Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/15/21

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 854 PM PDT Fri May 14 2021

.SYNOPSIS...14/454 PM.

Overnight and morning low clouds, patchy fog, and drizzle will
continue through midweek for most coast and valley areas with
light rain possible near the foothills this weekend. Some
locations will not have much clearing until the afternoon, if at
all. Afternoon temperatures will be cooler than normal.

&&

.SHORT TERM (FRI-MON)...14/853 PM.

Upper level low pressure system moving into northern California
tonight, brining increasing cyclonic flow and lowering heights
across Southwest California. This system is helping to reinforce
strong onshore flow near the surface, as evident by the LAX-
Daggett gradient peaking at +9 mb late this afternoon. These
two factors combined will bring a deepening marine layer to the
region tonight into Saturday. Current ACARS data showing the
marine layer depth around 2000 feet this evening, with NAM-12
model cross sections showing it could deepen to between 3000 and
4000 feet by Saturday morning. Current satellite imagery already
showing low clouds quickly expanding inland into the valleys.
With further deepening of the marine layer, low clouds expected to
extend well into the coastal slopes overnight into Saturday
morning. There will also likely be a few areas that will see very
light measurable rainfall by Saturday morning, especially in LA
county.

The deeper marine layer and strong onshore flow will bring
cooler temperatures to the entire region on Saturday. There will
likely be slow to no cleaning across some of the coastal areas on
Saturday afternoon. In addition, there will be gusty onshore
winds across the mountains and deserts where wind gusts between 25
and 40 mph will be common. Isolated gusts as high as 50 mph can
be expected across the Antelope Valley foothills.

*** From previous discussion ***

A 563 dm H5 low will track from just E of Monterey County Sat
evening to the central San Joaquin Vly by early Sun morning then E
into far srn NV by late Sun afternoon. This system will help to
further deepen the marine layer Sat night into Sun morning, when
it will be a deep moist layer over 5000 ft and cover much of the
forecast area except the higher mtns and most of the Antelope
Vly. The moist layer combined with cyclonic flow aloft will bring
drizzle and a slight chance to chance of light rain to much of
the area as well Sat night into Sun morning, with a few hundredths
of an inch possible. The clouds should thin out toward the coast
by Sun afternoon, but should linger over much of L.A./VTU
Counties from the coast to coastal slopes. However, there is also
the possibility of the clouds breaking up along the coast in a
reverse clearing pattern thru the afternoon. The breezy to gusty
onshore flow pattern will continue thru Sun afternoon. Temps will
remain quite cool for mid May across the region on Sun, as much as
8-15 deg below normal.

The upper level low will continue to move slowly E Sun night into
Mon, with upper level ridging expected to build just off the
coast by late Mon. The marine layer is expected to lower some but
widespread night and morning low clouds and fog can still be
expected for the coast, vlys and up to the lower coastal slopes.
Patchy drizzle also cannot be ruled out for the coast and vlys S
of Point Conception. The low clouds should clear back toward the
coast by afternoon, with mostly sunny skies for many inland
sections. Another round of breezy to gusty SW-W winds can be
expected Mon afternoon. Temps should turn a little warmer but
remain 4-10 deg below normal for most areas. The warmest vlys and
lower mtns should reach the lower to mid 70s.

.LONG TERM (TUE-FRI)...14/217 PM.

Upper level ridging with H5 heights up to 578-580 dm can be
expected on Tue over swrn CA. A fast-moving shortwave upper level
trof should move over the region on Wed, followed by a broad upper
level trof over CA on Thu, altho the EC is a bit stronger with
this system. The deterministic EC even brings in some rain to
SLO/SBA Counties on Thu while the mean ensembles remain dry.
There is additional uncertainty Fri, with the GFS building flat
upper level ridging into srn CA, while the EC hangs onto the upper
level trof. Decided to lean more toward the GFS during this fcst
period, especially for Thu and Fri.

The marine layer pattern will likely continue Tue thru Wed, with
plenty of night and morning low clouds and fog for the coast and
vlys, altho the S SBA County coast may stay free of the low clouds
Tue night into Wed morning due to northerly flow off the Santa
Ynez mtns. Northerly flow should help to clear out most of the low
clouds Wed night into Thu, except for the southern L.A. County
coast, then no low clouds are expected Thu night into Fri as
northerly flow persists. Otherwise, mostly clear skies for the
most part can be expected across the region Tue thru Fri. Temps
are forecast to remain largely several degrees below seasonal
norms thru the extended period, altho Tue and Fri should be the
warmest days with some inland areas near normal to perhaps even a
few degrees above normal.

&&

.AVIATION...15/0014Z.

At 2330z at KLAX... The inversion was around 1400 feet. The top
of the inversion was around 4600 feet with a temperature of about
18 degrees Celsius.

Overall... Moderate to high confidence in the current TAFs.
IFR/MVFR conditions will continue to linger at some locations this
afternoon and some will not clear or may lift above 3000 feet.
Most locations that achieve VFR will return to IFR/MVFR after 02z
with KPRB after 10z. There will be a similar stratus pattern
tonight with areas of drizzle and possible reduced VSBY 11z-17z
and clearing locally delayed until 20z-22z, if at all. KPMD and
KWJF will remain VFR throughout the forecast period.

KLAX... Moderate confidence in the current TAF. IFR/MVFR
conditions will return about 02z and linger until about 21z. There
will be periods of drizzle and possible reduced VSBY 11z-17z.
There will not be east winds greater than 8 kts during the
forecast period.

KBUR... Moderate confidence in the current TAF. IFR/MVFR
conditions will return about 04z and linger until about 20z.
There will be periods of drizzle and possible reduced VSBY 11z-
17z.

&&

.MARINE...14/823 PM.

Patchy fog and drizzle will continue across the coastal waters
through the weekend. There is a chance of measurable light rain
showers Saturday night into Sunday morning.

Across the outer waters... Small Craft Advisory (SCA) level winds
across the northern and central outer waters through late tonight.
The winds will likely reach SCA level across all of the outer
waters Monday through Wednesday. Otherwise and elsewhere,
conditions will be below SCA level.

Across the nearshore waters of the Central Coast... Winds will
likely reach SCA levels on Tuesday and Wednesday. Otherwise
conditions will be below SCA level.

Across the inner waters south of Point Conception... Winds near
Point Conception and western portions of the Santa Barbara Channel
will likely reach SCA level Monday through Wednesday. Otherwise
and elsewhere, conditions will remain below SCA level.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Saturday for
      zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (MON-FRI).

No significant hazards expected.

&&

$$

PUBLIC...Gomberg/Sirard
AVIATION...Kj
MARINE...Gomberg/Kj
SYNOPSIS...Kj

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