162
FXUS66 KLOX 171059
AFDLOX

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

.SYNOPSIS...17/358 AM.

A significant heatwave will impact Southern California for Friday
through this weekend and into next week. A large portion of the
Southern California mountains, foothills, and valleys away from
the coast will experience dangerously hot conditions, and very
warm conditions may extend toward the coast. Monsoonal moisture
will increase across the area through next week, which may have
the potential to bring thunderstorms to the interior mountains
and valleys. Also, gusty Sundowner winds will affect portions of
southern Santa Barbara County during the evening and overnight
hours through Friday night.

&&

.SHORT TERM (TDY-FRI)...17/358 AM.

For the rest of this week, an upper trough off the Pacific
Northwest coast will be repeatedly reinforced by the progression
of a series of deep lows coming from the Gulf of Alaska -- staying
well to the north of the local area. This will have a significant
influence on the evolution of downstream sub-tropical midlevel
ridging, causing the persistent midlevel anticyclone over the
western states to build and gradually elongate northward across
the Intermountain West from the central Great Basin and sharpen
with northward extent into adjacent western Canada.

In response to these developments, 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.
Temperatures are expected to rise by around 2-5 degrees each day
through the remainder of this week. By Friday, HeatRisk is
expected to become Major for the interior mountains and valleys,
as well as the Santa Ynez Range and the Santa Monica Mountains, as
high temperatures reach 95-110 degrees -- 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 -- for the onset of
what is expected to be a prolonged and significant heatwave.

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.

Patchy night and morning dense fog and/or low clouds will be a
possibility with the shallow marine layer late this week. Also of
note, 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.

Regarding the potential for convection, thunderstorm chances are
currently below 20% through late this week. However, minor
convection-induced perturbations embedded in an elongating fetch
of monsoonal moisture aloft may have the potential to bring
isolated showers and thunderstorms by Thursday or Friday to the
San Gabriels and Antelope Valley. These chances will be re-
evaluated in subsequent forecasts as confidence grows regarding
the mesoscale details of any such atmospheric perturbations, which
are currently accompanied by very low predictability.

Through the rest of this week, winds will primarily be light to
moderate across the region and diurnally driven. However, 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 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. A Wind Advisory has been issued for the
western Santa Ynez Range and the Santa Barbara County Southwestern
Coast late this afternoon through tonight. There is a 60-80%
chance for additional Wind Advisories to become necessary for
these areas again late Thursday afternoon through Thursday night,
and again late Friday afternoon through Friday night.

.LONG TERM (SAT-TUE)...17/358 AM.

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 around 596 dam from central
California to central Nevada this weekend and into next week.
This pattern will prolong the heatwave into next week, with daily
high temperatures 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 become Major to 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.

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 be in the 80s in many coastal
valleys, with temperatures rising well into the 90s in some of
these areas -- 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. 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
(less than 20% chance).

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...17/0626Z.

At 0531Z at KLAX, the marine layer was 600 feet deep. The top of
the inversion was at 4000 feet with a temperature of 27 deg C.

High confidence in TAFs for KSBA, KBUR, KVNY, KPMD, and KWJF.

Low confidence in coastal TAFs through 18Z, then high confidence.
Flight cat changes may be off by 1-3 hours. There is a a 20%
chance that KCMA and a 30% chance KSMO, KLAX, and KOXR become IFR
12Z-17Z. There is a 40% chance that KLGB remains VFR through the
period. There is a 10% chance for brief IFR cigs at KPRB between
12Z-16Z. There is a 30% chance that KSBP remains VFR through the
period, and a 10% chance KSMX and KSBP become VLIFR due to dense
fog.

KLAX...Low confidence in TAF through 18Z, then high confidence
through 06Z, then low confidence afterward. Low confidence in cig
arrival time, which may be as late as 12Z. 30% chance of flight
cat becoming BKN006-008 after 08Z, lifting to BKN010-015 MVFR
between 13Z-17Z. Good confidence in clearing by 18Z. There is a
30% chance of BKN008-010 cigs after 09Z Thurs. No significant east
wind component expected.

KBUR...High confidence in TAF.

&&

.MARINE...17/328 AM.

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 50%
chance of SCA level winds this afternoon and evening, with a 30%
chance Thursday afternoon. Each afternoon and evening Friday
through the weekend, SCA level winds and steep seas are likely.

For the Santa Barbara Channel, less confidence in the wind
forecast today with a 40-50% chance of SCA winds across the
western portion of the Channel this afternoon through tonight,
especially nearshore the Santa Barbara south coast this evening.
Then, there is a 20-40% chance of SCA winds and choppy seas each
afternoon/evening through the weekend.

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.

&&

.BEACHES...17/109 AM.

A series of Southern Hemispheric storms will send a long-period,
South to Southwest swell (around 18-second wave periods) to the
Southern California coastal waters through the middle of this
week. Wave periods will gradually shorten to around 15 seconds by
late week. This S to SW long-period swell will bring an extended
period of elevated surf -- surf heights of 4-6 feet -- along with
powerful and dangerous rip currents (over 80% chance) through
Thursday. The area of greatest concern for these impacts will be
across beaches with highest exposure to S to SW swell directions
-- especially across the L.A. and Ventura County beaches. These
 factors warrant the issuance of a Beach Hazards Statement for the
 L.A. and Ventura County beaches.

&&

.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 from 5 PM this afternoon to 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 from 3 PM this afternoon to 3
      AM PDT Thursday for zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM this morning to 3 AM
      PDT Friday for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen
AVIATION...Lund
MARINE...Lund/Phillips
BEACHES...Phillips/Lund
SYNOPSIS...Cohen

weather.gov/losangeles

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