861
FXUS66 KLOX 230721
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1221 AM PDT Thu May 23 2024

.SYNOPSIS...22/919 PM.

Strong onshore flow and a deep marine layer with a passing upper-
level trough will continue to keep low clouds and fog spreading
into coastal slopes of the mountains the next couple of days.
Night through morning drizzle will be possible the next couple of
night and mornings. Clouds will struggle to clear from the beaches
each day through Saturday. A warming trend will establish over
the weekend as high pressure aloft builds into the region. Better
clearing and near normal temperatures are expected by the middle
of next week.

&&

.SHORT TERM (THU-SUN)...22/953 PM.

Low clouds are advancing rapidly inland this evening as strong
onshore flow deepens the marine intrusion. The onshore push,
brought to us by broad upper-level trough aloft, will continue to
tighten through Friday. A shortwave trough will move south over
the region into Thursday, then a stronger but moisture-starved
will drop down out of the Gulf of Alaska through Friday. A cooling
trend will continue into Friday away from the coast. Coastal
locales will likely remain near persistence over the next couple
of days.

Low clouds and fog will be staple of the forecast through at least
Friday or Saturday. With the strong onshore pressure gradients
between 8 and 10 mb over the next several days, clouds will very
likely struggle to clear away from the land mass each afternoon.
There is a slight chance that enough cold air advection could
sneak in to bring better clearing north of Point Conception, but
local studies suggest the strong pressure gradients will likely
win out and keep a cloudier forecast in play.

The instability with the shortwave trough could lift the marine
layer deck and squeeze out some drizzle tonight and into Thursday
morning. EPS ensemble members and high-resolution multi-model
ensemble members do have some solutions with drizzle or very light
rain developing at some sites through Thursday morning. As the
stronger trough approaches, there is some concern that the drizzle
or light rain on Thursday night and Friday morning could turn into
light rain across the eastern Los Angeles County. PoPs may be
need to increased to account for this. The previous forecast did
introduce mountain showers, but there is a non-zero chance for
light rain across the San Gabriel Valley, as well.

With the strong onshore gradients, gusty onshore winds will
develop across the interior portions of the area Thursday
afternoon and evening, and again Friday afternoon and evening. The
strongest winds appears to be on Friday afternoon and evening when
a wide area of advisory level winds could develop across the
region. The highest confidence in gusty winds for Thursday will be
for the western Antelope Valley foothills, but ensemble members
hint that advisory level winds could not only develop across the
interior, but also across the Central Coast, southern Santa
Barbara County, and into the Ventura County coast. Future shifts
will need to take a closer look at this potential.

An update was issued earlier to tweak temperatures and cloud
coverage for Thursday. No additional updates are planned at this
time.

***From Previous Discussion***

Friday, a sharper shortwave trough will quickly pass over the
region, bringing colder air aloft. This will further increase
chances of drizzle and fog overnight into the morning. For most
locations except for the coasts, Friday will be the coldest day of
the forecast period, with most maximum temperatures in the 60s,
and deserts only reaching the 70s. These temperatures are 6 to 12
degrees below normal. Clearing is generally expected to be even
slower, if at all. However, there is a 20% chance that the
instability will cause fragmentation of the marine layer,
resulting in pockets of sunshine and slightly warmer temperatures
than forecast, even at the beaches. In addition, instability from
the trough will lead to a 10-15% chance of afternoon
shower/thunderstorms for inland mountain ranges. Finally, advisory
level gusts are possible during the afternoon and evening, for
the western Antelope Valley and foothills and the western portion
of the Santa Barbara South Coast.

Saturday the trough will start breaking down and upper level
heights will rise. Highs will a few degrees warmer than Friday,
kicking off a warming trend, and clouds are expected to be less
extensive, as higher upper level pressure compresses the marine
layer.

.LONG TERM (MON-THU)...22/213 PM.

The GFS and ECMWF and their respective ensemble means continue to
show little variation in the pattern for Sunday through Tuesday.
Sunday the persistent broad trough will finally break down,
giving way to a weak ridge. Onshore flow will weaken at this time,
and thus the inland reach of marine layer clouds is expected to
subside each day as upper level heights rise. Daytime chances for
clearing and sunshine will improve for the latter portion of
Memorial Day weekend into Tuesday. Maximum temperatures will trend
upwards, and will likely peak Tuesday or Wednesday. Daytime highs
for Memorial day will range from the mid 60s to mid 70s for
coasts, and up to the low 80s for valleys. This will put
temperatures just a touch below normal. Additionally warming on
Tuesday will increase conditions to near normal for most
locations.

Wednesday marks the start of mild disagreement between various
models. NBM clusters show the potential for the weak ridging
pattern to start to break down. The GFS leans towards falling
hieghts, while the ECMWF continues the weak ridge. The overall
impacts of these differences are expected to be minimal for
Wednesday, mostly impacting the extent of marine layer clouds and
slightly impacting high temperatures.

&&

.AVIATION...23/0719Z.

At 0505Z at KLAX, the marine layer was 2800 ft deep. The top of
the inversion was at 5400 feet with a temperature of 16 C.

High confidence in desert TAFs.

Moderate confidence in TAFs. There is a 20 percent chc of brief
SCT conds at any site with no clearing fcst. There is a 30 percent
chc of no IFR cigs at sites with them in the fcst.

KLAX...Moderate confidence in TAF. There is a 20 percent chc of a
few hours of SCT conds in the afternoon. Any east wind component
will be 6kt or less.

KBUR...Moderate confidence in TAF. VFR transition could occur any
time 18Z-21Z.

&&

.MARINE...22/823 PM.

For the Outer Waters, moderate to high confidence in current
forecast. Tonight through Saturday, there is a 70-90% chance of
Small Craft Advisory (SCA) level winds and seas. For Sunday and
Monday, there is a 30-50% chance of SCA level winds.

For the Inner Waters north of Point Sal, moderate confidence in
current forecast. Tonight through Thursday morning, high
confidence in winds and seas remaining below SCA levels. For
Thursday afternoon through Saturday night, there is a 40-50%
chance of SCA level winds, mainly in the afternoon and evening
hours.

For the Inner Waters south of Point Conception, moderate
to high confidence in current. For most of the area, high
confidence in winds and seas remaining below SCA levels through
Monday. However across the western half of the Santa Barbara
Channel, there is a 30-40% chance of SCA level winds through
Thursday then a 60-70% chance on Friday and Saturday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 2 PM this afternoon to 9 PM PDT
      this evening for zone 381. (See LAXNPWLOX).
PZ...NONE.

&&

$$

PUBLIC...Hall/Schoenfeld
AVIATION...Rorke
MARINE...RAT/Lund
SYNOPSIS...Hall

weather.gov/losangeles

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