845
FXUS66 KLOX 172337
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
437 PM PDT Wed Jul 17 2024

Updated Aviation Section

.SYNOPSIS...17/126 PM.

A significant heatwave will impact Southern California through
at least this weekend. A large portion of the mountains,
foothills, and valleys away from the coast will experience
dangerously hot conditions, and very warm conditions may extend
toward the coast. Gusty Sundowner winds will affect portions of southern
Santa Barbara County through Friday night.  Monsoonal moisture
will potentially bring thunderstorms to the interior mountains and
valleys over the weekend.

&&

.SHORT TERM (TDY-SAT)...17/302 PM.

Synoptically, there is a 557 dam Low in the Gulf of Alaska area
that will drop periodic troughs along the west coast, but not as
far south as SoCal. There is a strong (594 dam) upper level High
over the four-corners area which will build slightly Thursday, and
Friday. On Saturday this feature will drop back down to about 594
dam and begin a slow transit westward until it is centered
roughly over central California by Sunday where it will remain and
build (~598 dam) through next Wednesday.

Temperatures are down a little along south coast today as we saw
more cloud cover than than yesterday with the monsoonal moisture
from our southeast. Temperatures rose 2-5 degrees at higher
elevations and in the interior kicking off the return of a long
duration heat event that will impact SoCal through at least the
weekend. Heat will continue to build over interior areas for the
remainder of this week, with related impacts becoming increasingly
significant by the end of this week.

A small area of enhanced flow will be focused across Sundowner-
wind-favored locations in southwest Santa Barbara County each
late afternoon to overnight hours from today through Friday
night. Areas affected include the western Santa Ynez Range and
western parts of the Santa Barbara County South Coast. Despite
lacking upper support, enhanced surface ridging over the Pacific
coastal waters fostering the aforementioned coastal jet, combined
with modestly favorable offshore pressure gradients for Santa
Barbara-Santa Maria around 2-3 mb, should support 45-50 mph
Sundowner wind gusts at times through Friday night.

Thursday temperatures should be up 2-5 degrees. An increasingly shallow
marine layer closer to the coast will limit the degree of warming
there, with temperatures at the beaches only expected to
gradually rise through the 70s from day-to-day through the rest
of this week. However, with marine-layer depths generally
decreasing in response to rising heights aloft, the spatial extent
of the building heat will increase from day to day, with
temperatures by Friday expected to rise well into the 80s in many
coastal valleys, with a few 90s around the foothills.

Through the remainder of this week, the surface pressure pattern
will become increasingly favorable for the development of a
cyclonic eddy over the inner coastal waters of the Southern
California Bight -- flanking a pronounced coastal jet. Southerly
return flow on the eastern side of the eddy will have the
potential to reinforce the marine layer and related low clouds,
fog, and even drizzle for locations near the immediate coast south
of Point Conception -- where considerably cooler conditions may
exist in proximity to local beaches. There is a 60-80% chance
for additional Sundowner Wind Advisories again late Thursday
afternoon through Thursday night.

Friday temperatures will be up another 2-5 degrees and the HeatRisk
category is expected to increase to increase to "Major" for the
interior mountains and valleys, the Santa Ynez Range, and the
Santa Monica Mountains as high temperatures reach 95-110 degrees.
Temps will be highest across the Antelope Valley. An Excessive
Heat Watch has been issued for a large portion of the Southern
California mountains, foothills, and valleys away from the coast
starting Friday. Patchy night and morning dense fog and/or low
clouds will be a possibility with the shallow marine layer late
this week. There is a 60-80% chance for additional Sundowner Wind
Advisories again late Friday afternoon through Friday night.

Saturday will bring the highest (10-20%) chance of showers in the
short-term as monsoonal moisture aloft may have the potential to
bring isolated showers and thunderstorms to the San Gabriels and
Antelope Valley. Onshore flow could help bring the temperatures
down a little, but it will not be a permanent reprieve from the
heat.

.LONG TERM (SUN-WED)...17/258 PM.

The Excessive Heat Watch across a large portion of the Southern
California mountains, foothills, and valleys away from the coast
continues through this weekend and into next week, as heights
aloft are forecast to build along the north-south-oriented
expansive upper ridge across the western states. Heights at the
500-mb level are forecast to reach or exceed 594 dam from central
California to central Nevada this weekend and into next week.
Daily high temperatures are forecast to reach or slightly exceed
110 degrees across the Antelope Valley starting this weekend,
while reaching 95-110 degrees elsewhere across the Excessive Heat
Watch area, where the HeatRisk will range between Major and
Extreme. Warm overnight low temperatures generally in the 70s, to
the lower 80s in the Antelope Valley, will have the potential to
worsen heat impacts.  By Wednesday temperatures should peak.

Closer to the coast, patchy dense fog and low clouds will be a
possibility with the increasingly shallow marine layer this
weekend into next week. However, similar to late this week, the
areal extent of significant heat will continue increasing this
weekend into next week. High temperatures each day this weekend
into next week are expected to rise into the 90s in many coastal
valleys, with temperatures around 100 degrees in the warmest
valleys especially closer to the foothills. For the immediate
coast and especially south of Point Conception, significantly
cooler conditions, with more persistent marine stratus and fog,
are expected through this weekend, perhaps aided by the
circulation around an eddy over the inner coastal waters.

Present indications are that the gradual influx of rich deep-
layer moisture will continue through the weekend and next week. As
monsoonal moisture continues to stream into the region from the
deeper sub-tropics, precipitable water values may be exceeding 1
inch in most areas by early next week -- perhaps upwards of
1.25-1.50 inches in Los Angeles and Ventura Counties on Tuesday.
While the overall upper pattern may not be particularly conducive
for significant coverage of thunderstorms, the combination of the
substantially increased moisture and ample heating resulting in
terrain-driven ascent will have the potential to conditionally
favor strong thunderstorms and locally heavy rains. Confidence is
very low in monsoonal thunderstorms developing, and forecast
chances for such activity are presently less than 20%. Regardless,
the low-predictability potential for increased thunderstorm
impacts does exist from this weekend into next week over the
higher terrain and interior valleys of Los Angeles and Ventura
Counties -- perhaps even extending into Santa Barbara County by
early next week. The conditional potential for strong, erratic
outflow winds causing rapid spread rates of fires, lightning-
induced fire ignitions, and flash flooding from localized intense
rainfall rates will exist -- if thunderstorms were to develop.

While the influx of the monsoonal moisture brings some
uncertainty regarding how extreme the heatwave will become, this
will be a significant heatwave accompanied by Major to Extreme
HeatRisk. Even if the monsoonal moisture overspreading the area
were to modestly limit the degree of heating, the coverage of
related convection and convective debris are not expected to be
high enough to substantially inhibit the degree of heating --
without even richer deep-layer moisture in place. As such, even if
the resident moisture were to have a modest effect of limiting
daily high temperatures, such effect will be small compared to the
boost in apparent temperatures and overnight low temperatures that
will come with with the added moisture content. This should
facilitate prolonged Major to Extreme HeatRisk regardless of the
monsoonal moisture influx. Those susceptible to heat impacts
should consider preparing for a dangerous heatwave.

&&

.AVIATION...17/2336Z.

At 23Z over KLAX, the marine layer was 900 feet deep. The top of
the inversion was at 3700 feet with a temperature of 29 Celsius.

High confidence in VFR conditions through Thursday over KBUR,
KVNY, KPMD, and KWJF.

There is a 10-20 percent chance of -TSRA at KPMD KWJF Thursday
20-24Z. There is a 5-10 percent chance of -TSRA Thursday 12-18Z
for KLGB KLAX KSMO KBUR KVNY.

LIFR-IFR ceilings possible tonight at KPRB (30%) KSBP (70%) KSMX
(80%) KSBA (20%). IFR-MVFR ceilings possible tonight at KOXR
(30%) KCMA (30%) KSMO (40%) KLAX (50%) KLGB (60%). Low confidence
on timing (+/- 5 hours).

KLAX...50% chance of BKN008-012 ceilings tonight, starting as
early as 06Z and as late as 15Z. If ceilings form, moderate
confidence of VFR by 17Z. There is a 10% chance of -TSRA with
brief erratic winds 12-18Z Thursday over the airport, with
localized -TSRA likely over the nearby mountains after 20Z.
Moderate confidence that any southeast winds will stay under 08
knots.

KBUR...High confidence in VFR conditions through Thursday with
seasonal winds. There is a 5% chance of -TSRA with brief erratic
winds 12-18Z Thursday.

&&

.MARINE...17/301 PM.

For the outer waters, an extended period of Small Craft Advisory
(SCA) level conditions are expected at least through the weekend,
and possibly through the early portion of next week. SCA winds
will overspread the outer waters this morning, and continue at or
above advisory levels through the weekend. Winds may briefly drop
below SCA levels Thursday morning, but will restrengthen to
advisory levels by the afternoon. Winds look to peak this weekend,
with a 20% chance of Gale Force gusts Saturday evening through
night and possibly Sunday. Seas will become steep and hazardous
over the weekend, as early as Friday afternoon, and continue at
SCA levels through at least Monday evening.

For the inner waters along the Central Coast, there is a 30%
chance of SCA level winds and or seas each afternoon and evening
through the weekend.

For the Santa Barbara Channel, SCA winds will form over the
western portion by this evening, with steep seas through most of
the channel. There is a 40% change that it will be widespread
enough to warrant a Small Craft Advisory tonight, and a smaller
chance on Thursday.

For the nearshore waters off of Los Angeles and Orange Counties,
the chance for SCA conditions is less than 20 percent through most
of the week, although local gusts to 20-25 knots are possible in
the San Pedro Channel, mainly during the afternoon to evening
hours Friday and Saturday. Choppy seas will be building each day
as well, but areas away from the mainland coast.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Watch in effect from Friday morning through
      Wednesday evening for zones
      38-88-342>345-351>353-358-369>372-374>383. (See LAXNPWLOX).
     Wind Advisory in effect until 5 AM PDT Thursday for zones
      349-351. (See LAXNPWLOX).
     Beach Hazards Statement in effect through Thursday evening
      for zones 354-362-366. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for
      zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for
      zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen/jld
AVIATION...Kittell
MARINE...RK/Lund
SYNOPSIS...jld

weather.gov/losangeles

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