Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 202046
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
146 PM PDT Tue Oct 20 2020

.SYNOPSIS...20/145 PM.

Very quiet weather is expected through the week with overnight
through morning low clouds across the coasts and valleys.
Temperatures will cool through the week and be several degrees
below normal. Light rain or drizzle is possible later this week
into the weekend with a much deeper marine layer, mainly coastal
and valleys areas. Gusty Santa Ana winds possible early next week.

&&

.SHORT TERM (TDY-FRI)...20/118 PM.

Slow clearing marine layer today despite light offshore trends.
500 mb heights lower slightly tonight as a weak trough moves
through, however models show light offshore trends continuing one
more day. Overall probably a very similar day, perhaps a little
warmer up north with some light northeast flow in the lower levels
and a little earlier clearing but otherwise not a big departure
from today.

The cooling trend will resume Thu and continue into the weekend as
the West Coast trough deepens and a small upper low approaches
slowly from the west. Increasing onshore flow and a deepening
marine layer will result in cooler temps and slowing morning
clearing, and possibly no clearing by Friday in some areas.
Increasing afternoon sea breezes are expected across the interior
as well. Can`t rule out some spotty drizzle at times, though
better chances for that over the weekend.

.LONG TERM (SAT-TUE)...20/141 PM.

Deep marine layer pattern over the weekend as that previously
mentioned slow moving upper low is expected to be passing through
late Saturday into Sunday. Marine layer likely to push upwards of
4000` and possibly higher by Sunday. There is a little energy with
the low and the combination of that and a 7+mb onshore gradient
and the deep inversion are typically the necessary ingredients
for a widespread drizzle/light rain scenario south of Pt
Conception. Ensembles have been hinting at this for some time and
now a majority of the solutions show at least a little precip
across the coast/valley areas with the best chances across
southeastern LA County up along the foothills of the San Gabriels
Saturday night into Sunday morning.

With or without precip it`s going to be a cool, gloomy weekend
with minimal clearing at lower elevations. By Sunday afternoon
assuming the trough passes by then there should be better chance
of clearing and slightly warmer temps but still well below normal.

Sunday night and Monday we rapidly shift away from a deep marine
layer pattern to a Santa Ana as a cold trough drops out of Canada
and into the Great Basin and Rocky Mountains. This is a much more
favorable upper level pattern than we`ve seen with the earlier
events this season and likely to generate stronger and more
widespread north to northeast winds through Tuesday. The GFS is
definitely farther west with the trough than the ECMWF and would
even hint at a chance for showers in the eastern mountains Monday
though moisture currently is completely absent. So for now it just
looks like a moderate Santa Ana event, peaking Tuesday morning at
likely advisory levels and possibly warning in the mountains if
the deterministic GFS is correct.

This will be a much cooler Santa Ana wind event than we`ve seen
with much more cold advection which will be the driving force to
get stronger winds to the coast. Temps will be as warm or warmer
at the coast than in the valleys or other inland areas due to the
compressional heating effects from the winds coming down the
mountain slopes. But temps will mostly top out in the 70s. And the
dry air and longer nights in combo with the cold air from the
trough could bring near freezing temps to some of our far interior
areas like inland SLO County and the Antelope Valley as well as
many mountain areas that are protected from the northeast winds.

&&

.AVIATION...20/1910Z.

At 1744Z at KLAX, The marine layer was 1900 ft deep. The top of
the inversion was at 3900 feet with a temperature of 24 C.

Moderate confidence in the coastal and valley TAFs. Good
confidence in the desert TAFs. All coastal/valley TAFs have cleared
to VFR conditions except KOXR and KCMA which may clear later this
afternoon. Otherwise, all coastal TAF sites will see a return of
marine clouds early this evening, followed by the LA valley TAF
sites later this evening or overnight. Conditions shoud be MVFR
with the marine clouds, except the LA valleys where IFR conditions
can be expected overnight.

KLAX...Moderate confidence in the 18z TAF. There is a 30 percent
chance of marine clouds arriving plus or minus two hours from the
forecasted time. Very good confidence of no easterly wind
component over 5KT.

KBUR...Moderate confidence in the 18z TAF. There is a 30 percent
chance of marine clouds arriving plus or minus two hours from the
forecasted time.

&&

.MARINE...20/122 PM.

The Small Craft Advisory (SCA) winds that were affecting the
waters from Point Conception to San Nicolas Island are expected to
return late this afternoon or this evening to the same waters, so
the SCA remains in effect through late tonight. The winds will
become confined to the northwest portion off the Central Coast on
Wednesday. Short period northwest to west swell will be on the
rise through mid to late week.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Wednesday for
      zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (FRI-TUE).

Strong winds from a moderate Santa Ana pattern are possible late
Sunday through Tuesday across LA/Ventura Counties, leading to
elevated to brief critical fire weather conditions. Near freezing
temperatures are possible across interior areas by Tuesday
morning.

&&

$$

PUBLIC...MW
AVIATION...Sweet/Stewart
MARINE...Sweet
SYNOPSIS...MW

weather.gov/losangeles


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