298
FXUS66 KLOX 120459
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
959 PM PDT Tue Jun 11 2024

.SYNOPSIS...11/933 PM.

Low clouds and patchy night to morning fog are expected through
late this week over the coast and coastal valleys. This will
bring highs in the 60s near the coast to 80s and 90s over the
interior. Gusty southwest winds will affect the Antelope Valley
through late this week. Winds will shift to northwest to north
Friday night into the weekend, with moderate to locally strong
winds over the higher terrain. Warming temperatures are expected
this weekend, with fairly good clearing to the coast.

&&

.SHORT TERM (TUE-FRI)...11/959 PM.

The latest water vapor imagery shows an upper-level cutoff trough
of low pressure sitting near 29N and 122W this evening. This
trough, about 400 miles south-southwest of Los Angeles, will
remain nearly stationary through early Wednesday morning, then the
trough will begin to lift out to the northeast. The trough center
will move over southern California between Thursday and Thursday
night.

Onshore flow will strengthen through Thursday as the trough
moves closer to the region. A cooling trend with a deepening
marine layer depth will likely occur through at least Wednesday,
and possibly into Thursday, depending upon the exact movement of
the trough center. Earlier solutions brought the trough across Los
Angeles County, while the latest solutions take it a bit farther
to the south near Orange-San Diego County line. NAM BUFR and
local 3-km WRF time height sections indicate the marine layer
depth deepening substantially tonight and into Wednesday morning.
Both solutions suggest the marine layer depth deepening to between
2000 and 2500 feet deep at KLAX, and to between 1500 and 2000
feet for KSMX. Given that low clouds and fog are already well-
entrenched and making an aggressive evening surge into the San
Fernando Valley this evening, it is hard to argue with the NAM
BUFR and local 3-km WRF solutions stance. In the upcoming update,
the forecast deepens the marine layer depth and introduces clouds
into the Santa Clarita Valley.

With instability with trough moving north, night through morning
drizzle cannot be ruled out. Local 3-km WRF solutions add some
weak lift through the mixed layer and both NAM BUFR and local 3-km
WRF time height sections indicate a favorable flow pattern through
the depth of the marine layer for drizzle to develop at KLAX. In
the upcoming update, PoPs were increased and mention of drizzle
was added for areas south of Point Conception to the overnight and
early morning hours.

***From Previous Discussion***

Water vapor loops show that the upper low has moved well south-
southwest of the region, now centered off the coast of Baja
California. From tonight through Wednesday night, the upper low
will gradually edge northward before turning more eastward into
Thursday, spreading weak height falls over the region.
Correspondingly, the marine layer will lift and deepen to some
extent through Wednesday night, causing an increase in the
coverage of low clouds along with night/morning fog over the
coasts and coastal valleys, extending toward some of the nearby
foothills. The potential for drizzle will accompany the marine
layer Wednesday night into Thursday morning, when midlevel height
falls maximize -- albeit still remaining modest in magnitude.

With marine stratus likely remaining entrenched over coastal
locations, and only partially clearing elsewhere across the
coastal valleys and nearby foothills each afternoon, there will be
a large range in high temperatures between the coast and interior
sections each day through Thursday. These temperatures will range
from the middle and upper 60s over the coast, to the 70s over
coastal valleys, to the 80s and 90s over the foothills and
interior valleys. However, a cooling trend by around 5 degrees is
expected across interior locations from Wednesday into Thursday,
in response to cooling aloft ahead of the approaching upper low.

Later on Thursday, the center of the upper low is expected to
come in closest proximity to the forecast area along its eastward
track south of the area. This is when cooling aloft will have the
greatest chance to erode the top of the inversion surmounting the
marine layer, which could disrupt the structure of the marine-
layer thermal profile and clear out low stratus over the coasts
and coastal valleys. If this were to occur, greater diurnal
heating over coastal areas and coastal valleys on Thursday could
result in warmer temperatures than currently anticipated --
specifically around a 30% chance of high temperatures reaching
into the upper 70s to lower 80s close to the coast. However,
continued significant onshore pressure gradients limit confidence
in both substantial clearing of marine stratus and significantly
warmer temperatures materializing.

Then on Friday, the upper low will accelerate east-northeastward
toward the central Rockies, with midlevel heights rebounding in
its wake. The marine layer will likely remain entrenched at the
coasts and coastal valleys, as the inversion aloft is reinforced
by rebounding midlevel heights, and onshore pressure gradients
continue. Thus, temperature gradients across the forecast area
will once again increase -- ranging from the middle 60s close to
the coast, to near 100 degrees in the Antelope Valley.

Regarding other weather elements, aside from a few areas of
afternoon cumulus build-ups over the higher terrain, midlevel
tropospheric conditions will be too dry for precipitation
development through the period. The onshore pressure gradients
will maximize during the afternoon hours, when wind gusts --
generally from the southwest to west-southwest -- could reach
30-40 mph over interior areas, perhaps locally to 45 mph over the
Antelope Valley. However, without more appreciable upper support,
wind headlines are unlikely at this time through Friday (less
than 20% chance).

Late Friday will feature the onset of a significant change in the
wind pattern across the area, which will also usher in warmer
conditions and clearing skies all the way to the coast for this
weekend. Also late this week, the southern periphery of enhanced
cyclonic flow aloft, south of a compact upper low approaching the
Pacific Northwest coast, will begin to graze the region. Surface
high pressure, resulting from subsidence upstream of the upper
low, will build north of the area causing SBA-SMX and SBA-BFL
pressure gradients to transition decidedly offshore Friday night,
specifically around 4.0-4.5 mb offshore gradients. Winds will
become northwesterly to northerly and strengthen from the Santa
Ynez Range and adjacent foothills to the I-5 Corridor vicinity and
western Antelope Valley and Foothills. And with modest upper
support given enhanced midlevel flow, Wind Advisories will be
likely (60-80% chance), and there is a small but non-zero chance
for High Wind Watches/Warnings (20% chance) over the Santa Ynez
range and surrounding foothills. In addition, the influence of the
strengthening offshore flow will cause the marine layer to
retreat toward the coast, or even offshore particularly for the
Santa Barbara County South Coast, by late Friday night.

.LONG TERM (SAT-TUE)...11/205 PM.

Offshore flow will continue for the Santa Barbara County South
Coast vicinity through a large portion of the weekend, in
response to the enhanced offshore pressure gradients being
maintained for SBA-BFL and SBA-SMX. Moderate to locally strong
northwest to north winds will continue over the higher terrain
through at least the first part of the weekend -- enhanced by
nocturnal drainage flow during the evening, overnight, and early
morning hours. However, gusty winds will likely persist into the
daytime hours, aided by the modest upper support. The strongest
winds are expected over the Santa Ynez range and surrounding
foothills, where High Wind Watch/Warning chances are low but non-
zero (around 20% chance) into the weekend. Otherwise, at least
Wind Advisories will be likely from these areas through the I-5
Corridor and western Antelope Valley and Foothills into the
weekend.

Any night/morning marine stratus and fog are expected to clear
during the day, especially in areas where offshore flow is
strongest -- specifically Santa Barbara South Coast and vicinity.
This will correspond to a moderate to strong warming trend on
Saturday, with high temperatures reaching the middle 70s to middle
80s across many coastal locations and coastal valleys. Where
local sea-breeze circulations form -- such as the beaches around
Santa Monica Bay in LA County -- and amid a weak onshore-flow
component for SLO County and SBA County Central Coast, high
temperatures may only remain in the middle and upper 60s this
weekend.

Also of note, the combination of the enhanced Sundowner winds and
reduced humidity from downslope-flow drying could increase the
fire-weather risk this weekend especially in southern Santa
Barbara County. Please reference the Fire Weather Planning
Forecast for additional information.

Through the day Sunday, the trough surrounding the aforementioned
Pacific Coast upper low is expected to be amplifying, causing
midlevel heights to fall over the forecast area. Corresponding
surface pressure falls north of the area will cause offshore
pressure gradients to weaken, and temperatures will cool by a few
to several degrees going into Sunday -- largest cooling trends
over interior sections.

Then for early next week, the trough will remain over the area and
is expected to lose organization, as the associated upper low
moves well north of the CONUS. Offshore gradients pressure
gradients are expected to be further weakening, though light to
locally moderate northwest to north winds will continue over
many interior areas, with diurnally-enhanced onshore winds
returning to most coastal areas and vicinity. This will have the
potential to bring more widespread marine stratus and fog, along
with cooler temperatures, for coastal locations and coastal
valleys early next week.

&&

.AVIATION...12/0003Z.

At 2300Z at KLAX, the marine layer was 1700 ft deep. The top of
the inversion was at 3900 ft with a temperature of 26 C.

High confidence in VFR conds and winds for KPRB, KPMD and KWJF.

For the remainder of the sites, low to moderate confidence. The
arrival times of CIGs may differ by up to 2 hours from TAF times,
and there is lower confidence in flight categories. Generally low
IFR conds expected for the Central Coast sites, with a 30 percent
chance of LIFR conds. Further south, IFR to low MVFR conds
expected with a better chance of afternoon clearing by 21-23Z.
Low confidence in KBUR and KVNY with a 30 percent chance that VFR
conds persist through the period.

KLAX...Moderate confidence in the 00Z TAF. There is a 20-30
percent chance that conds remain in MVFR category overnight
through Wed morning. There is a 30 percent chance that CIGs do not
clear Wed afternoon. No significant easterly wind component is
expected.

KBUR...Low confidence in the 00Z TAF. There is a 30-40 percent
chance that VFR conds prevail through the period. Otherwise, lower
confidence in the onset time and dissipation time of CIGs.

&&

.MARINE...11/752 PM.

Good confidence in current forecast through Thursday afternoon
with Small Craft Advisories (SCA) not expected. Increasing winds
are expected Thursday night, with SCA winds likely through the
weekend, and Gales possible Saturday afternoon/night.

Moderate confidence the forecast for the nearshore waters along
the Central Coast. SCA conditions are likely (60 percent chance)
between Friday afternoon and Sunday. There is a chance (30 percent)
of GALE force winds Saturday afternoon through Sunday evening.
Wave heights will likely peak between 8 to 12 feet.

For the outer waters, SCA winds will likely (60-70 percent)
develop Thursday night over most of the area. GALE force winds are
possible Friday through Sunday, but most likely Saturday afternoon
and night. If GALES do not develop there will be strong SCA
conditions. An extended period of steep, short period hazardous
seas with overall wave heights of 10 to 15 feet are likely between
Friday and Sunday.

For the western Santa Barbara channel SCA winds and seas will
likely develop Friday night and continue through Sunday. There is
a 30-40 percent of a period of GALE force winds focused Saturday
afternoon and evening.

For the inner waters, including the eastern Santa Barbara
Channel, wave heights along with steep choppy seas will build
Friday and continue into the weekend with a 20-30 percent chance
of SCA winds or seas.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

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

weather.gov/losangeles

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