Frost Advisory 4, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1106 AM PST Sat Nov 7 2020

.SYNOPSIS...07/851 AM.

Periods of mostly light rain, mountain snow, and widespread gusty
winds expected through the weekend, with well below normal
temperatures. Partly cloudy skies and below normal temperatures
are in store starting Monday and continuing through most of next
week.

&&

.SHORT TERM (TDY-MON)...07/851 AM.

***UPDATE***

Organized band of showers currently moving through Los Angeles
County, and should be out and to the east by late morning.
A few isolated showers will continue to develop through at least
the afternoon over the region, but the flow becomes more
northwesterly by noon which should focus those showers over SLO
and northern SBA Counties, as well as the mountains of Los
Angeles and Ventura Counties. Snow levels have lowered already
this morning to around 4000 feet, and should stay that way through
the day despite daytime heating as the core of the low
approaches. That means any showers will result in mountain snow
flurries. This includes the Tejon Pass on I-5, although
accumulations there today should be minimal to none. There are a
few forecast challenges to figure out. One question: How much
clouds clearing will occur tonight and where, which would result
in potential freezing temperatures in the valleys. The other
challenges are include the chances of rain and wind advisories
for tomorrow. Still looks like rain amounts and coverage tomorrow
will be much less than today, but winds will be stronger and snow
levels could be lower. This should actually add up to more
impacts than today, and the Tejon Pass could get up to an inch of
accumulations which would result in significant delays.

***From Previous Discussion***

A cold 539 dam upper low over Sacramento will move south to Fresno
and then pivot to the east an into NV. As it does this it will
sweep a trof and a large lobe of PVA through the 4 county forecast
area. Showers will be likely everywhere today moreso in the
morning across SLO and SBA counties and then in the afternoon for
VTA and LA counties. The low and the PVA are moving a little
faster than previously forecast and the rain chc will diminish
later in the afternoon. 500 mb temps will fall to -28 C which will
destabilize the atmosphere enough to bring a slight chc of TSTMs
to the area as well. Rainfall totals will mostly be in the quarter
inch to half inch range with local amounts to one inch in the San
Gabriel foothills and coastal slopes due to an extended period of
upslope flow. Not too much concern about the burn areas as
rainfall rates will not be too intense. While the rainfall rates
under any TSTM that forms will higher the storms will be moving
fast enough to prevent any flooding problems.

Snow levels should drop to around 4000 feet along the northern
slopes and slope up to 5000 ft closer to the coast as 700 mb
temperatures cool to near -8 C. Areas above 6000 ft will likely
see 3 to 6 inches of snow with local amounts to 9 inches across
the higher elevations of the eastern San Gabriels. A winter
weather advisory remains in effect through 10 pm Sunday night.

The area will be in between systems tonight and this will allow
skies to clear and temps to fall to well below normal. Some areas,
such as the Ojai and the Cuyama Vlys, will likely see freezing or
near freezing temperatures tonight.

A second low will spin down the CA/NV state line on Sunday. This
track will produce more of a windy day than a rainy one. Still
there will be chc of showers everywhere with the best chc across
the north slopes of the mtns. Due to the overland trajectory of
the flow rainfall amounts (and rates) will not be too great and
expect amounts to range form the trace to a tenth of an inch. Snow
levels may be a little lower than today and the Grapevine area
could see a dusting of snow. It will be a blustery day and there
will likely be advisory level gusts in the mtns and the Antelope
Vly.

It will be a chilly two days with max temps across the coasts and
vlys struggling to see temps in the lower 60s. Most max temps will
end up 10 to 20 degrees blo normal. Needless to say the record
streak of max temps 70 degrees or more at downtown LA will end.

The low will move to the east by Monday, bringing in dry northwest
flow through Wednesday. Monday morning should be rather chilly
under mostly clear skies. Frost advisories or Freeze warnings are
looking increasingly likely for the inland and interior valleys.

.LONG TERM (TUE-FRI)...07/317 AM.

Very benign weather forecast for next week as both the EC and the
GFS agree that dry NW flow will set up and continue over the area.
A little energy moves in Thursday night and might bring some rain
to nrn SLO county but there is little mdl or ensemble consensus on
this. Apart from that slight deviation skies will vary from
mostly clear to partly cloudy. Max temps will warm slightly but
will remain blo normal through the week. The cool temps will
prevent any marine inversion from forming and thus no low clouds.

&&

.AVIATION...07/1853Z.

At 1717Z at KLAX, there was no marine layer and no inversion.

Low confidence in TAFs through 23Z. A cold storm system will
bring highly variable conditions to all sites through 23Z,
although conditions will not likely drop below MVFR in most cases.
Locally heavy showers/TSTMS could briefly cause IFR conditions and
locally gusty winds. Cigs will vary the most. There is a 20
percent chc of a TSTM through 23Z. Good confidence in dry VFR
conds aft 23Z. Surface winds will remain rather gusty out of the
west to northwest over the next couple of days, especially in the
afternoon and evening.

KLAX...Low confidence in 18z TAF. There will be intermittent MVFR
cigs through 23Z. Brief IFR conditions are possible with passing
showers and isolated TSTMS through 22-23z. Good confidence in TAF
aft 23Z. Good confidence that any east wind component will be
under 3 kt.

KBUR...Low confidence in 18z TAF. There will be intermittent MVFR
cigs through 23Z. Brief IFR conditions are possible with passing
showers and isolated TSTMS through 22-23z. Moderate confidence in
TAF aft 22Z with a chc of bkn040-050 conds.

&&

.MARINE...07/915 AM.

There is a slight chance of thunderstorms over the coastal waters
later this morning through this afternoon.

Across the outer waters... Conditions will be gale force level
through Sunday night with seas over 10 ft expected. Winds will
likely diminish to Small Craft Advisory (SCA) level Monday through
at least Wednesday.

Across the nearshore waters of the Central Coast... Conditions
will be SCA level through Sunday night. Winds could be borderline
gale force in western portions of the East Santa Barbara Channel
and far western areas south of Point Mugu to San Mateo Point this
afternoon before weakening overnight. Conditions will likely
diminish to below SCA level Monday through Wednesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 4 PM PST Monday for zones
      34-35. (See LAXCFWLOX).
     High Surf Advisory in effect until 4 PM PST Sunday for zones
      39>41. (See LAXCFWLOX).
     Winter Weather Advisory in effect until 10 PM PST Sunday for
      zones 52>54. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Monday for
      zones 645-650-655. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PST Monday for zones
      670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (MON-FRI).

Sub-freezing temperatures are possible in the coldest valleys
Monday morning.

&&

PUBLIC...Rorke/Kittell
AVIATION...Sweet/Rorke/Phillips
MARINE...Sweet/Phillips
SYNOPSIS...RK

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