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497
FXUS66 KLOX 230655
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1155 PM PDT Sat Jun 22 2024

.SYNOPSIS...22/920 PM.

Dangerously hot weather will continue away from the coast as high
pressure aloft remain in place across much of the southern United
States. There will be a gradual cooling trend over many locations
during the upcoming week, except for dangerously hot conditions
continuing across the Antelope Valley into mid-week. While mostly
dry conditions are expected, there is the possibility for a few
thunderstorms to form tonight through Monday, as moisture from the
remnants of Tropical Cyclone Alberto affects Southern California.

&&

.SHORT TERM (SAT-TUE)...
-- Changed Discussion --
22/858 PM.

Heatwave well underway across the interior with triple digit
readings common across some of the warmer valleys, lower
mountains, and Antelope Valley. Palmdale Airport reached a high
temperature of 106 degrees today. Current satellite imagery
showing a surge of low clouds and fog returning to immediate
coastal areas this evening which will likely spread a few miles
inland across portions of the coastal plain. With a low and
strong marine inversion in place, patchy dense fog will be likely
across most coastal areas tonight, with low clouds likely not
reaching the valleys. A dense fog advisory may be needed for
portions of the coast if it becomes more widespread.

Remnant mid level moisture from Tropical Cyclone Alberto is
currently focused across northern Baja this evening and is
beginning to spread into San Diego and Imperial counties. As this
mid level moisture and elevated instability tracks to the
northwest, there will be a 10-15 percent chance of thunderstorms
over LA county and the adjacent coastal waters late tonight, with
this same small threat of thunderstorms extending across much of
our forecast area through the day on Sunday. In the evening
update, have added a 10-15 percent chance of thunderstorms for
Sunday night for areas north of Point Conception. Model soundings
over San Luis Obispo County showing MUCAPE values in the 1000-1700
J/kg range during the late afternoon/night time frame on Sunday,
indicative of the elevated instability. With the lower levels of
the atmosphere still remaining fairly dry, DCAPE values are as
high as 1500 J/kg, indicative of the potential for gusty downdraft
winds. Since this appears to be a mostly elevated convection
event, the main threats with any thunderstorms that do form will
be cloud to ground lightning strikes with limited rainfall and
gusty downdraft winds. There is a smaller potential for small hail
and brief heavy downpours (mainly in the mountains). With the
recent heat continuing to dry the fuels across the interior, any
dry lightning strikes with minimal rainfall will have the
potential for new fire ignitions. Please see Fire Weather Planning
Forecast discussion for more details on the elevated fire weather
concerns across the interior through early next week.

Projected temperatures on Sunday look on track with no change in
heat products expected at this time.

*** From previous discussion ***

A large and strong midlevel anticyclone will expand westward
across the southern United States this weekend, as diffuse
vorticity from the remnants of Tropical Cyclone Alberto circulate
around the southern and then western peripheries of the
anticyclone. By Tuesday, 500-mb heights are forecast to rise to
594 dam over AZ and NM, with the accompanying ridge axis
expanding over the forecast area this weekend into early in the
upcoming week. As midlevel heights rise this weekend, temperatures
will increase to dangerously hot levels from the Antelope Valley
and nearby foothills westward to the Santa Barbara County
interior mountains. Excessive Heat Warnings have been posted in
these areas, where high temperatures are expected to reach 95-105
degrees (locally to around 108 in the Antelope Valley) producing
Major to Extreme HeatRisk. For the Antelope Valley and eastern
Antelope Valley Foothills, these conditions are expected to be
prolonged into the upcoming week, and the Excessive Heat Warning
extends through Thursday.

Elsewhere across the region away from the coast, HeatRisk will be
relatively lower compared to the Excessive Heat Warning area,
though it will still be significant in many areas. In these areas,
a Heat Advisory is in effect as potential heat impacts are
anticipated, with Moderate to Major HeatRisk. Earlier today, the
Heat Advisory was expanded toward (though not including) the coast
across Los Angeles, Ventura, Santa Barbara, and San Luis Obispo
Counties. Consistent with the latest observational data in these
areas, the diurnal erosion of the marine layer will cause its
cooling influence to retreat closer to the immediate coast each
day this weekend, resulting in a broader area of significant heat
impacts. High temperatures in the Heat Advisory area should reach
the 90-100 degree range this weekend -- across a large portion of
the region away from the coast and outside of Excessive Heat
Warning headlines.

Overall day-to-day variability in temperatures should be small
through early in the upcoming week. However, an expected increase
in midlevel moisture from the remnants of Tropical Cyclone
Alberto could be accompanied by a couple degrees of cooling day-
to-day from Sunday into Monday and Tuesday. As a result, outside
of the Antelope Valley and eastern Antelope Valley Foothills where
dangerously hot temperatures are expected to continue through
Thursday, Excessive Heat Warnings and Heat Advisories are
currently set to expire Sunday evening. However, extensions of
these headlines in time will be possible in later forecasts (up to
60% probability in some areas). Also, with the increase in
moisture tempering nocturnal radiational cooling, overnight low
temperatures will remain elevated this weekend -- 70s in many
areas -- which will have a compounding effect on heat impacts this
weekend.

For locations near the coast, outside of ongoing heat headlines,
temperatures will be relatively cooler given the influence of the
marine layer. Within the shallow marine layer, areas of nighttime
and morning dense fog are expected near the immediate coast and
nearby coastal valleys through early next week.

Gusty south to southwest winds are expected over the Interstate-5
corridor and the western Antelope Valley and nearby foothills
each afternoon and evening. While local gusts could reach 35-40
mph, Wind Advisories are not currently anticipated.

Present indications are that the aforementioned increase in
midlevel moisture accompanying the remnants of Tropical Cyclone
Alberto will still have the potential to bring a few thunderstorms
across the forecast area. Most locations will remain dry, and
chances for thunderstorms are currently very low, less than 20
percent. There is a good chance that no storms form anywhere
across the region. However, if enough moisture is able to move
into the area this weekend, isolated thunderstorms will have the
potential to form. Right now, the most-likely timing of this
activity will be tonight, though a couple storms cannot be ruled
out as late as Sunday afternoon. If thunderstorms were to form,
cloud-to-ground lightning would be the main concern, with highly
conditional potential for small hail, gusty winds, and brief
heavy downpours. By Sunday night into Monday, the thunderstorm
risk will be decreasing, as weak, positive theta-e advection aloft
decreases behind the leading edge of the elevated moisture surge.
However, a couple thunderstorms could also occur in Santa Barbara
and Ventura Counties with the lingering moisture on Monday (15%
chance).

.LONG TERM (WED-SAT)...22/222 PM.

There remains high confidence that the persistent midlevel
anticyclone over the southwest states will prolong Major to
Extreme HeatRisk across the Antelope Valley and eastern Antelope
Valley Foothills through the middle of the upcoming week. The
Excessive Heat Warning is currently in effect for these areas
through Thursday. This will occur as 500-mb heights peak around
596 dam over the southern Rockies by the middle of the upcoming
week. Meanwhile, strong heating across the Intermountain West,
combined with an approaching midlevel trough over the Pacific
Northwest, will drive strengthening onshore flow and perhaps an
increase in marine stratus near the coast. In addition to bringing
gusty southwest winds over interior sections, this will cool off
the coast and coastal valleys by a few degrees into the middle of
the upcoming week, with highs in the 70s and 80s. This will
confine significant heat impacts to interior sections
progressively farther away from the coast from day to day.
However, aside from Excessive Heat Warnings extending through
Thursday in the Antelope Valley and the eastern Antelope Valley
Foothills, additional heat headlines will be possible across
interior mountains and valleys following the expiration of heat
headlines this weekend (60% chance). By the end of the upcoming
week and into next weekend, medium-range model guidance indicates
lowering 500-mb heights corresponding to a cooling trend for
interior areas, as well. High temperatures for next Friday and
Saturday may not rise above 100 degrees over the local area.

Also, for middle to latter parts of the upcoming week, offshore
surface ridging beneath subsidence upstream of the aforementioned
midlevel trough, may favor increasing Sundowner Winds for
southern Santa Barbara County. However, the potential for wind
headlines is currently unlikely (less than 20% chance).

Present indications are for dry conditions to prevail through
middle and latter parts of the upcoming week and into next
weekend. However, the strong heating across the Intermountain West
ahead of the aforementioned trough will foster an additional
northward influx of monsoonal moisture across the Desert
Southwest. Richer tropospheric moisture is expected to reside east
of the forecast area for the upcoming week. However, given
anticipated daily increases in convection over the higher terrain
of Arizona, there is a remote potential for residual convective
debris to be advected over the local area for the middle of next
week. Very low predictability in convection-augmented vorticity
fields and moisture enhancements will still be accompanied by a
non-zero risk for showers and thunderstorms across the forecast
area, as minor midlevel impulses and moisture enhancements
circulate around the larger-scale anticyclone. However,
confidence in such activity is too limited for mention in the
forecast at this time.
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
23/0655Z.

At 0515Z at KLAX, the marine layer was 600 feet deep. The top of
the inversion was at 1800 ft with a temperature of 28 deg C.

Very good confidence in valley and desert TAFS.

Moderate confidence in all coastal TAFS. Arrival timing of low
clouds could be off by 1 hour while dissipation timing could be
off by 2 hours. Vis at sites will likely vary frequently between
1/4SM and 3/4SM through 16Z.

There is a 10-15 percent chance of SHRA or TSRA starting in LA
county late tonight, overspreading the forecast area on Sunday.

KLAX...Moderate confidence in TAF. 2SM BR BKN004 conds could last
as late as 18Z. There is a 25 percent chc BKN005 conds arriving as
early as 00Z. Vis will likely vary frequently through 16Z. There
is a 15 percent chance of SHRA or TSRA aft 12Z

KBUR...High confidence in TAF. There is a 15 percent chance of
SHRA or TSRA aft 12Z.
-- End Changed Discussion --

&&

.MARINE...22/919 PM.

High confidence in the winds and sea forecast. Low confidence in
the expected weather and possible gusty winds associated with any
thunderstorm activity.

For waters southwest through northwest of the Channel Islands and
along the Central Coast, winds and seas will remain below Small
Craft Advisory (SCA) levels through at least early Wednesday.
There is a 40-60 percent chance of SCA level winds developing
between Wednesday and Friday morning.

Inside the southern California bight, winds and seas will remain
below Small Craft Advisory (SCA) levels through at least early
Wednesday. There is a 20-40 percent chance of SCA level winds
developing between Wednesday afternoon and evening, and again
Thursday afternoon and evening.

In addition, there is a slight chance of thunderstorms over the
waters late tonight through at least Sunday afternoon, as remnant
moisture aloft from Tropical Cyclone Alberto advances into the
region. Areas of dense fog will likely continue at times through
early next week. Please see the Marine Weather Statement
(LAXMWSLOX) for information of the thunderstorm potential, as well
as the potential for dense fog.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory remains in effect until 8 PM PDT Sunday for
      zones 38-88-342>345-352-356>358-368>375-380-548. (See LAXNPWLOX).
     Excessive Heat Warning remains in effect until 8 PM PDT
      Sunday for zones 353-376>379-381. (See LAXNPWLOX).
     Excessive Heat Warning remains in effect until 8 PM PDT
      Thursday for zones 382-383. (See LAXNPWLOX).
PZ...NONE.

&&

$$

PUBLIC...Gomberg/Cohen
AVIATION...Rorke
MARINE...Hall
SYNOPSIS...Hall/Cohen

weather.gov/losangeles

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