543 
FXUS66 KLOX 052209
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA 
209 PM PST Sat Dec 5 2020

.SYNOPSIS...05/205 PM.

Clear to partly cloudy skies and dry weather will prevail for at 
least the next seven days. High temperatures will be a little 
warmer than normal and most lows will be a few degrees lower than 
normal. There will be moderate to strong Santa Ana winds Monday 
through Tuesday morning.

&&

.SHORT TERM (TDY-TUE)...05/204 PM.

Mostly sunny skies covered the forecast area early this afternoon
with little change expected thru sunset except a few hi clouds
should move into SLO/SBA Counties. Offshore pressure gradients to
the E continued early this afternoon (20Z LAX-DAG was -3.1 mb). 
Breezy NE winds prevailed over portions of the foothills and mtns 
of VTU/L.A. Counties, with local gusts up to 25 mph. These winds 
will persist thru sunset, while elsewhere breezy onshore flow can 
be expected. The offshore gradients will help many areas away 
from the coast to lower mtns to top out in 70s to around 80 this 
afternoon, which are about 5 to 10 deg above normal for this time 
of year.

A weak upper level trof will prevail over SW CA this afternoon, 
with H5 heights around 578 dm. Upper level ridging should build 
in tonight into Sun, with H5 heights rising to around 581 dm by 
late Sun morning. A 562 dm upper level low is forecast to develop 
over nrn NV on Sun, then track S Sun night into Mon morning. The
dry upper level low will reach L.A. County by 21Z Mon before 
moving off the coast S of the area by late Mon. This system will 
move to a position off the nrn Baja coast Mon night then remain 
nearly stationary thru Tue.

There may be some hi clouds over the area at times thru Mon, 
otherwise mostly clear skies should prevail thru Tue. Marine 
layer clouds should remain off the coast thru Sun, altho patchy 
low clouds and fog cannot be entirely ruled out along the 
immediate Central Coast thru Sun morning. Increased offshore flow
will then push any low clouds well off the coast later Sun night 
into Mon then continue thru Tue.

Pressure gradients will be weaker offshore to the N and E tonight 
into Sun, then strengthen Sun night into Mon. By 12Z Mon, the NAM 
forecasts the LAX-DAG gradient to be around -5.4 mb, with LAX-BFL 
around -4.7 mb. Breezy offshore flow can be expected tonight and Sun 
morning, especially for the foothills and mtns. The increased 
offshore gradients combined with some cold air advection and upper 
level support will bring strong and gusty NE winds to the mtns 
and some vlys by early Mon morning. The winds should remain quite 
gusty thru Mon afternoon as the offshore gradients increase 
further. Further strengthening of the winds can be expected Mon
night into Tue morning as offshore gradients to the N and E are 
forecast by the NAM to increase by 12Z Tue to -8.9 mb from LAX- 
DAG, and to -7.3 mb from LAX-BFL. 850 MB winds over VTU/L.A.
Counties are forecast to increase to 45-50 kt Mon into Tue, with 
950 mb winds forecast to peak around 50 kt Mon morning, and around
50-55 kt late Mon thru Mon night. Significant 850 mb winds will 
also affect SLO/SBA Counties and increase out of the east up to 
40 kt later Mon and Mon night. 

Damaging wind gusts up to 60 to 70 mph will be possible at times 
for the mtns of VTU/L.A. Counties including the Santa Monica Mtns 
from late Sun night thru midday Tue. A High Wind Watch will be
issued with the afternoon zones for these areas. Widespread high-
end Wind Advisories will eventually be needed for the rest of
VTU/L.A Counties, except for the Antelope Vly, from Mon morning 
thru Tue morning, and possibly for the SLO/SBA County mtns from 
Mon afternoon thru early Tue morning. 

High temps across the region are forecast to remain quite mild 
and several degrees above normal for many areas Sun. Temps should 
cool some for the mtns and deserts on Mon, while S of the mtns it 
should be a few degrees warmer with the increased offshore flow 
and adiabatic warming. Highs in the vlys and coastal areas should
reach the 70s Sun, and mostly in the mid to upper 70s on Mon.
Temps Tue S of the mtns should warm up another 5 to 10 deg and 
reach the low to mid 80s for a lot of the coast and vlys. There
will also be cold overnight lows in sheltered areas. Lows should
fall into the mid teens to lower 20s in the Antelope Valley 
tonight where a Hard Freeze Warning will be issued.

.LONG TERM (WED-SAT)...05/159 PM.

The upper level low off Baja will slowly move E Tue night and Wed
while weak upper level ridging builds into the forecast area. Flat 
upper level ridging will linger Thu and Fri. The EC builds in 
stronger upper level riding into the area by Sat, while the GFS 
keeps weak flat upper ridging over the region. The mean ensembles 
forecast weaker upper level ridging for Sat. Offshore gradients to
the N and/or E will persist thru the extended period. 

Dry weather will continue Wed thru Sat over SW CA with mostly
clear skies thru the period. Breezy offshore flow can be expected
night and morning hours, altho may be weak to light onshore on
Fri, and a little bit stronger on Sat. Temps will be warmest on 
Wed, about 5-10 deg above normal for many areas, then cool 
slightly thru Sat but still remain a few degrees above normal each
day, especially S of the mtns.

&&

.AVIATION...05/1749Z.

At 17Z at KLAX, there was no marine layer. The surface based
inversion top was at 1000 feet and 17 degrees Celsius.

There is a 20 percent chance of LIFR FG impacting KSMX Sunday
10-16Z. Otherwise, high confidence in VFR everywhere with weaker
than normal winds. Widespread moderate to strong northeast winds
will develop Monday and Tuesday.

KLAX...High confidence VFR conditions through Sunday, with no
significant east winds. 

KBUR...High confidence VFR conditions through Sunday.

&&

.MARINE...05/101 PM.

High confidence in winds staying below Small Craft Advisory (SCA)
through early Sunday morning. High confidence in SCA winds from 
the Central Coast to San Nicolas Island forming by Sunday 
afternoon and continuing through the night. High confidence of 
northeast winds forming over most waters on Monday, and moderately
confident that they will become unusually widespread and gusty by
Monday afternoon or evening...then gradually weakening Tuesday 
and Wednesday. There is a 50 percent chance for Gale Conditions 
from Ventura to Santa Monica from Monday afternoon through Tuesday
morning. Otherwise expecting solid SCA winds and seas over most 
if not all zones, including the Central Coast and possibly Avalon
Harbor.

Meanwhile, a large long period west to northwest swell will impact
all waters Monday through early Wednesday, lowering but staying
elevated through the rest of the week. Tuesday will be the
largest, when swells will range from around 15 feet off the
Central Coast, to around 7 feet from Santa Barbara to Orange
County. This will create hazardous breaking waves near shore,
possibly including the Morro Bay and Ventura Harbor entrances.
While this long period swell will be dominant, it is worth
mentioning that there will also be short period waves on top of
this swell when and where the gusty NE winds (mentioned above) 
are blowing.

&&

.BEACHES...05/1237 PM.

A very large and long period west to northwest swell will impact
the area later Monday through early Wednesday. The forerunner 
waves will have periods longer than 20 seconds Monday afternoon 
and night, Breaking waves between 15 and 20 feet are possible on
the Central Coast during the Tuesday peak, and 6 to 12 feet for
Los Angeles, Ventura and parts of southern Santa Barbara Counties.

Surf of 20 feet and greater can damage coastal structures like 
piers and jetties, so a High Surf Warning may be required for the
Central Coast. Otherwise, expecting High Surf Advisories with 
dangerous rip currents at most beaches. Widespread northeast to 
east winds are expected during this time as well, especially 
Monday afternoon and night which could amplify some waves. 
Vulnerable harbor entrances, like Morro Bay and Ventura, have a 
history of dangerous breaking waves under such conditions. High 
tides should stay below 5 feet during the event, so the threat of 
coastal flooding is minimal.

&&

.FIRE WEATHER...05/933 AM.

Red flag warnings remain in place today over the mountains of Los
Angeles and Ventura Counties, and the Santa Clarita Valley due to
northeast to east winds gusting 20 to 30 mph coupled with very 
low humidities of 5 to 10 percent. Offshore winds will continue to
decrease tonight and Sunday. 

A cold low pressure system over Nevada on Sunday night will dive 
southwestward through Southwest California on Monday, shifting to 
the south of the area by Monday night. This system will likely 
bring increased higher level clouds and an uptick of surface 
humidities through Monday morning, then a drying trend by Monday 
afternoon into Tuesday when humidities in the teens and single 
digits will be common. Gusty Santa Ana winds will likely develop 
late Sunday night into Monday morning, then peak in coverage and 
strength Monday afternoon into Tuesday morning when wind gusts of 
40 to 55 mph will be common across the wind favored coastal and 
valley areas of Los Angeles and Ventura counties, and damaging 
gusts of 55 to 70 mph possible in the mountains. Also of note with
this upcoming event is that the upper level wind support will 
likely bring unusually strong northeast to east winds across 
portions of Santa Barbara and San Luis Obispo counties, especially
in the mountains and foothills, where wind gusts of 40 to 60 mph 
will be possible from Monday afternoon through Tuesday morning. 
Widespread critical fire weather conditions will be likely with 
this event for Los Angeles and Ventura counties, and possibly 
portions of San Luis Obispo and Santa Barbara counties.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from Monday morning through
      Wednesday evening for zones 34-35-39>41-87. (See LAXCFWLOX).
     High Wind Watch in effect from late Sunday night through
      Tuesday morning for zones 46-53-54. (See LAXNPWLOX).
     Hard Freeze Warning in effect from midnight tonight to 9 AM
      PST Sunday for zone 59. (See LAXNPWLOX).
     Fire Weather Watch in effect from Monday afternoon through
      Tuesday morning for zones 234-235-239-251-252. (See LAXRFWLOX).
     Red Flag Warning in effect from 4 AM Monday to 10 PM PST
      Tuesday for zones 240-241-244>246-253-254-288-547-548. 
     (See LAXRFWLOX). 
PZ...Gale Watch in effect from Monday afternoon through Tuesday 
      morning for zones 650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from noon Sunday to 6 AM PST 
      Monday for zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

Periods of gusty offshore winds and dry conditions could lead to 
critical fire weather conditions Monday night and Tuesday or 
Wednesday, then again by next Saturday. High surf or strong rip 
currents may create hazardous ocean conditions Monday through 
Wednesday. Sub-freezing conditions are possible for interior 
valleys each morning at least through Wednesday.

&&

$$

PUBLIC...Sirard
AVIATION...Kittell
MARINE...Kittell
BEACHES...RK
FIRE...Gomberg
SYNOPSIS...30/Sirard

weather.gov/losangeles