overall... Moderate to high confidence in the current TAFs.
At KSBP and KSMX, LIFR/IFR conditions will return after 08z.
Otherwise and elsewhere, VFR conditions will prevail. Periods of
moderate to strong low-level wind shear and turbulence possible
through 22z, especially for Los Angeles County valley terminals.

KLAX... Moderate confidence in the current TAF. VFR conditions
will prevail with periods of moderate to strong low-level wind
shear and turbulence possible through 22z. There is a ten percent
chance of east winds greater than 8 kts during the 13z-18z
period.

KBUR... Moderate confidence in the current TAF. VFR conditions
will prevail with periods of moderate to strong low-level wind
shear and turbulence possible through 22z.

&&

.MARINE...18/149 PM.

Patchy fog, locally dense with visibility under one mile, will
continue through midday Monday off the Central Coast and across
the outer waters, then will be widespread across all waters through
Thursday. The poorest visibility and most widely spread conditions
will occur in the overnight and morning hours.

Across the outer waters... Winds will continue at Small Craft
Advisory (SCA) level through Wednesday with a forty percent
chance of gales Monday night and Tuesday. The strongest winds will
be in the afternoon and evening hours. Seas will reach SCA level
Tuesday. Conditions will be below SCA level Thursday and Friday.

Across the nearshore waters of the Central Coast... Winds will
increase to SCA level on Tuesday with a forty percent chance that
seas will reach SCA level on Tuesday as well. Otherwise,
conditions will remain below SCA level.

Across the inner waters south of Point Conception... Offshore
winds will be SCA level from Point Mugu to Santa Monica through
this evening. Near Point Conception and through the Santa Barbara
Channel, west winds will be SCA level this afternoon and evening.
Otherwise, conditions will remain below SCA level.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for zone
      650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PDT this evening
      for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

No significant hazards expected.

&&

$$

PUBLIC...Sirard
AVIATION...Kj
MARINE...Kj
SYNOPSIS...Sirard

weather.gov/losangeles



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