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709
FXUS66 KLOX 220448
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
948 PM PDT Fri Jun 21 2024

.SYNOPSIS...
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21/945 PM.

A hot weather pattern will develop across the area away from the
coast over the weekend. Dangerously hot conditions are forecast
across the interior portions of Southern California this weekend.
Very hot conditions will persist into next week away from the
coast. While temperatures will not be as hot closer to the coast,
very warm temperatures are expected this weekend into early next
week for some of the coastal valleys and nearby foothills. While
mostly dry conditions are expected, there is the possibility for a
few thunderstorms to form over the region late Saturday into
Saturday night and Sunday, as moisture from the remnants of
Tropical Cyclone Alberto affects Southern California.
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&&

.SHORT TERM (FRI-MON)...
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21/944 PM.

The latest satellite imagery shows a broad upper-level ridge of
high pressure centered near the Texas-Arkansas border this
evening. This ridge will build west into Far West Texas and the
Borderland region of New Mexico through Monday. 500 mb heights are
already climbing across the region with heights at KLAX up by
about 5 decameters versus this time last night, and 1000-500 mb
thickness values up by about 7 decameters. 500 mb heights will
likely climb through the weekend. Subsiding air aloft with the
high pressure system will press down on the marine intrusion,
thin the marine layer depth, and reduce the effects of the sea
breeze each day. The latest NAM-WRF solutions indicate onshore
flow remaining in place, albeit weaker than the last several days
and weeks. While onshore flow and night through morning low clouds
and fog will continue at the coast and into the Santa Ynez Valley
each night and morning, areas outside the marine influence will
be hot through at least the weekend and likely into much of next
week.

A few tweaks were made for low clouds and fog to bring some clouds
into the Santa Ynez Valley late tonight and into Saturday morning.
Some low clouds were tweaked for the coastal areas south of Point
Conception, as well to introduce a bit more coverage along the
beaches.

Otherwise, the package is in agreeable shape, and no additional
updates are planned.

***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 Monday, 500-mb heights are forecast to rise to 593 dam over
AZ and NM, with the accompanying ridge axis overspreading the
forecast area this weekend. As the midlevel heights rise this
weekend, temperatures will increase to dangerously hot levels
from the Antelope Valley and nearby foothills and 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 next week, and the
Excessive Heat Warning extends through next Thursday.

HeatRisk will be more moderated in a surrounding area from the
eastern San Gabriels and the San Gabriel Valley through the San
Fernando Valley and the Santa Clarita Valley, and across the
Cuyama Valley and the San Luis Obispo County mountains and
interior valleys. In these areas, a Heat Advisory is in effect as
heat impacts are still anticipated, though impacts are not
expected to be as significant as in the Warning area. High
temperatures in the Heat Advisory area should reach the 90-100
degree range this weekend.

Overall day-to-day variability in temperatures should be limited
through Monday, though the increase in midlevel moisture from the
remnants of Tropical Cyclone Alberto could be accompanied by a
couple degrees of cooling from Saturday into Sunday and Monday.
As a result, outside of the Antelope Valley and eastern Antelope
Valley Foothills, Excessive Heat Warnings and Heat Advisories are
set to expire Sunday evening, though extensions cannot be ruled
out in later forecasts (20-40% chance). However, with the
increase in moisture tempering nocturnal radiational cooling,
overnight low temperatures will remain elevated this weekend --
lower to middle 70s in many areas -- potentially worsening the
magnitude of heat impacts this weekend.

Also of note, temperatures closer to the coast, away from
the ongoing heat headlines, will be relatively cooler given the
influence of the marine layer. However, the marine layer will be
shallow given the rising heights aloft, and will efficiently mix
out of the coastal valleys and retreat toward the immediate coast
each day. While uncertain, this could potentially bring hazardous
heat impacts closer to the coast, and there is a chance for areal
expansion of the Heat Advisories this weekend (30% probability).
And within the shallow marine layer, areas of nighttime and
morning dense fog will be possible near the immediate coast and
nearby coastal valleys tonight through this weekend and 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. However, 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
to the local 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
Saturday night, though a couple storms cannot be ruled out as
early as Saturday afternoon or as late as Sunday afternoon. If
thunderstorms were to form, cloud-to-ground lightning would be the
main concern, though locally small hail, gusty winds, and brief
heavy downpours cannot be ruled out -- highly conditional at this
time. Areas south of Point Conception will have the greatest
chance of experiencing this activity. 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.

.LONG TERM (TUE-FRI)...21/247 PM.

There is 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 next week. The Excessive Heat Warning is
currently in effect for these areas through next Thursday. This
will occur as 500-mb heights build to around 596 dam over the
southern Rockies by the middle of next 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 next week,
with highs in the 70s and 80s, limiting the spatial extent of
significant heat to interior sections. However, additional heat
headlines will be possible across other interior areas next week.
By the end of next week, medium-range model guidance indicates
lowering 500-mb heights corresponding to a cooling trend for
interior areas, as well.

Present indications are for dry conditions to prevail through
middle and latter parts of next week. 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 next 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 vorticity and
moisture enhancements have a non-zero risk of bringing about a
potential for showers and thunderstorms to the forecast area, as
they circulate around the larger-scale anticyclone. However,
confidence in such activity is too limited for mention in the
forecast at this time.
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&&

.AVIATION...21/2351Z.

At 23Z at KLAX, the marine layer was 1000 feet deep. The top of
the inversion was at 2500 ft with a temperature of 25 deg C.

Very good confidence in valley and desert 00Z TAFS. Moderate
confidence in coastal 00Z TAFS due to uncertainty in arrival and
category of cigs. There is a 30 percent chance of no cigs at
KSBA, KOXR, KCMA, KLAX, KSMO, and KLGB. Otherwise, cig
arrivals/departures could be off by +/- 2 hours and cig heights
could be off by 300 feet.

KLAX...Moderate confidence in 00Z TAF. There is a 30 percent
chance of no cigs at KLAX through period. If cigs do arrive, they
could be +/- 2 hours of TAF time, with a 20% chance that it will
be in MVFR category. Any east wind component will be less than 6
kt.

KBUR...High confidence in TAF.

&&

.MARINE...
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21/727 PM.

For the Outer Waters, high confidence in current forecast. Small
Craft Advisory (SCA) winds will continue through this evening.
Thereafter, SCA conditions are not expected through at least early
next week, though there is a 40% chance for SCA winds to return by
next Wednesday.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. SCA winds are likely this
evening. Thereafter, SCA conditions are not expected into next
week.

For the inner waters south of Point Conception, moderate to high
confidence in current forecast. SCA winds are not expected over
most of the area, though the western third of the Santa Barbara
Channel will have local gusts to 25 kts this evening. Otherwise,
SCA conditions are not expected into next week.

In addition, there will be some possibility for a few thunderstorms
to impact the local waters this weekend, as remnant moisture aloft
from Tropical Cyclone Alberto advances into the region. Please
reference the Marine Weather Statement (LAXMWSLOX) for additional
information regarding the thunderstorm potential, as well as the
potential for night and morning dense fog.
-- End Changed Discussion --

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory remains in effect from 10 AM Saturday to 8 PM
      PDT Sunday for zones 38-88-343>345-372-373-548. (See LAXNPWLOX).
     Excessive Heat Warning remains in effect from 10 AM Saturday
      to 8 PM PDT Sunday for zones 353-376>379-381. (See LAXNPWLOX).
     Heat Advisory now in effect from 10 AM Saturday to 8 PM PDT
      Sunday for zone 380. (See LAXNPWLOX).
     Excessive Heat Warning now in effect from 10 AM Saturday to 8
      PM PDT Thursday for zones 382-383. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Saturday for
      zones 645-670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/Cohen
AVIATION...Sirard
MARINE...Cohen/Sirard
SYNOPSIS...Hall/Cohen

weather.gov/losangeles

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