FXUS66 KLOX 222110

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
210 PM PDT Fri Sep 22 2023

.SYNOPSIS...22/205 PM.

Increasing cloud coverage is expected through tonight, with
perhaps a few sprinkles in Los Angeles County, as a weather
disturbance moves across the area. Skies will gradually clear
through the day on Saturday. Temperatures will slowly warm this
weekend into early next week, but will remain below normal. More
substantial warming is expected over interior valleys and
foothills Tuesday into Wednesday, with the potential for gusty
offshore winds over the mountains of southern Santa Barbara


.SHORT TERM (TDY-MON)...22/205 PM.

A steady stream of mid/upper-level moisture continues to extend
northeastward into the area from the remnants of Tropical Cyclone
Kenneth, located well off the coast of central Baja California.
Diffuse, convectively enhanced vorticity centers drifting through
the flow aloft are interacting with the moisture plume to support
showers south of the area. However, recent trends in satellite
imagery and composite radar data imply that flow fields aloft are
becoming increasingly meridional ahead of a more prominent
impulse, thereby reorienting the moisture plume and resulting in
light radar returns extending farther north toward LA. This trend
is evident in some recent high-resolution model guidance (e.g.,
both HRRR and NAM), and supports the addition of a slight chance
of sprinkles across a broad area of LA County and nearby Ventura
County for the rest of today into this evening. Chances for
sprinkles occurring at any one location are less than 25 percent,
while chances for measurable precipitation with this activity will
remain below 15 percent. Otherwise, the interaction between weak
ascent preceding the impulse and the moisture plume will
facilitate an increase in mid/high clouds this evening and tonight.
Correspondingly weaker radiational cooling tonight should prevent
temperatures from cooling off as much as they did last night,
with lows from the mid 40s to the mid 50s over the mountains and
foothills, except mid 30s to mid 40s over the Ventura County
mountains, with lows in the mid 50s to lower 60s closer to the

The primary axis of the midlevel impulse will be shifting east of
the area by Saturday afternoon, with drier air working into the
region from upstream. Skies are expected to be gradually clearing
by the afternoon leading to ample sunshine and a few degrees of
warming compared to Friday. However, fringes of the midlevel cold
pool accompanying the impulse will still linger over the area and
temper the degree of warming. Saturday high temperatures are
expected to reach as warm as the upper 70s to lower 80s over the
interior valleys, with readings in the upper 60s to the mid 70s
closer to the coast, and cooler readings in the mountains.

Then for Sunday into Monday, additional warming will take place
as midlevel heights rebound in association with a migratory ridge
that will amplify and congeal as it approaches the Rockies.
Temperatures will most prominently warm across the interior
valleys and foothills -- reaching the mid and upper 80s in these
areas by Monday. Warming closer to the coast will be more modest
(upper 60s to the 70s), as onshore gradients foster the influx of
a shallower marine layer into these areas accompanied by areas of
low clouds and fog.

.LONG TERM (TUE-FRI)...22/205 PM.

By late Monday night, the subtropical component of the
aforementioned midlevel ridge will become increasingly west-east-
oriented to the south of the region. Meanwhile, the tail end of a
weakening front will graze the central California coast well to
the north of the forecast area before fully dissipating -- within
the base of a deep trough impacting the Pacific Northwest. Cloud
coverage and precipitation with this system are not expected to
affect the area. However, surface ridging behind the weakening
front, aided by large-scale subsidence circulating around the
attendant deep-layer trough, are expected to support a decided
offshore gradient setting up across southern Santa Barbara County
Monday night, more prominently increasing Tuesday, peaking on
Wednesday, then lessening by late next week. Global model guidance
suggests that Santa Barbara-Bakersfield and Santa Barbara-Santa
Maria offshore gradients could intensify to -5 mb and -4 mb,
respectively, by early Wednesday. This regime will favor the
potential for Sundowner wind events in the vicinity of the
southern Santa Barbara County mountains including the Santa Ynez
range. And with upper support along the southern rim of the deep
cyclone centered well north of the area, there appears to be
increasing probabilities (35 percent) for wind gusts over 45 mph
to materialize by Tuesday and especially Tuesday afternoon into
Tuesday night and again Wednesday afternoon into Wednesday night.
Moreover, the influence of upper support may translate to enhanced
drying phasing with these winds to create elevated fire-weather
conditions. Please reference the Fire Weather Planning Forecast
for additional information.

Otherwise through Wednesday, the offshore-flow pattern beneath
500-mb heights around 590 dam will correspond to an additional few
degrees of warming. Widespread high temperatures in the 80s are
expected across the interior valleys and foothills, with a few
spots of lower 90s becoming possible. These readings are above
normal by at least a few degrees. Though once again, a relatively
shallow marine layer accompanied by low clouds and fog will
mitigate such warming while keeping temperatures slightly below
normal closer to the coast. Amply dry deep-layer conditions are
expected to prevent the development of any precipitation areawide.

Then for Thursday into Friday, offshore gradients are expected to
be subsiding as broadly cyclonic mean flow over the northwest
states and adjacent Pacific waters becomes reinforced by another
potentially stronger disturbance. Confidence regarding the details
of the large-scale pattern evolution at that time range is
presently low. However, present indications are that the overall
pattern will support weakening of the midlevel ridge across the
forecast area by the end of next week with gradually cooling
temperatures, along with a deeper marine layer over coastal areas.
Exact magnitudes of these changes are uncertain. However,
precipitation chances will remain near zero, given the
displacement of any meaningful large-scale ascent well north of
the region based on the vast majority of deterministic and
ensemble model guidance.



At 1615Z, the marine inversion at KLAX was based at 3100 feet.
The top of the inversion was 5100 feet with a temperature of
18 degrees Celsius.

Overall, low to moderate confidence in 18Z coastal TAFs, but high
confidence in valley/desert TAFs. Lower confidence for coastal
sites due to uncertainties with the marine layer stratus. Deep
inversion and onshore gradients support stratus development
overnight, but high clouds could greatly hinder development.

KLAX...Overall, moderate confidence in 18Z TAF. Through this
evening, high confidence in VFR conditions. Overnight, low
confidence in return of MVFR CIGs as well as low confidence in
timing of potential return. No significant easterly wind component
is expected.

KBUR...High confidence in 18Z TAF. There is a 20-30% chance of
MVFR CIGs in the 10Z-16Z time frame.


.MARINE...22/1251 PM.

For the Outer Waters, moderate to high confidence in current
forecast. Through tonight, high confidence in winds and seas
remaining below Small Craft Advisory (SCA) levels for all the
Outer Waters. For Saturday through Sunday, there is a 30-50%
chance of SCA level winds around Point Conception. For Monday
through Wednesday, winds and seas will increase with high
confidence in SCA level winds and seas for all the Outer
Waters by Tuesday and Wednesday.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. Through Sunday night, high
confidence in winds and seas remaining below SCA levels. On
Monday, there is a 30-40% chance of SCA level winds in the
afternoon and evening hours. For Tuesday and Wednesday, there
is a 40-60% chance of SCA level winds and seas.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Generally for a majority of the
southern Inner Waters, winds and seas are expected to remain below
SCA levels tonight through Wednesday. However, across the western
half of the Santa Barbara Channel, there is a 40-50% chance of SCA
level winds Saturday and Sunday then a 60-80% chance of SCA level
winds Monday through Wednesday.

With the gusty winds expected to develop this weekend and into
early next week, choppy, short-period, seas can be expected.







Experimental Graphical Hazardous Weather Outlook at:

Chino Hills, Ca. Weather | www.ChinoHillsWeather.com