FXUS66 KLOX 011709
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1009 AM PDT Sun Sep 1 2024

.SYNOPSIS...01/415 AM.

A warming trend will occur for the rest of this weekend and into
early parts of the upcoming week, before significant warming and
dangerous heat materialize for mid to late week. While the
significant heat will start as early as Tuesday for the Western
San Fernando Valley, the most dangerous heat will be on Thursday
and Friday. Areas of low clouds and patchy night through morning
dense fog will affect areas near the coast through Monday,
persisting along the Central Coast throughout the week, with clear
skies elsewhere for the remainder of the week. Precipitation is
not expected. A cooling trend is expected next weekend.

&&

.SHORT TERM (TDY-TUE)...01/751 AM.

***UPDATE***

Patchy dense fog is occurring along the Central Coast and is
likely to clear within the next few hours. After the patchy
morning clouds clear, today is expected to be sunny with
temperatures a few degrees below normal along the Central Coast,
near normal for the beaches south of Point Conception, and
several degrees above normal elsewhere. HeatRisk today will be
low-to-moderate, with higher values for away from the immediate
coast. No major changes with this forecast update.

***From Previous Discussion***

A compact upper low and surrounding trough are forecast to track
from the waters off the northern California and Oregon coasts to
the northern Rockies through Tuesday. This track will be to the
north of an upper anticyclone re-focusing from the central
Rockies and Great Basin to the Desert Southwest.

Subsidence-induced surface ridging over the east Pacific waters
upstream of the upper low will maintain offshore pressure
gradients for SBA-SMX through early in the upcoming week. Modest
Sundowners are expected to bring generally sub-advisory-level
winds to southwest Santa Barbara County. However, wind-prone
areas in the Santa Ynez range near Gaviota and Refugio could
experience occasional 45-mph wind gusts during the evening hours
through Monday, and there is a 20% chance for short-duration Wind
Advisories to become necessary.

Meanwhile, the east Pacific surface ridge will continue to favor
a modest north-northwesterly coastal jet off the Central Coast
into the upcoming week. Speed shear east of this jet across the
Southern California Bight will sustain a broad and diffuse
cyclonic eddy over the coastal waters south of Point Conception,
with multiple circulation centers. On the east flank of this
broader circulation, return flow will maintain the inland
presence of the marine layer near the coast of Los Angeles and
Ventura Counties, where night through morning low clouds are
expected through Monday, dissipating by each afternoon. However,
for the Central Coast, more prominent onshore flow along the edge
of the coastal jet will foster a stronger influx of the marine
layer. Also, patchy night through morning dense fog is expected
in the marine layer, and there is a 20% chance for localized
short-duration Dense Fog Advisories to become necessary.

The eastward progression of the aforementioned upper low and
upstream subsidence will correspond to an offshore trend in
pressure gradients early in the upcoming week. The marine layer
edge will correspondingly retreat closer to the coast, especially
south of Point Conception, which will lessen the coverage of
clouds and fog. By Tuesday, any night through morning low clouds
and fog are expected to be confined to the Central Coast vicinity,
with clear skies prevailing elsewhere. Given the offshore trend
in pressure gradients and retreating marine-layer edge,
temperatures will warm by 1-3 degrees from day to day from the
beaches to the coastal valleys. High temperatures by Tuesday will
generally range from the middle and upper 70s at the beaches, to
the 80s and 90s and locally near 100 degrees over the coastal
valleys, with highs upwards of 95 to 103 degrees over the interior
valleys. Cooler conditions will persist near the Central Coast
where onshore flow will be stronger.

There is a notable exception regarding these Tuesday temperatures,
however, and that is across the Western San Fernando Valley.
Present indications are that the greatest day-to-day warming will
be over the Western San Fernando Valley. This is a mesoscale zone
where encircling hills and mountains will locally favor greater
downslope flow associated with the offshore-trending pressure
gradients, and related adiabatic warming will locally accentuate
heating rates. By Tuesday, high temperatures around Woodland
Hills are projected to increase to around 107 degrees, bringing
Major HeatRisk to the Western San Fernando Valley as early as
Tuesday. This will mark a localized early onset of the significant
heat, which will affect a much broader portion of Southern
California later in the week. Given the strong synoptic and
mesoscale signals favoring the early onset of significant heat
across the Western San Fernando Valley on Tuesday, combined with
high confidence that some of the most significant heat impacts
occur in this zone later in the week, an Excessive Heat Watch has
been issued for the Western San Fernando Valley starting on
Tuesday. There is also a 30% chance for heat headlines to become
necessary as early as Tuesday over nearby locations from the Santa
Clarita Valley through the Eastern San Fernando Valley through
Calabasas and Agoura Hills, as well as the Santa Monicas --
especially if temperatures were to trend warmer in subsequent
forecasts over these areas.

Dry conditions are expected to prevail through the remainder of
this weekend and into early parts of the upcoming week. Diurnally-
enhanced orographic circulations into modest midlevel moisture
could yield a few high-based cumulus build-ups during the afternoon
hours each day around the San Gabriel Mountains. However, with
precipitable water over the higher terrain remaining below 1 inch,
precipitation is not expected.

.LONG TERM (WED-SAT)...01/415 AM.

By middle and latter parts of the upcoming week, the continued
re-focusing of the aforementioned upper anticyclone across the
Desert Southwest will bring rising midlevel heights over the
region. Medium-range model guidance suggests 500-mb heights rising
upwards of 595-597 dam over the area by Thursday. This will occur
after the previously mentioned marine-layer edge retreats closer
to the immediate coast while pressure gradients continue to trend
offshore. This will cause significant warming area-wide, with the
most dangerously hot conditions expected Thursday and Friday. One
exception will be near the Central Coast, where more prominent
onshore flow along the edge of the coastal jet will continue to
maintain stronger presence of the marine layer.

By Wednesday, high temperatures are expected to reach 100-105
degrees in many interior sections, and the 90s to around 103
degrees across most coastal valleys, with 70s and 80s near the
coast. Another few degrees of warming will occur going into
Thursday and Friday, bringing widespread Moderate to Major
HeatRisk -- even to locations nearing the coast, aside from the
Central Coast. Present indications are that isolated locations
could experience Extreme HeatRisk on Thursday and Friday across
Los Angeles and eastern Santa Barbara Counties away from the
immediate coast. Warm overnight low temperatures in the lower to
middle 70s will bring little relief from the heat in many
locations.

The most prolonged heatwave and most significant heat impacts are
expected to focus through late week in the Western San Fernando
Valley due to the aforementioned mesoscale effects. This is where
the Excessive Heat Watch, which starts on Tuesday, continues
through Friday. High temperatures Thursday and Friday will likely
exceed 110 degrees around Woodland Hills, and there is a 20%
chance for these temperatures to reach Top-10 readings at Pierce
College -- in excess of 115 degrees. Needless to say, this has the
potential to be a very dangerous heatwave for the Western San
Fernando Valley. Elsewhere across much of the region, there is
a greater than 95% chance for heat headlines to be issued for
Wednesday and/or Thursday through Friday in subsequent forecasts.
This includes a greater than 95% chance for Excessive Heat
headlines to become necessary in some areas.

By next weekend, model guidance suggests midlevel heights
lowering over the region as an upper trough crosses the Pacific
Coast. While model solutions vary considerably regarding the
depth, amplitude, and phasing of this disturbance, there is at
least medium confidence that the significant heat will have
concluded prior to next weekend.

Aside from persistent marine stratus and night through morning
fog along the Central Coast where onshore flow will be strongest,
most areas will experience clear skies for much of the upcoming
week. Precipitation is not forecast to occur through next
weekend, as the upper pattern does not favor any appreciable
influx of monsoonal moisture.

&&

.AVIATION...01/1708Z.

At 1615Z at KLAX, the marine inversion was 1000 feet deep. The
top of the inversion was at 4100 feet with a temp of 27 deg C.

For 18Z TAF package, high confidence in valley/desert TAFs, but
low to moderate confidence in coastal TAFs. Return of CIG/VSBY
restrictions tonight could be +/- 4 hours of current forecasts.
There is a 40% chance that CIG and/or VSBY restrictions do not
develop tonight at any and all coastal sites.

KLAX...Overall, moderate confidence in 18Z TAF. High confidence
in VFR conditions through this evening. For tonight, high
confidence in return of CIG/VSBY restrictions, but low confidence
in timing (could be +/- 4 hours of current 12Z forecast) and
flight category (equal chances of IFR or LIFR). No significant
easterly wind component is expected.

KBUR...High confidence in 18Z TAF as CAVU conditions are
anticipated through the period.

&&

.MARINE...01/737 AM.

For the Outer Waters, moderate to high confidence in current
forecast. Today through tonight, high confidence in Small Craft
Advisory (SCA) level winds. On Monday, high confidence in SCA
level winds continuing across PZZ673/676, but only a a 30% chance
of SCA level winds across PZZ670. For Tuesday through Thursday,
there is a 40-60% chance of SCA level winds across PZZ673/676 and
a 20-30% chance of SCA level winds across PZZ670.

For the Inner Waters north of Point Conception, moderate to high
confidence in current forecast. Today through Monday, there is a
20-30% chance of SCA level winds in the afternoon and evening
hours. For Tuesday through Thursday, high confidence in winds and
seas remaining below SCA levels.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Across the western half of the
Santa Barbara Channel, there is a 60-80% chance of SCA level winds
late this afternoon and evening...a 40-60% chance of SCA level
winds late Monday afternoon and evening...and a 20-30% chance of
SCA level winds Tuesday through Thursday in the late afternoon and
evening hours. For the remainder of the Inner Waters south of
Point Conception, high confidence in winds and seas remaining
below SCA levels through Thursday.

Patchy dense fog will continue to impact the coastal waters the
next few days, mainly in the night and morning hours. Visibilities
of one nautical mile or less will be possible.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Watch in effect from Wednesday morning through
      Friday evening for zones 38-343>345-348-351>353-376>378. (See LAXNPWLOX).
     Excessive Heat Watch in effect from Tuesday morning through
      Friday evening for zone 372. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect from 5 PM this afternoon to 3
      AM PDT Monday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Monday for zone
      670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen/Schoenfeld
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Cohen

weather.gov/losangeles

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