850
FXUS66 KLOX 190748
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1248 AM PDT Thu Sep 19 2024

.SYNOPSIS...18/824 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 into early
Friday, 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.

&&

.SHORT TERM (THU-SUN)...18/845 PM.

***UPDATE***

Interesting weather pattern the next 24-36 hours as an
unseasonable cold upper low moves through the area. 00z models
continue to zero in on the interior part of SLO County through
interior SBA/Ventura Counties as the focus of the most active
weather beginning Thursday, then shifting southeast into the
remainder of Ventura and LA Counties Thursday night into Friday
morning. It won`t rain everywhere, but there could be periods of
heavy rain and thunderstorms with hail in the most active areas,
which again models are focusing in the interior areas bordering
Kern County, west of LA County. It`s a very tricky and low
confidence forecast but the potential is there for some
significant impacts in those areas. For this reason a flood watch
was issued earlier, emphasizing the potential for debris flows
around the recent burn scars.

For LA and southern Ventura County much of Thursday may remain dry
with on and off sunshine. Hi res models do develop a fairly deep
marine layer later tonight that could generate some morning
drizzle across coast/valleys/foothills, but a weak inversion
should allow for at least some sunshine in the afternoon.

Overall, no significant changes to the forecast other than to
decrease rain chances south of Pt Conception Thursday during the
day (aside from the morning drizzle potential).

***From Previous Discussion***

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 (MON-THU)...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.

&&

.AVIATION...19/0747Z.

At 06Z at KLAX, there was a 5000 ft deep moist layer and no
inversion.

Good confidence in TAFs for KPMD and KWJF.

Low to moderate confidence in the remainder of the TAFs. Cigs
heights will vary frequently due to presence of upper level low
moving over the region.

For KPRB, KSMX, KSBP, and KSBA -DZ is possible through the
morning. Then there is a 20% chance of TSTMs with brief heavy
rain and gusty and erratic winds 17Z through the period. Timing of
flight cat changes and rain timing could be off by +/- 3 hours.

For Ventura and Los Angeles County TAFs (except desert TAFs),
Cigs will vary between MVFR and VFR through 16Z, then lift to VFR.
-DZ is possible at all sites through the morning due to the
marine layer rapidly deepening. There is a 10% chance of isolated
TSTMs at KOXR and KCMA after 00Z Fri, and a 20% chance of VCSH at
all sites after 04Z. There is a 20% chance of MVFR cigs at L.A.
sites after 04Z Fri.

KLAX...Low to moderate confidence in TAF. Cigs may vary between
BKN015-035 frequently through 17Z as upper level clouds move
overhead. -DZ is possible through 17Z Thurs. There is a 20%
chance of BKN025-030 cigs after 04Z Fri. VCSH could occur as early
as 06Z. No significant east wind component is expected.

KBUR...Low to moderate confidence in TAF. There is a 20% chance
of BKN022-030 conds through 15Z Thurs and after 04Z Fri.

&&

.MARINE...18/938 PM.

Low confidence in the current forecast for the period. Higher
confidence exists in the forecast for seas. Lower confidence in
the current forecast for winds and thunderstorms.

Isolated to scattered thunderstorms will be possible over all of
the coastal the marine zones Thursday afternoon through Thursday
evening as a potent upper-level low pressure system drops from
Northern California to near Pt. Conception by Thursday afternoon.
The trough will move southeastward across the southern California
bight Thursday night and onshore over southern California through
Friday. A marine weather statement is in effect to communicate
the hazards from thunderstorms to the marine community Thursday.

Winds and seas diminished a little earlier than expected this
evening and the Small Craft Advisory (SCA) was cancelled early.
There is a moderate chance of localized wind gusts at SCA levels
through midnight tonight. Winds and seas will likely remain below
SCA levels through the period, except for a moderate-to-high
(30-50 percent) chance of SCA level winds each afternoon and
evening for the southern waters from Point Conception south to San
Nicolas Island, in the vicinity of the Channel Islands, and
inside the southern California bight. The highest chance will be
on Friday afternoon and evening when marginal to widespread low
end SCA level winds could develop.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Flood Watch in effect from 5 AM PDT early this morning
      through this evening for zones 38-344-345-377. (See LAXFFALOX).
PZ...NONE.

&&

$$

PUBLIC...MW/Cohen
AVIATION...Lund
MARINE...Hall
SYNOPSIS...MW/Cohen

weather.gov/losangeles

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