Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/20/23

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 904 PM PDT Fri May 19 2023

.SYNOPSIS...19/904 PM.

Warmer temperatures are expected this weekend with increasing
thunderstorm chances in the mountains and Antelope Valley. Night
and morning low clouds and fog are expected to clear by afternoon.
A cooling trend is expected next week with temperatures falling
to 5 to 10 degrees below normal by Wednesday.

&&

.SHORT TERM (FRI-MON)...19/902 PM.

Strong onshore flow and a deep marine layer have brought low
clouds rapidly inland this evening, already reaching many of the
valley areas as of 8 pm. The current LAX-Daggett pressure gradient
is at +8 mb and ACARS data is showing the marine layer depth
around 2200 feet. As you would expect with this pattern, gusty
onshore winds continue this evening across the interior, with
gusts over 40 mph being observed at Lake Palmdale. Little change
in the marine layer pattern and temperatures through the weekend.

The main short term weather concern will remain the convective
potential through the weekend. Some residual showers and isolated
thunderstorms continue across the San Bernardino and eastern Kern
county deserts this evening. High resolution models have this
activity dissipating as it moves southwestward, but there is
a 10 percent chance of showers in the Antelope Valley. For
Saturday and Sunday, there will an influx of moisture across
our local mountains and deserts, with precipitable water values
increasing to 1.2 to 1.3 inches, which is about 200% of normal
for this time of year. 00Z NAM model cross sections showing
this moisture is more distributed through the lower and mid
levels of the atmosphere, which will increase the probability
of thunderstorms to 20-30% across the LA/Ventura county mountains
and eastern Antelope Valley. Plenty of instability forecasted
over the weekend, with lifted index values of -5 to -7 and
CAPE values in the 1500-2000 J/kg range.

If storms develop, steering flow remains under 10kt so periods of
heavy rain are possible along with gusty outflow winds, hail, and
lightning. With such weak steering flow, will have to keep an eye
on the possibility of localized flash flooding on Saturday
afternoon and again Sunday afternoon for the mountains of
LA/Ventura counties, and eastern Antelope Valleys. Given the light
northeast flow aloft there`s a 5-10 percent chance of storms
spilling over into the valleys of LA/Ventura counties. There is
also a chance that the increased moisture aloft and storm activity
will cause a significant reduction in the marine layer coverage
south of Pt Conception. These patterns are difficult to predict
but could result in a several degree increase in daytime
temperatures. Models do indicate weakening onshore flow this
weekend so it`s likely temperatures will be warming up across the
coast and valleys.

Moisture starts dropping off late Sunday into Monday but can`t
rule out an afternoon storm or two over the mountains. Otherwise,
very little change in sensible weather.

.LONG TERM (TUE-FRI)...19/151 PM.

The persistent Rex Block pattern on the west coast will break down
next week as a trough moves into the Pac NW. This will shift the
light easterly flow aloft back to to west, resulting in decreasing
moisture aloft and a more stable air mass through at least the
middle of next week (ie no thunderstorms). NAEFS pressure
gradients continue to show increasing onshore flow next week,
matching up well with the deterministic models, some of which have
the LAX-DAG gradient as high as 10mb by mid week. So confidence
is high in several degrees of cooling across inland areas and
continued (or a resumption) of May Gray conditions across the
coast/valleys. Highs expected to be 5-10 degrees below normal
(especially in the valleys).

The 12z GFS deterministic model shows an upper low dropping
through California late next week with some moisture. Ensembles
and model clusters would suggest a more conservative, but still
below normal weather pattern, however around 30 percent of the
solutions favor the cutoff low scenario so there is a possibility
of even cooler temperatures and possibly some showers by around
next Friday, mainly in the mountains.

&&

.AVIATION...19/2352Z.

At 2350Z, the marine inversion at KLAX was 2300 feet. The top of
the inversion was 3500 feet with a temperature of 19 degrees
Celsius.

Overall, moderate confidence in coastal/valley TAF sites and high
confidence in desert TAFs. MVFR cigs remaining across many coastal
locations late this afternoon. High confidence in CIG/VSBY
restrictions tonight for coastal/valley sites, but only moderate
confidence in flight categories and timing of category changes.

KLAX...Moderate confidence in 00Z TAF. MVFR cigs are now expected
to persist at KLAX through this evening, with IFR cigs likely
returning sometime around 06z, however that timing could be +/- 2
hours. During the overnight hours, there is a 30% chance of LIFR
conditions 10Z-16Z. No significant easterly wind component is
expected.

KBUR...Moderate confidence in 00Z TAF. Timing of return of
CIG/VSBY restrictions could be +/- 2 hours of current 10Z
forecast. There is a 40% chance of LIFR conditions 10Z-16Z.

&&

.MARINE...19/1255 PM.

For the Outer Waters, moderate to high confidence in current
forecast. For PZZ670/673, high confidence in Small Craft Advisory
(SCA) level winds through Saturday afternoon with an 80% chance
of the SCA level winds continuing through Sunday night. For
Monday through Wednesday, winds and seas will remain below SCA
levels for PZZ670/673. For PZZ676, high confidence in winds and
seas remaining below SCA levels through Saturday with a 50-60%
chance of SCA level winds Saturday night through Sunday night. For
Monday through Wednesday, high confidence winds and seas
remaining below SCA levels.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Overall, winds and seas are
expected to remain below SCA levels through Wednesday. However,
there is a 20-30% chance of SCA level winds Sunday afternoon
and evening.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Through Wednesday, winds and seas
are generally expected to remain below SCA levels. However, there
is a 40-50% chance of SCA level winds across the western half of
the Santa Barbara Channel Sunday afternoon and evening.

Long period south swells with short period wind chop will continue
to affect the waters through the weekend.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect until 11 PM PDT this
      evening for zones 87-354-362-366. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 4 AM PDT Sunday for
      zones 670-673. (See LAXMWWLOX).

&&

$$

PUBLIC...Gomberg/MW
AVIATION...Gomberg
MARINE...RAT
SYNOPSIS...MW

weather.gov/losangeles

Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox