-- Discussion containing changed information from previous version are highlighted. --
684
FXUS66 KLOX 221942
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1242 PM PDT Sat Jun 22 2024

.SYNOPSIS...22/207 AM.

A heatwave is ongoing, with dangerously hot conditions expected
for locations away from the coast, especially the deserts and
mountains. While temperatures will peak this weekend, heat will
continue though much of next week. Overnight lows will be warm
and offer little relief for inland areas. There is a slight
chance of showers or thunderstorms late Saturday through Sunday.

&&

.SHORT TERM (TDY-TUE)...
-- Changed Discussion --
22/934 AM.

***UPDATE***

The Heat Advisory has been expanded toward (though not including)
the coast across Los Angeles, Ventura, Santa Barbara, and San
Luis Obispo Counties. This includes Downtown Los Angeles. Across
areas added to the Heat Advisory, temperatures through mid morning
have already climbed into the upper 70s to the middle 80s. Around
9AM PDT at KLAX, the marine layer depth was quite shallow -- around
600 feet. And with only modest surface pressure gradients,
confidence is high that the marine layer will remain confined to
only the immediate coast this afternoon -- even with sea-breeze
development -- allowing significant warming farther inland. As a
result, confidence in Moderate to Major HeatRisk impacts has
increased to the south and west of previous headlines, as
temperatures today climb to 90-100 degrees. Low temperatures will
remain elevated tonight owing to the influx of moisture aloft --
lows in the 70s in most areas, thereby compounding heat impacts.
Similar high temperatures are expected on Sunday, and thus the
Heat Advisory now covers a large portion of Los Angeles, Ventura,
Santa Barbara, and San Luis Obispo Counties away from the coast.
Excessive Heat Warnings also remain in effect for areas farther
away from the coast across interior locations, where weekend high
temperatures upwards of 100-108 degrees are expected -- highest
across the Antelope Valley. The forecast has been updated to
account for the expanded Heat Advisory. For additional information
on the forecast, please reference discussion sections below.

***From Previous Discussion***

The very broad region of high pressure, that has been impacting
the eastern United States, will swoop into the region from the
east today. Upper level heights will rise rapidly in response,
with 500 mb heights reaching 592 dam late tonight. Sunday and
Monday heights will be similar, as the center of the high pressure
system migrates to the west and settles over Arizona and New
Mexico.

The rapid rise in heights and the dramatically warming airmass
will allow highs today to soar to 8 to 15 degrees above normal
for all areas, except for the beaches where persistent marine
layer clouds will keep conditions cooler. Daytime highs will peak
for most areas this weekend, as relatively little change is
expected in temperatures Sunday. Highs will trend downward
slightly for Monday with an increase in onshore flow.

Only the beaches will see highs in the 70s this weekend, and the
temperature difference between the immediate coast and inland
areas will be drastic. A compounding factor for heat impacts will
be overnight lows and rising humidities. Overnight lows will be
warm and offer little relief from the heat, especially for the
deserts and mountains, where lows Saturday and Sunday night will
be near 80 degrees. Also, moisture from the remnants of Tropical
Cyclone Alberto will arrive tonight, driving humidites up, which
will increase apparent temperatures. Heat precautions and
awareness are advised for this weekend, and into next week.

Excessive Heat Warnings have been issued this weekend for the
Antelope Valley and Foothills, the San Gabriel Mountains, the
Ventura County Mountains, and the inland mountains of Santa
Barbara County. The deserts and mountains will see dangerously
hot conditions, with highs across the Antelope Valley reaching
102 to 108 degrees. For the Antelope Valley and eastern Antelope
Valley Foothills, hazardous daytime heat and warm overnight lows
are expected to continue into next week, and the Excessive Heat
Warning extends through next Thursday.

A Heat Advisory has been issued this weekend, for the Eastern San
Gabriels and the San Gabriel Valley through the San Fernando
Valley and the Santa Clarita Valley, and across the Cuyama Valley
and the San Luis Obispo County mountains and interior valleys.
High temperatures will be in the 90s up to around 102 degrees
this weekend. Heat impacts are expected to drop off for Monday,
however there is a small chance that the heat advisory may be
extended for some areas.

With very high pressure aloft and a warm airmass, the marine
layer will be shallow under a strong temperature inversion. This
compression may produce dense fog across the coastal waters, and
over the coastal plains overnight into the mornings. Overall, the
marine layer influence will be minimal, with lower onshore flow
this weekend. Expect very good clearing for all areas but the
beaches.

As the previously mentioned moisture enters the region tonight,
there will be a slight chance (< 20 percent) of thunderstorms.
The best chance will be Saturday night through Sunday, south of
Point Conception. If thunderstorms were to form, cloud- to- ground
lightning would be the main concern, though locally small hail,
gusty winds, and brief heavy downpours are possible. However the
most likely scenario will be build ups of cumulus clouds over the
mountains. By Sunday night, the thunderstorm risk will be
decreasing for most areas, except for the Santa Barbara South
Coast. The latest NAM model run indicates a increase in vorticity
on Monday over the Santa Ynez range. The PVA could interact with
lingering moisture and produce a 15-20 percent chance of a shower
or thunderstorm.

.LONG TERM (WED-SAT)...22/332 AM.

For the extended period, the GFS and ECMWF and their respective
ensemble members remain in good agreement that high pressure will
persist over the region. High pressure will likely peak on
Wednesday with 500 mb heights between 591 to 593 dam. Thursday and
Friday heights will fall to between 585 to 588, as the high
pressure centered over the desert southwest is dampened by a
trough passing through the Pacific Northwest. However looking to
next weekend, both the GFS and ECMWF indicate that very high
pressure will again build by next Sunday or Monday. This event has
the potential to surpass the current heatwave, as the ECMWF has
heights peaking at 594 to 595 dam. This hints at a possibility of
an even stronger heatwave beyond the forecast period.

Even with small changes to the upper level hieghts Tuesday
through Friday, the upper level pattern will stay relatively
constant. Additionally, strong onshore surface pressure gradients
will persist. Thus the weather pattern is expected to be similar,
with a slight cooling trend each day due to the falling hgts.
Thursday and Friday temperatures will be within 5 degrees of
normal across the region. A shallow marine layer will produce low
clouds and patchy dense fog, that will advance inland each night
through the coastal plains. Stratus will clear quickly for all
areas but the beaches, where clouds may cling throughout the day.

There is a small (< 10% chance) of thunderstorms over the
mountains Tuesday, as mositure is expected to linger in the area.
By Wednesday however, precipitable water is expected to trend
significantly downward. There is high confidence that dry SW flow
aloft will set up late Tuesday and this will end any threat of
thunderstorms.
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
22/1941Z.

At 19Z at KLAX, the marine layer was 0800 feet deep. The top of
the inversion was at 2100 ft with a temperature of 26 deg C.

Very good confidence in valley and desert TAFS.

Moderate confidence in KSBA and KOXR and low confidence for all
other coastal TAFs including KLAX. There is a 10-20 percent chance
that VFR conds prevail at KSBA and KOXR. For other coastal TAFs, there
is a 40-50 percent chance of IFR or lower cigs/vsbys between
06-16Z where VFR conds are advertised and 20-30 percent chance of
VFR conds where IFR or lower conds are advertised.

There is a 10-20 percent chance of SHRA or TSRA late tonight
through at least Sunday afternoon.

KLAX...Low confidence in TAF. There is a 40-50 percent chance of
IFR or lower cigs/vsbys between 06-16Z. No significant east wind
expected. There is a 10-20 percent chance of SHRA or TSRA late tonight
through at least Sunday afternoon.

KBUR...High confidence in TAF. There is a 10-20 percent chance of
SHRA or TSRA late tonight through at least Sunday afternoon.
-- End Changed Discussion --

&&

.MARINE...22/933 AM.

No significant changes with the morning update.

For the Outer Waters, high confidence in current forecast. Small
Craft Advisory (SCA) winds are not expected through at least
early next week, though there is a 40% chance for SCA winds to
return by next Wednesday.

For the Inner Waters north of Point Sal, high confidence in
current forecast. SCA conditions are not expected into next week.

For the inner waters south of Point Conception, high confidence
in current forecast. SCA winds are not expected through at least
early next week.

In addition, there is a slight chance of thunderstorms over the
waters south of Pt Conception this weekend, as remnant moisture
aloft from Tropical Cyclone Alberto advances into the region.
Please see the Marine Weather Statement (LAXMWSLOX) for
information of the thunderstorm potential, as well as the
potential for dense fog.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory remains in effect until 8 PM PDT Sunday for
      zones 38-88-342>345-352-356>358-368>375-380-548. (See LAXNPWLOX).
     Excessive Heat Warning remains in effect until 8 PM PDT
      Sunday for zones 353-376>379-381. (See LAXNPWLOX).
     Excessive Heat Warning remains in effect until 8 PM PDT
      Thursday for zones 382-383. (See LAXNPWLOX).
PZ...NONE.

&&

$$

PUBLIC...Cohen/Schoenfeld/Rorke
AVIATION...Munroe
MARINE...Rorke/RM
SYNOPSIS...RS

weather.gov/losangeles

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