163
FXUS66 KLOX 010625
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1125 PM PDT Sun Jun 30 2024

.SYNOPSIS...30/219 PM.

A significant heatwave will impact a large part of the area this
week and continue into next weekend. Temperatures will become
dangerously hot across much of the area. High temperatures by mid
to late week are expected to reach 95 to 105 degrees in many areas
away from the coast, with highs upwards of 105 to 115 over
interior valleys and foothills including the Antelope Valley.
Also, by late week, very warm to hot conditions could extend
closer to the coast. Patchy night and morning dense fog will be
possible near the coast early this week.

&&

.SHORT TERM (SUN-WED)...30/823 PM.

***UPDATE***

There was a very weak upper level trough over the area early this
evening with H5 heights around 590-591 dam. The high H5 heights
was helping to promote a strong and shallow marine inversion this
evening. The depth of the inversion was around 800 ft at both VBG
and LAX. A warm layer was noted in the soundings at around
2400-2600 feet or so with temps in the low 80s.

Skies were mostly clear across the region early this evening,
except for some low clouds and fog mainly along the Malibu coast
and portions of the L.A. County beaches. Thru the overnight
hours, low clouds are expected to expand over the SoCal Bight as
an eddy develops, with low clouds pushing into much of the coast
from L.A. county this evening to the VTU and S SBA County coast
later tonight. Low clouds are also expected to quickly develop
along the Central Coast overnight and move inland to the Santa
Ynez Vly late tonight. Patchy dense fog will likely accompany the
low clouds overnight, but there is also a chance the dense fog
could have a larger aerial extent which would lead to the
issuance of a Dense Fog Advisory. Otherwise and elsewhere, mostly
clear skies can be expected thru the night.

As a result of the aforementioned warm layer in the soundings,
low temperatures tonight in the foothills and lower mtns should
be quite mild. Temps are expected to fall only into the lower 70s
for much of this area altho a smattering of mid 70s would not be
out of the question.

***From Previous Discussion***

Midlevel heights will continue to gradually rise early this week,
as an upper-level ridge builds over Southern California. Then by
Wednesday, a strong upper-level anticyclone over the east Pacific
will begin expanding eastward over California, bringing more
significant midlevel height rises. In response to these
developments, the warming trend will continue early this week, and
will become more significant on Wednesday. Tuesday high
temperatures are expected to have warmed to 95-105 degrees in most
areas, 105-110 over the Antelope Valley, and the middle 70s to
the middle 80s near the coast. Temperatures are expected to
increase further, by another few to several degrees, for
Wednesday.

Much of the earlier Excessive Heat Watch has been upgraded to an
Excessive Heat Warning for interior areas starting on Tuesday,
with the onset of the significant heatwave. Thereafter, with the
continued warming trend into Wednesday, confidence continues to
increase that significant heat impacts will extend farther toward
the coast as the week progresses. Correspondingly, by Wednesday
across portions of the coastal ranges, foothills, and valleys of
Southern California closer to (but not including) the coast, high
temperatures are forecast to become dangerously hot -- with highs
up to 95 to 105 possible, and upwards of 105 to 110 possible over
valleys well away from the coast. An Excessive Heat Watch has
been issued starting on Wednesday for these areas -- from the San
Fernando Valley through the Santa Monicas, Calabasas, and Agoura
Hills, to the central Ventura County valleys and through the Santa
Ynez Range and Santa Lucia Range.

Local surface pressure gradients will be neutral to moderately
onshore through the middle part of this week. This overall
pattern will maintain night and morning low clouds and fog in the
marine layer near the coast and over coastal valleys, scattering
out and clearing during the afternoon. With the continued rising
heights over the region, patchy dense fog will remain possible in
the shallow marine layer over coastal areas and some coastal
valleys.

Otherwise, dry conditions are expected into middle parts of this
week. Occasionally gusty southwest to northwest winds are
expected each afternoon over interior areas. The strongest winds,
up to 25-35 mph, are expected over the Interstate-5 corridor as
well as the Antelope Valley and nearby foothills. However, the
issuance of Wind Advisories is not expected.

.LONG TERM (THU-SUN)...30/219 PM.

Dangerously hot conditions are expected to continue over the
region through the remainder of this week and into next weekend,
as the strong upper-level anticyclone continues expanding
eastward over California. Medium-range model guidance continues
to suggest 500-mb heights rising to around 597-600 dam to the
north of the area by late this week and into next weekend. This
should correspond to daily high temperatures of 95-105 degrees in
many areas away from the coast, with highs upwards of 105-115
over interior valleys and foothills including the Antelope Valley.
Warm overnight low temperatures will worsen the effects of
extreme heat, with lows remaining above 80 degrees at some
locations over interior valleys and nearby foothills. Excessive
Heat Warnings and Watches continue through late this week, next
weekend, and into early Monday morning, July 8. HeatRisk is
forecast to become Major to Extreme in most areas away from the
coast during latter parts of the week and into next weekend.

In addition, very warm to hot conditions will have the potential
to expand and bring significant heat impacts toward the coast,
especially for Thursday and Friday. This would occur as the marine
layer becomes increasingly shallow and retreats toward the coast
provided the rising heights aloft, which could also eventually
limit the chances for night and morning dense fog. Heat headlines
may need to be expanded to include a larger portion of Southern
California in later forecasts. This is especially the case for
southern Santa Barbara County, where local SBA-SMX pressure
gradients are forecast to become offshore around 1.5-2 mb for
Thursday and Friday -- behind a weak wave tracking over the
northern Rockies late this week. The mass response to the offshore
pressure gradient -- including enhanced Sundowner winds -- will
have the potential to transport the extreme heat over interior
areas toward the Santa Barbara County coast, where some model
projections are indicating 950-mb temperatures reaching upwards of
around 40C. This could bring Major to Extreme HeatRisk toward or
even right up to the immediate coast of southern Santa Barbara
County (60% chance).

Present indications are that precipitation will be unlikely
(below 20% chance) through this week and into next weekend.
However, given how hot and strongly mixed the boundary layer will
be over interior sections during latter parts of the week and into
next weekend, any low-predictability monsoonal moisture influx
south of the midlevel ridge axis could introduce chances for
showers or thunderstorms over higher terrain. Chances are too low
for mention in the forecast at this time, though this scenario
will continue to be monitored.

The combination of the significant heatwave, with dry boundary
layer conditions, and diurnally enhanced southwest to northwest
winds over interior areas, will increase the fire-weather risk
this week and into next weekend. This will also be of particular
concern in southern Santa Barbara County for Thursday into Friday,
when elevated to critical fire-weather conditions will be
possible in conjunction with the Sundowner winds. For additional
information, please reference the Fire Weather discussion below.

&&

.AVIATION...01/0625Z.

At 06Z at KLAX, the marine layer was 600 feet deep. The top of
the inversion was at 2300 feet with a temperature of 26 Celsius.

High confidence in VFR conditions for for KPRB, KWJF and KPMD
through at least Monday.

Moderate confidence in all airports that get ceilings seeing LIFR
conditions at times, low confidence on coverage and timing. VLIFR
dense FG possible anywhere, especially before 12Z. There is a
30-40 percent chance of improving flight categories after 10Z due
to an eddy focused across KLAX, KSMO, KLGB.

For KBUR and KVNY there is a 10-20 percent chance that the eddy
is strong enough for at least brief LIFR/VLIFR cigs/vsbys in the
10-15Z time frame.

KLAX...Low confidence in TAF with a moderate to strong eddy
providing a 30-40 percent chance of improving flight category
after 10Z. There is a 30 percent chance of a brief return to VFR
conds between 07Z-10Z. Return to VFR later this morning may be off
by 3 hours. High confidence in any easterly wind component
staying well below 06 knots. Note that the TAF will reflect vsby
on the ground, not at the tall tower at 300 ft agl.

KBUR...Moderate confidence in VFR conditions with a 10-20 percent
chance of LIFR cigs/vsbys between 10-15Z.

&&

.MARINE...30/812 PM.

For the Outer Waters, moderate to high confidence in current
forecast. Through Monday night, high confidence in Small Craft
Advisory (SCA) level winds from San Nicolas Island northward. For
the waters north of Point Conception (PZZ670/673), there is a
60-70% chance of Gale force winds tonight and 30-40% chance of
Gale force winds Monday afternoon and Monday night. A GALE WARNING
will remain in effect through tonight for PZZ670/673. For Tuesday
through Friday, high confidence in winds and seas remaining below
SCA levels.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Today through Monday, there is a
50-70% chance of SCA level winds in the afternoon and evening
hours. For Tuesday through Friday, high confidence in winds and
seas remaining below SCA levels.

For the Inner Waters south of Point Conception, moderate
confidence in winds staying under Small Craft Advisory (SCA)
levels as the winds are already lowering at the mid-channel buoy.
So the previous SCA was let go. There is a 20% chance of SCA
level winds Monday afternoon/evening but likely will stay very
localized. Otherwise, high confidence in winds and seas remaining
below SCA levels through Friday across the Inner Waters south of
Point Conception.

Patchy dense fog (with visibilities one nautical mile or less)
will continue to impact the coastal waters through the week. The
areal coverage of the dense fog will lessen from day to day.

&&

.FIRE WEATHER...30/429 PM.

Elevated fire weather conditions will continue across the
interior valleys, mountains, and deserts through Monday due to hot
temperatures, low humidities, and locally gusty onshore winds.

Beginning on Tuesday, and continuing into next weekend, high
pressure will build into the region from the Eastern Pacific. This
will likely bring a significant and long duration heat wave away
from the coast from Tuesday through next weekend. High
temperatures between 100 and 110 degrees will be common across the
interior during this period, with temperatures potentially rising
to between 110 and 115 degrees across the Antelope Valley and San
Luis Obispo County interior valleys during the peak of the heat
Thursday through Saturday. Overnight lows will also remain quite
warm, adding to the potential for dangerous heat mid to late week.
Widespread humidity values between 7 and 15 percent will likely
occur across the interior during this period, along with poor
humidity recoveries, especially in the mountains, foothills, and
Antelope Valley. Typical gusty onshore winds are expected across
interior sections in the afternoon and evening hours, with gusts
of 20 to 35 mph common. There is also the potential for gusty
sundowner/I5 corridor winds late Thursday into Friday night which
could bring hot and dry conditions into portions of southern Santa
Barbara county.

Given the very hot temperatures, low relative humidity, and
locally gusty winds, an extended period of elevated to brief
critical fire weather conditions are likely from Tuesday through
Sunday for valley, foothill, mountain, and desert locations.
Elevated to critical fire weather conditions may expand into
southern Santa Barbara county if the sundowner winds develop late
Thursday into Friday night. The hot and unstable conditions will
also increase the threat of large vertical plume growth for any
fires that develop across the interior. The very hot and dry
conditions will also cause a significant drop in both live and
dead fuel moisture levels this week. These drying fuels combined
with the expected fire weather conditions and increased outdoor
activities associated with the 4th of July will cause the large
fire threat to become high for areas away from the coast Tuesday
through next weekend.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning remains in effect from 11 AM Tuesday
      to 5 AM PDT Monday for zones 38-343>345-353-376>379-381>383. (See LAXNPWLOX).
     Excessive Heat Watch remains in effect from Wednesday morning
      through late Sunday night for zones
      88-342-348-351-352-356>358-369>375-380. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 6 AM PDT Monday for zone
      645. (See LAXMWWLOX).
     Gale Warning in effect until 6 AM PDT Monday for zones
      670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zone 676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen/Sirard
AVIATION...Munroe
MARINE...RAT/RK
FIRE...Gomberg
SYNOPSIS...Cohen

weather.gov/losangeles

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