Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/28/19

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 808 PM PDT Fri Sep 27 2019

.SYNOPSIS...27/712 PM.

A low pressure system will approach the region from the north,
bringing a cooling trend to most areas through the weekend. There
is also a chance of drizzle or light rain during the night to
morning hours through Saturday, especially in Los Angeles County.
Temperatures will start warming up slightly by the middle of next
week as onshore flow weakens.

&&

.SHORT TERM (FRI-MON)...27/807 PM.

Satellite imagery showing low clouds rapidly advancing inland this
evening, already overspreading much of the coastal mountain
slopes. 00z Vandenberg sounding showed marine layer depth around
3500 feet this evening, and over 4000 feet across the LA Basin
as shown on the latest ACARS data. Unusually strong upper trough
for this time of year will deepen across California later tonight
into Saturday, which will help to further increase the marine
layer depth, with low clouds likely filtering into portions of the
Antelope Valley by Saturday morning. Last night, measurable rain
was mainly confined to the foothills and coastal slopes, but with
the better synoptic lift in place, there will likely be more
widespread light rain and drizzle across the coastal and valley
areas south of Pt Conception. Some areas could even see local
amounts up to 0.10 inches. In evening update, will bump up
pops/QPF for LA/Ventura counties for Saturday.

Expecting another gloomy start to the day Saturday, however
increasing northerly flow in the afternoon along with continued
cooling aloft will help scour out the low clouds across SLO/SB
Counties. Still enough onshore flow and temperature inversion
across LA/Ventura County coast/valleys to keep a fair amount of
clouds through the day. Temperatures will remain well below
normal with highs in the mid 60s to mid 70s in most areas,
including the Antelope Valley which will likely see around 10
degrees of cooling from today`s highs. Gusty southwest to west
winds will likely reach advisory levels in the Antelope Valley
and LA county mountains, where wind gusts of 45 mph will be
common. Isolated gusts up to 55 mph will be likely in the
foothills of the Antelope Valley. Will be issuing wind advisories
for these areas with the evening update, with potential for
reduced visibilities in Antelope Valley due to blowing dust. The
northerly flow developing over the western part of the forecast
area by evening may generate advisory level winds in southwestern
SB County, mainly in and around the Gaviota area.

*** From previous discussion ***

Additional cold air dropping into the trough Sunday as well as
the northerly flow spreading over the remainder of the forecast
area should scour out the remaining low clouds by Sunday afternoon
and help warm temps a couple degrees. And with no marine layer
and a fairly cool/dry air mass, overnight lows should be the
coldest we`ve seen since before summer began. Northerly winds
could again generate low end advisory level winds across srn SB
County.

The trough will remain over CA/NV through Monday maintaining
cyclonic flow but it will likely be too cold aloft to support a
marine layer so skies are expected to be mostly clear. Despite the
ample sunshine temps will stay below normal with highs still
mostly in the 60s and 70s, though warmer than Sunday with weaker
onshore flow. Northerly flow will continue as well with more low
grade Sundowners expected in SB.

.LONG TERM (TUE-FRI)...27/138 PM.

One last relatively cool day Tuesday before the trough finally
exits to the east Wednesday. Onshore flow expected to remain very
weak through the period, even turning lightly offshore Wed/Thu.
That should help warm temps back up to normal and even a few
degrees above normal in some areas for the latter half of the
week. With the trough exiting northerly flow will decrease so
Sundowners should be much weaker. Will keep skies clear through
the period given the weak onshore flow but we could start seeing
some low clouds returning to southern coastal areas towards the
end of the week. All in all a pretty quiet week expected weather-
wise.

&&

.AVIATION...28/0011Z.

At 00Z at KLAX, the marine layer was 4200 feet deep. The top of
the inversion was 5200 feet with a temperature of 17 degrees C.

Moderate confidence that MVFR/VFR CIGS will be common through
Saturday, with little or late clearing/early arrival. Low
confidence on timing of CIG changes and evolution, with ranges of
OVC015-035. High confidence in peak gusts at KPMD KWJF 30-40KT
with mountain waves possible, especially on Saturday. -DZ/-SHRA
possible late tonight through Saturday, especially LA County.

KLAX...Moderate confidence in MVFR CIGS reforming by 02-05Z. Low
confidence on when CIGS go back above VFR, with window of 09-16Z.
No significant easterly wind component expected. 30 percent chance
-DZ/-SHRA 10-18Z Saturday.

KBUR...VFR CIGS expected to transition to MVFR cigs tonight,
with VFR cigs likely occurring Saturday afternoon. 40 percent
chance -DZ/-SHRA 10-18Z Saturday.

&&

.MARINE...27/714 PM.

High confidence in conditions staying under Small Craft Advisory
(SCA) though tonight. High confidence in west to northwest winds
increasing quickly everywhere on Saturday, with at least SCA
likely over portions of all zones by Saturday afternoon or
evening. This includes the western portions of the Santa Barbara
Channel and the other inner waters. There is a 60 percent chance
for low-end Gale Force winds 20 miles offshore of San Luis Obispo
County, with a 30 percent chance of the remaining outer waters
reaching Gale Force. All areas will see very choppy and building
seas by Saturday evening, even areas that do not get that windy.
Similar but slightly weaker conditions are expected on Sunday and
Monday, except with the stronger winds shifting focus from off the
Central Coast down to the Channel Islands and Point Conception
where there is a 20 percent chance for low-end Gales.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect from 3 PM Saturday to 3 AM PDT
      Sunday for zones 645-650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 AM to noon PDT Saturday
      for zone 670. (See LAXMWWLOX).
     Gale Warning in effect from noon Saturday to 3 AM PDT Sunday
      for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM Saturday to 3 AM PDT
      Tuesday for zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (MON-FRI).

No significant hazards expected.

&&

$$

PUBLIC...Gomberg/MW
AVIATION...Gomberg/Kittell
MARINE...Kittell/Gomberg
SYNOPSIS...Smith

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