657
FXUS66 KLOX 170113
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
613 PM PDT Tue Jul 16 2024

.SYNOPSIS...16/137 PM.

Another heatwave is expected to impact Southern California for Friday
through this weekend and into next week. The most dangerous heat
is expected over the interior, though very warm conditions may
extend toward the coast. Monsoonal moisture will increase across
the region this weekend and next week, which will have the
potential to bring thunderstorms over the interior mountains and
valleys. Also, gusty Sundowner winds will affect portions of
southern Santa Barbara County during the evening and overnight
hours through Wednesday night.

&&

.SHORT TERM (TUE-FRI)...16/137 PM.

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 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 at least a couple of degrees
each day through the remainder of this week. By Friday, HeatRisk
is likely 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 100-110 degrees -- highest across the
Antelope Valley. There is at least an 80% chance that these
conditions warrant heat headlines, starting on Friday.

One element of uncertainty regarding the degree of warming, and
onset timing of most significant HeatRisk, is the influx of rich
monsoonal moisture on the western flank of the aforementioned
midlevel ridge. Present indications are that re-orientation of the
ridge aloft over greater latitudinal expanse will favor a
moisture influx from a source region deeper into the sub-tropics.
Thus, with the onset of more significant heat late in the week,
the influx of this moisture could have a tendency of limiting the
degree of surface heating -- aided by any convection or convective
debris clouds that move across the region. These factors cast
uncertainty on the onset of more significant heat.

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. Patchy night and morning dense
fog will be a possibility with the increasingly shallow marine
layer late this week. However, with marine-layer depths 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, and a few 90s near the foothills.

Regarding the potential for convection, thunderstorm chances are
currently below 20% through the end of this week. However, minor
convection-induced perturbations embedded in the long 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 late today
into tonight and again late Wednesday into Wednesday 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 waters will
favor offshore Santa Barbara-Santa Maria pressure gradients
around 2-3 mb through mid-week. Local gusts may reach 45 mph late
today into tonight, with a 20% chance for Wind Advisory issuance.
Slightly more pronounced offshore gradients late Wednesday into
Wednesday night correspond to a higher probability of Wind
Advisory issuance then (40% chance). These offshore pressure
gradients are expected to weaken by late week.

.LONG TERM (SAT-TUE)...16/137 PM.

The potentially dangerous heatwave will be ongoing this weekend
and continue into next week, as heights aloft build along the north-
south-oriented expansive upper ridge over the western states.
Heights at 500 mb are forecast to reach around 596 dam from
central California to central Nevada this weekend and into early
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 100-105 in most interior mountains, valleys, and
foothills. These conditions will likely result in Major to Extreme
Heat Risk, and there is at least an 80% chance for heat headlines
to be issued continuing into next week.

A continuing element of uncertainty will be the potential for
rich monsoonal moisture over the area to limit the degree of
heating. Such uncertainty mainly applies to the maximum degree of
warming and just how extreme this heatwave becomes. Regardless,
this will likely be a significant heatwave, and those susceptible
to heat impacts should consider preparing for dangerous heat.

In addition, closer to the coast, patchy dense fog will be a
possibility with the increasingly shallow marine layer this
weekend into next week. 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.

Present indications are that the gradual influx of rich deep-layer
moisture will continue through the weekend and next week. As
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).

&&

.AVIATION...17/0112Z.

At 00Z at KLAX, the marine layer was 800 feet deep. The top of
the inversion was at 5200 feet with a temperature of 26 deg C.

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

Cigs arrival times may be delayed 1-3 hours. Moderate confidence
in KSMO, KLAX, and KLGB. 30% chance KSMO and KLAX become IFR after
08Z. 10% chance for brief cigs at KPRB. Low confidence for KSMX,
KSBP, KSBA, KOXR, and KCMA. There is a 10% chance KSMX and KSBP
become VLIFR due to dense fog. There is a chance KOXR (30%) and
KCMA (20%) become IFR in the morning. Flight category changes may
be off +/- 3 hours.

KLAX...Moderate to low confidence in TAF. Low confidence in
cig arrival time, which may be as late as 12Z. 30% chance of
flight cat becoming IFR between 08-17Z due to cigs becoming lower
than 1000 ft. Cigs should dissipate and become VFR by 18Z at the
latest. No significant east wind component expected.

KBUR...High confidence in CAVU TAF.

&&

.MARINE...16/215 PM.

High confidence in Small Craft Advisory (SCA) conditions (winds
and steep seas) developing by Wednesday for the offshore waters
from the Central Coast to San Nicolas Islands (The Outer Waters).
High confidence for SCA conditions continuing through at least
early next week, peaking Friday and Saturday. There is a 30
percent chance of SCA conditions for the nearshore Central Coast
waters each afternoon and evening through Thursday, with chances
increase Friday and Saturday for winds and steep seas.

For the Santa Barbara Channel, while SCA winds are likely for the
far northwest portion (west of El Capitan), there is a 40 percent
chance of winds pushing through most of the western half. While
confidence is on the lower-end for the current SCA, we will keep
it going and reassess this evening. The chances look to decrease
tomorrow, but choppy seas will be increasing each day.

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 25 knots are expected in the
San Pedro Channel. Choppy seas will be building each day as well.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...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 Wednesday for
      zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM Wednesday to 6 AM
      PDT Thursday for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 6 AM PDT Thursday for
      zones 673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen
AVIATION...Phillips
MARINE...Kittell
SYNOPSIS...Cohen

weather.gov/losangeles

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