Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/05/19

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 825 PM PDT Sat May 4 2019

.SYNOPSIS...04/643 PM.

A low pressure system approaching the area will deepen the marine
layer tonight, bringing low clouds into the coasts and valleys.
Clouds are expected to remain over the area Sunday and Monday
along with a chance of showers. Tuesday and Wednesday will be dry,
and more showers are possible by Friday. Temperatures will remain
cool through the period.

&&

.SHORT TERM (SAT-TUE)...04/825 PM.

Closed upper level low pressure system centered around 400 miles
west of Point Conception this evening. This approaching upper
level low pressure system combined with strong pressure gradients
(LAX-Daggett peaking at +7.3 mb this afternoon) has resulted in
a deepening of the marine layer and some cooling across the
region. ACARS data showing the marine layer depth around 2000 feet
this evening across the LA basin, but is expected to deepen to
around 4000 feet by Sunday morning. Satellite imagery showing
low clouds surging in quickly this evening across coastal areas,
and is expected to spread rapidly inland overnight, reaching the
lower coastal slopes by Sunday morning. This rapid deepening
of the marine layer should bring areas of drizzle for areas
of south Point Conception overnight into Sunday morning,
especially in the foothills and coastal slopes. In addition,
temperatures will be much cooler on Sunday.

A weak frontal band is establishing ahead of the low pressure
system. This system will bring a threat of rain showers, mainly
north of Point Conception and the adjacent coastal waters on
Sunday. While typical convective parameters (Lifted Index and CAPE
values) are not overly impressive with the system, 500 mb
temperatures at the core of the low pressure system are as low as
-23 degrees Celsius which is unseasonably cold for an early May
system. With this cold air aloft, a high May sun angle, and some
jet dynamics, have included a slight chance of thunderstorms on
SUnday for areas north of Point Conception as well as the
adjacent coastal waters. The upper low wobbles around to our
north on Sunday night and is then expected to track southeastward
across our forecast area on Monday. While the exact track of the
upper low for Monday is still in question, the models generally
bring the low center and the associated cold air aloft into our
area on Monday, bringing a threat of convective showers,
especially in the mountains. Later shifts may need to include
a slight chance of thunderstorms over portions of the mountains
on Monday afternoon as latest NAM model trending more unstable.

*** From previous discussion ***

Some residual moisture with a broad northerly flow aloft will
continue a slight chance to chance of showers over VTU/L.A. Counties
later Mon night. By early Tue it looks like some drier air will move
into the area with any leftover showers moving E of the region.
However, lingering low level moisture with onshore flow will keep
skies partly to mostly cloudy thru the day on Tue.

Rainfall amounts for Sun thru Mon night will be highly variable due
to the showery nature of the weather system. For now it appears less
than 0.25 inch can be expected with most areas receiving just a few
hundredths of an inch. Snow levels should drop to as low as 6000
feet by Mon morning, with an isolated inch or two of snow not out of
the question for the higher mountain elevations.

Plenty of clouds will help to bring cooler temps to the area Sun
thru Tue, with highs at least 5 to 10 deg below normal for many
areas. The warmest vlys and inland coastal areas on Sun should be in
the upper 60s to low 70s, then cool further to the mid to upper 60s
on Mon before rebounding slightly to the upper 60s to low 70s for
Tue.

.LONG TERM (WED-SAT)...04/233 PM.

The 12Z GFS and EC are in pretty good agreement with the extended
forecast. A upper level trof is expected to move into srn CA from
the N on Wed, then another upper level trof should develop over
central CA Thu and strengthen into an upper level low near Point
Conception by early Fri. This system should drift off to the SE Fri
night and be near the lower Colorado River Vly on Sat.

A few showers will be possible over extreme eastern L.A. County
early Wed, otherwise dry conditions with partly cloudy skies should
prevail thru Wed evening. A slight chance of showers is expected for
VTU/L.A. Counties later Wed night then spread into the rest of the
forecast area on Thu, with a better chance of showers over the mtns
Thu afternoon. The upper level low will continue to bring a slight
chance to chance of showers over most of the forecast area Thu night
thru Sat as well, with the best chance of showers expected in the
mtns Fri afternoon and evening. There could also be a few
thunderstorms with this system, but being this far out there is a
fair amount of uncertainty so will keep thunderstorms out of the
fcst for now pending further model runs.

Temps are expected to remain several degrees below normal across the
region during the extended period. Highs in the warmest vlys and
inland coastal areas should be in the mid 60s to around 70 Wed and
Thu, in the upper 60s to low 70s Fri, and in the 70s on Sat.

&&

.AVIATION...04/2350Z.

At 2340Z, the marine inversion was around 2000 ft deep at KLAX.
The top of the inversion was around 4200 ft with a temperature
around 16 degrees C.

An approaching upper level low pressure system combined with
strengthening onshore flow will continue to deepen the marine
layer to around 4000 feet by Sunday morning. There will be
a chance of showers for areas north of Point Conception on
Sunday, with areas of drizzle for areas south of Point Conception
on Sunday morning. Ceilings tonight into Sunday morning are
generally expected to remain in MVFR category, but could see
brief IFR conditions for KSBP, KPRB, and KSMX tonight. Increasing
onshore winds at KPMD and KWJF on Sunday, with a 20 percent chance
of gusts reaching 35 knots by the afternoon hours.

KLAX...Moderate confidence in the 00z TAFs. Mostly expecting MVFR
cigs tonight into Sunday morning, with a 30 percent chance of cigs
lingering into Sunday afternoon. There will be the potential for
drizzle and S-SE winds of 5-7 knots on Sunday morning.

KBUR...Moderate confidence in the 00z TAFs. Mostly expecting MVFR
cigs overnight into Sunday morning, with a 20 percent chance of
brief IFR conditions tonight. There is the potential for drizzle
on Sunday morning.

&&

.MARINE...04/751 PM.

For the Outer Waters...Moderate to high confidence in the current
forecast. Winds and seas should remain under Small Craft Advisory
(SCA) thresholds through Wednesday. However, there is a 30%
chance of occasional SCA-level gusts around the Channel Islands
through late this evening.

For the Inner Waters N and S of Point Conception...Generally high
confidence in the current forecast. Winds and seas should remain
under SCA thresholds through Thursday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

No significant hazards expected.

&&

$$

PUBLIC...Gomberg/Sirard
AVIATION...Gomberg
MARINE...Smith
SYNOPSIS...Stewart

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