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680
FXUS66 KLOX 182127
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
227 PM PDT Wed Sep 18 2024

.SYNOPSIS...
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18/225 PM.

An unsettled weather pattern is on the way for the next couple of
days, as a compact atmospheric disturbance crosses the region. It
will turn much cooler across the area for Thursday, when showers
and thunderstorms will move across the region. Not every location
will get rain, though many will, and thunderstorms producing
possible flooding rains will impact interior portions of Ventura,
Santa Barbara, and San Luis Obispo Counties for Thursday and
Thursday evening. Some showers or thunderstorms could linger into
Friday before the disturbance exits the region, and then it
should be dry for several days starting this weekend when
temperatures warm up again.
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&&

.SHORT TERM (TDY-SAT)...
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18/225 PM.

A record-caliber, upper-level cold core low is headed to Southern
California. Satellite loops show the tightly coiled upper vort
center diving southward over the central California waters, about
100 miles west of San Francisco. Multiple mesoscale swirls are
drifting southward beneath the synoptic cold core, suggesting that
sub-grid-scale processes are probably influencing what is
evolving into a very unseasonably deep, cut-off upper low that
will affect the region during the next couple of days. Model
solutions continue to catch up to this considerably deeper system
and its disconnection from westerly steering currents at higher
latitudes. What this means is that the potent upper system --
accompanied by record-cold upper-level temperatures based on
sounding climatology data -- will take a more southern trajectory
and achieve greater depth as it tracks across the bulk of the
local area, with core 500-mb heights around 566 dam and core
500-mb temperatures around -19C.

In response to these developments, confidence continues to grow in
a potentially high-impact convective event taking place over
interior portions of Ventura, Santa Barbara, and San Luis Obispo
Counties -- where a Flood Watch has been posted for Thursday
morning through Thursday evening. This does include the potential
for debris flows on the Hurricane and Apache burn scars. However,
this is not a particularly high-confidence event.

As the strongest cooling aloft overlies diurnally-enhanced
orographic ascent along the interior slopes across the Watch area,
expect thunderstorms to quickly form by mid-day Thursday. Sub-
cloud-layer humidities will be elevated by onshore flow, and
DCAPE will be modest, so individual potential-temperature deficits
in convective cold pools will be weak and permit fast cycles of
upshear boundary-layer recovery. And with the slow-moving large-
scale ascent, conditions will be optimal for regenerative
and backbuilding convection that will facilitate convective
training across the Watch area, as individual convective plumes
slowly drift northward to eastward at 5-15 kt. So even though
precipitable water will be modest -- on the order of 0.75 to 1.00
inch -- this convective scenario will be able to fully leverage
any moisture available in advance of the upper low. Given surface-
based CAPE around 500 J/kg and the convective training scenario,
and low echo centroids maximizing rainfall efficiency (i.e.,
limited sub-cloud evaporation of precipitation), rainfall rates of
one-half inch per 30 minutes and one inch per hour will be
possible, capable of producing flash flooding in the Watch area.
The most-likely time period for this activity will be 1 PM PDT
through 7 PM PDT Thursday. Potential impacts across the Flood
Watch include flooding of rivers, creeks, streams, and other low-
lying and flood-prone locations. The thunderstorms will have the
potential to impact the Hurricane and Apache burn scars during the
period of the Watch. Residents near the Hurricane and Apache burn
scars should prepare for the possibility of debris flows and
flooding impacts. Of note, given the dominant convective character
to precipitation on Thursday, and expected narrow convective
plumes limiting the predictability of their evolution, confidence
in related flood impacts -- if they were to occur -- could
escalate rather quickly with limited lead-time prior to the onset
of these impacts.

In addition to potential flooding and debris-flow impacts, with
low freezing-level heights around 9 kft AGL and ample upper-
tropospheric flow offering strong ventilation to updrafts and a
modest increase of effective shear to 20-30 kt, small hail --
perhaps accumulating in some areas -- will be possible with these
thunderstorms, and local wind gusts to 45 mph will be possible as
well. Moreover, stretching of ambient vertical vorticity around
the deeply-stacked cold core low could support a couple of cold-
air funnel clouds, as well, though tornadoes are highly unlikely
(less than 2% chance). POPs have been increased to 100% across a
large portion of the Flood Watch area, in conjunction with the
lobe of strongest ascent forced by differential cyclonic vorticity
advection ahead of the upper low favorably phasing with the
strongest orographic ascent.

Through Thursday evening, in locations where the heaviest rain
falls for the longest duration, total QPF could locally exceed 2
inches in spots. However, there may be numerous sharp gradients
in total precipitation amounts given anticipated narrow convective
plumes. As a result, total rain amounts and related impacts may
vary considerably over short distances. Nevertheless, instability
created by the anomalously cold upper-level temperatures and
sufficient ascent will provide at least slight chances for showers
and thunderstorms across much of the remainder of the forecast
area for Thursday into Thursday night, as activity gradually
develops eastward through in tandem with the track of the upper
low. Any locations that experience heavier showers could pick up
a quick tenth or two tenths inch of rain.

Similarly, there should be a relatively sharp gradient in
convective coverage between the Watch area and elsewhere across
the region for Thursday into Thursday night -- owing to the
compact lobe of synoptic-scale ascent and lack of greater deep
moisture. The exact coverage pattern of this convective
precipitation will be heavily influenced by the precise track of
the upper low, for which some uncertainty still exists owing to
the aforementioned sub-grid-scale effects. Minor deviations in its
track could result in sizable spatial shifts in higher convective
coverage and related impacts. For example, the Lake burn scar is
southwest of the most-likely area of Thursday thunderstorms.
However, there is some potential for this activity to drift over
the Lake burn scar Thursday afternoon and evening. If this were to
occur -- around 20 percent chance -- flash flooding and debris
flows could also impact the Lake burn scar. This area, and also
other areas peripheral to the Flood Watch, will be monitored
closely for possible expansions to flood headlines.

All of the above is to say -- Thursday into Thursday evening has
the potential to be a localized high-impact, low-confidence
event.

Shower and thunderstorm coverage will gradually lessen by late
Thursday evening. However, this activity will not completely
dissipate, owing to the very pronounced ascent and slight
nocturnal cooling at the top of the moist layer that will
maintain marginal boundary-layer-based buoyancy into the
overnight hours. Isolated to widely scattered showers and
thunderstorms will have the potential to be spreading across
Ventura and Los Angeles Counties through the overnight hours.
However, with enough nocturnal stabilization (though incomplete),
the potential for flooding and strong thunderstorms will decrease
with southward extent of a line from Point Conception to Gorman.
Going into Friday, chances for showers and slight chances for
thunderstorms will linger across the eastern half of the forecast
area -- given an anticipated slower-moving and deeper upper low
prolonging its effects over the local area. A low tracking even
slower than expected could bring potential hydrological impacts to
the Post and Bridge burn scars -- 10% chance. By Friday evening,
showers and thunderstorms will have exited the region, giving way
to several days of dry conditions.

Significant erosion of the inversion surmounting the marine-
influenced moist boundary layer -- owing to the large-scale
ascent -- will greatly disrupt marine-layer maintenance and
related low stratus and fog. As a result, cloud coverage,
especially later tonight through Saturday, will likely be driven
solely by convection and related convective debris. Once
convection exits the region, skies will mostly clear by late
Friday into Saturday.

Temperatures will be coolest on Thursday, owing to the greatest
influence of the cold core aloft, and will likely be running 10-15
degrees below normal -- i.e., high temperatures in the lower 70s
in many areas, with several locations over higher terrain and
near the beaches staying below 70 degrees. High temperatures are
forecast to warm a few to several degrees on Friday, though slower
motion of the upper low could considerably mute the warming trend
(30% chance). Confidence is comparatively higher that rebounding
midlevel heights in the wake of the departing upper low will
facilitate a more significant warming trend by several degrees for
Saturday, with highs topping out in the 80s for many locations.

.LONG TERM (SUN-WED)...18/225 PM.

A much quieter period is currently expected in the extended
forecast, as the general consensus amongst medium-range model
solutions is for an upper ridge to remain nearly stationary over
the southwest states from late this weekend into next week.
Differences in the strength of this ridge are significant between
the various model solutions, casting uncertainty regarding the
amount of warming that takes place. High temperatures are expected
to reach the 80s and 90s by early next week in many areas away
from the coast, and some model solutions indicate even warmer
temperatures materializing (30% chance). Pressure gradients
trending somewhat offshore owing to surface ridging behind the
departing upper low will have the potential to enhance the warming
trend late in the weekend into Monday. Alternatively, some model
solutions indicate additional wave amplifications over the central
and northern CONUS carving out an upper low that retrogrades over
the southwest CONUS around early next week. This would limit the
degree of warming (10-20% chance).

Overall, the upper flow over the CONUS will remain highly
amplified into next week. While the most-likely outcome is for
upper ridging to affect southern California, short wavelengths to
the amplified pattern aloft tend to suggest limited predictability.
Thus, current forecast temperatures are of only low to medium
confidence at this time. However, there is strong consensus among
model solutions that a dry air mass will prevail across the
region, with an unfavorable flow pattern for deep-layer moisture
to return -- keeping dry conditions in place. However, the marine
layer should more significantly re-establish across coasts and
coastal valleys by Sunday and into next week -- accompanied by
night through morning stratus and fog.
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&&

.AVIATION...
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18/1952Z.

At 20Z at KLAX, There was a deep moist layer, about 3000 feet
deep, with no inversion.

Good confidence in TAFs for KPMD and KWJF.

Fairly good confidence in TAFs for KOXR, KCMA, KBUR, and KVNY,
with a 30 percent chc of MVFR cigs after 06Z Thu.

Low to moderate confidence in KPRB, KSBP, and KSMX TAFs with a 30
percent chc of MVFR cigs lifting to VFR with VCSH possible after
12Z.

Low confidence in the remainder of the TAFs with a 20 percent chc
of no low clouds. Areas of cigs will be noted across the coastal
strips but could be very patchy in nature at times and even could
dissipate and reform frequently, esp after 10Z Thurs.

KLAX...Low to moderate confidence in TAF. There is a 15 percent
chc of no low clouds at all. If low clouds do form, they may
scatter and reform frequently. There is a 30 percent chc of BKN025
cigs aft 08Z Thurs. No significant east wind component is
expected.

KBUR...Moderate to high confidence in TAF. There is a 20% chance
of BKN020 conds aft 06Z Thurs.
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&&

.MARINE...
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18/1253 PM.

Scattered thunderstorms will be possible over all of the marine
zones from Thursday afternoon through Thursday evening, as a
potent upper-level low pressure system drops from Northern
California to near Pt. Conception, and then moves eastward across
SoCal through Friday. A marine weather statement has been issued
to communicate the hazards from thunderstorms to the marine
community.

For the Outer Waters, SCA winds will continue across the southern
two zones south of Point Sal to San Nicolas Island (PZZ673 and
PZZ676) through late tonight and accompanied by steep and choppy
seas. For Thursday and Friday through this weekend, SCA headlines
will be unlikely, though a few spots of marginal SCA winds cannot
be ruled out (30% chance) from areas around San Miguel and Santa
Rosa Islands toward Pt. Conception each afternoon and evening.

For the Inner Waters north of Pt. Sal, SCA conditions are not
expected through the period.

For the Inner Waters south of Pt. Conception, SCA winds are
expected across portions of the Santa Barbara Channel --
especially the western two-thirds -- this afternoon and evening,
accompanied by steep and choppy seas. For Thursday and Friday
through this weekend, SCA headlines will be unlikely, though a few
spots of marginal SCA winds cannot be ruled out (30% chance) over
the Los Angeles and Orange County waters within 20 NM from shore.
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&&

.LOX WATCHES/WARNINGS/ADVISORIES...
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CA...Flood Watch in effect from late tonight through Thursday
      evening for zones 38-344-345-377. (See LAXFFALOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for
      zones 650-673-676. (See LAXMWWLOX).
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&&

$$

PUBLIC...Cohen
AVIATION...JB
MARINE...JB
SYNOPSIS...Cohen

weather.gov/losangeles

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