218
FXUS66 KLOX 141948
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1248 PM PDT Sun Jul 14 2024

.SYNOPSIS...14/918 AM.

There is a slight chance of a thunderstorm this afternoon over the
mountains, otherwise skies will be clear save for the night
through morning low clouds across the coasts. A cooling trend
will continue across the area through at least Tuesday, but extreme
heat is possible across the interior by next weekend.

&&

.SHORT TERM (TDY-WED)...14/1248 PM.

***UPDATE***

Here`s an update on the convective chances this afternoon and
early evening across the local area -- highest in the Antelope
Valley.

A comparison of moisture fields from GOES satellite imagery
during the past 24 hours reveals a rather significant influx of
tropospheric moisture. Even in the last 12 hours, precipitable
water values across LA County have increased by about a quarter
inch -- 19Z readings now up to 1 inch across the Antelope Valley
ranging to almost 1.4 inches in the southern part of LA County.
This is from the continued influx of monsoonal moisture around the
western periphery of the expansive upper anticyclone centered
over the southern Rockies -- locally enhanced by a subtle,
mesoscale impulse that moved through the region this morning
bringing thunderstorms to parts of the LA Basin and points
northward. Highlighting how moist the environment is,
temperatures have been breaching 100 degrees in the Antelope
Valley, yet dewpoints are still holding around 50F in some spots
despite the ample mixing.

All this to say, there is copious deep-layer moisture across
portions of LA County, where the focus for afternoon thunderstorms
will be today. The moisture tapers off with northward extent across
the NWS Los Angeles / Oxnard County Warning Area, limiting the
thunderstorm risk elsewhere. One exception will be the northern
Ventura County mountains, where visible satellite imagery already
indicates cumulus build-ups developing early this afternoon that
could eventually yield a thunderstorm. The stable marine layer
over the coastal areas and coastal valleys should largely prevent
convective development there. Farther inland, strong heating
across the San Gabriels and points northward will be where the
greatest surface- based buoyancy will develop and focus thunderstorm
chances, though pockets of uninhibited surface-based buoyancy
will exist farther south over foothills locations from Hollywood
to Pasadena -- along the edges of richer marine-layer moisture
and desert heat.

Meanwhile, despite the weak forcing for ascent at the synoptic
scale, orographic circulations resulted from intense heating of
elevated terrain especially around the San Gabriels vicinity--
locally augmented by pockets of more extreme heat around the Vista
Fire -- will be the focus for the development of convection
between 1PM and 3PM PDT. Visible satellite imagery is detecting
cumulus fields becoming increasingly agitated and widespread
within this regime. In fact, there is an 80-100% chance for at
least isolated thunderstorms to form somewhere between the San
Gabriels and the Antelope Valley. Of particular note, surface
observations highlight a well-defined confluence axis becoming
established across the Antelope Valley -- between channeled west-
southwesterly flow through the Highway 14 corridor and southerly
flow farther east channeled through Cajon Pass. This confluence
axis will be a particularly favorable zone for convective
initiation and re-development this afternoon. For these reasons,
point-probabilities of thunderstorms in this regime have been
increased to around 40%. Much lower probabilities (around 10%) for
thunderstorms do exist farther south toward the Hollywood to
Pasadena foothills vicinity, as previously mentioned.

Regarding convective hazards, thermodynamic and kinematic
parameters will conditionally support strong thunderstorm wind
gusts. Aloft, midlevel winds have decreased by around 5-10 knots
over the past 24 hours, given the lessened influence from the east
Pacific upper low. Correspondingly weaker deep shear will only
support 10-20 knots of effective shear, resulting in less
convective organization and more of a pulse-convective mode where
thunderstorms develop. This will tend to shorten the longevity of
any individual convective cores. However, with the increased
moisture, and anticipated strong convective downbursts from ample
sub-cloud evaporation amid deep inverted-V thermodynamic profiles,
strong thunderstorm outflow will be a concern. This environment
will produce localized intense cold pools with peripherally-
flanking regenerative convection -- enhancing both convective
coverage and localized outflow strength. As a result, there is a
greater chance today compared to yesterday (60-80% chance today)
for very localized 45-55-mph wind gusts to occur with the most
intense convection. This is a conditionally favorable environment
for hybrid wet-dry microbursts, and there is a small chance (less
than 5%) for a damaging wind gust to 60 mph -- consistent with
DCAPE exceeding 1500 J/kg across the Antelope Valley. In addition,
with the weaker midlevel winds resulting in 5-10-knot northward
to northeastward mean storm motions, individual convective cores
will be moving slower today compared to yesterday. This will
increase the chances for pockets of heavy rain to develop in the
moister environment. There will be a small chance for localized
flash flooding to occur (less than 10%) if a thunderstorm were to
backbuild over an area more prone to flooding, such as more
urbanized areas around Lancaster and Palmdale.

Thunderstorms should continue through around 7PM PDT or 8PM PDT,
after which time nocturnal stabilization will yield the weakening
and dissipation of convection. Significant Weather Advisories
will be a possibility this afternoon and early evening especially
around the Antelope Valley, and an isolated Severe Thunderstorm or
Flash Flood Warning cannot be ruled out (less than 10% chance).

***From Previous Discussion***

There was an active system that moved south to north through Los
Angeles County this morning with many lightning strikes and
producing over 0.1" of rain in some areas. This has moved into
Kern County now and there shouldn`t be more activity until the
afternoon heating could generate some convection.

Temperatures are significantly cooler than this time
yesterday in the mountains (5-10 degrees) and Antelope Valley (10
or more degrees), and generally down 2-5 degrees elsewhere. Have
added in a slight chance (~15%) of thunder in the northern
mountains of Ventura County for this afternoon to go along with
the slight chance in the eastern San Gabriels and Antelope Valley.
While the main activity should stay to our east on Monday, there
is a slight chance on Monday as well for those areas.


A small vort lobe has rotated up and into LA county from the SSE.
It has produced a clutch of small showers that are moving from the
north to the south. There is a 20 percent chc that these showers
could grow into TSTMs. This shower activity will likely be clear
of the area by mid morning.

The marine layer has shrunk to under 900 ft. There are also 2 mb
offshore trends from both north and east. These two things have
caused the marine layer stratus to form slower this morning and
will likely stay confined to the coasts.

The upper high has pushed far enough to the east that only area at
risk of monsoon flow is the eastern LA mtns and eastern Antelope
Vly where there is a 20 percent chc of a thunderstorm this
afternoon. The moisture is quite high based which will greatly
increase the chc for dry lightning.

As the warmth and subsidence from the upper high moves eastward
temps should cool across the area. There is, however, a 25
percent chc that the sunnier skies and the offshore trends will
actually bring some warming to some areas - notably the LA/VTA
csts/vlys and SLO and SBA counties.

Benign weather is on tap for Mon and Tue. Srn CA will be in
between an upper high to the east and troffing to the NW. This
will result in dry SW flow aloft over the area. Hgts will be near
592 dam. There will be weak onshore flow on Monday with a little
stronger push to the east on Tuesday. Night through morning low
clouds will persist across all of the coasts and may penetrate
into the lower vlys.

Due to the offshore trends some areas (mostly in SLO and SBA
counties) are now forecast to warm some on Monday. The SBA south
coast may see the biggest warm up as local northerly flow develops
across the county. Tuesday remains the coolest day of the next 7
as onshore flow increases again. Look for 1 to 3 degrees of
cooling with most areas except the Antelope Valley coming in 2-4
degrees below normal.

.LONG TERM (THU-SUN)...14/309 AM.

Deterministic and ensemble solutions continue to show the upper
high strengthening and pushing to the west. Hgts will push up to
near 596 dam again. Moderate onshore flow to the east will
continue but the N/S gradient will turn offshore in the morning.
This will kick off a warming trend starting Wednesday and
persisting into next weekend, especially for interior areas.

By Saturday max temps across the Antelope Vly are fcst to be near
110 degrees again and another round of excessive heat warnings
looks likely. The lower elevation mtn locations and other
interior areas are fcst to see highs between and 100 and 105,
which would put them just under warning criteria.

While not in the best of agreement there is decent consensus among
the long range mdls that the upcoming heat wave will break next
Monday as the upper high diminishes.

&&

.AVIATION...14/1811Z.

At 17Z at KLAX, the marine layer was 1300 feet deep. The top of
the inversion was at 3800 feet with a temperature of 26 deg C.

High confidence in CAVU TAFs for KPRB and KVNY.

Moderate confidence for all other TAFs.

Isolated to scattered thunderstorms this afternoon across eastern
and northern parts of Los Angeles County will have the highest
coverage across the Antelope Valley. Direct impacts will be
possible (around 20% chance) at KPMD and KWJF between 21Z this
afternoon and 03Z this evening -- reflected by VCTS mention. The
strongest thunderstorms will be capable of producing variable-
direction wind gusts upwards of 40-50 kt, locally heavy rainfall,
frequent lightning, abundant turbulence, and very small hail.
Outside of thunderstorms, CAVU conditions are expected at KPMD and
KWJF, with west-southwest winds gusting around 25 kt this
afternoon.

For remaining TAFs over the coasts and coastal valleys, MVFR to
LIFR cigs with MVFR to IFR vsbys will return tonight through
Monday morning. The timing of flight category changes may vary
+/- 2-3 hours from current forecasts. There is a 30% chance for
minimum vsbys/cigs to be off by a category.

KLAX...Moderate confidence in TAF. There is a 30% chance for cigs
to become LIFR tonight and Monday morning. No significant east
wind component is expected.

KBUR...Moderate confidence in TAF. There is a 30% chance for MVFR
vsbys and IFR cigs to develop tonight and Monday morning.

&&

.MARINE...14/1111 AM.

In the outer waters, Small Craft Advisory (SCA) conditions are
not expected thru Sun night. In the northern zone (PZZ670), there
is a 30% chance of SCA winds Mon afternoon thru Wed night, mainly
during the afternoon/evening hours. SCA conds are likely (60%
chance) Thu thru Fri. In the southern two zones (PZZ673/676),
there is a 40% chance of SCA level winds Mon thru Fri, especially
during the afternoon/evening hours. SCA conds are likely (70%
chance) Wed afternoon thru Fri.

In the inner waters N of Pt. Sal, there is a 20% chance of SCA
conds in the afternoon/evening hours Mon thru Wed, with a 40-50%
chance Thu afternoon/eve. Otherwise, SCA conds are not expected
thru Fri.

In the inner waters south of Pt. Conception, there is a
20-30% chance of SCA winds across the far western portion of the
Santa Barbara Channel late each afternoon thru the late night
hours Tue thru Fri. Otherwise, SCA conds are not expected.

Patchy fog will reduce visibilities to below one mile during the
overnight and morning hours each day through Fri.

&&

.BEACHES...14/1149 AM.

A series of Southern Hemispheric storm systems will send a long-
period, southerly to southwesterly swell (around 17-second wave
periods) to the Southern California coastal waters by the middle
of this week. Wave periods will gradually taper off to around 15
seconds by late week. In response to these developments, an
extended period of elevated surf -- surf heights of 4-6 feet --
along with powerful and dangerous rip currents will be possible
(50-70% chance) from Tuesday through Thursday. The area of
greatest concern for these impacts will be those beaches with
most exposure to southerly to southwesterly swell directions --
especially the Los Angeles and Ventura County beaches. These
factors warrant the issuance of a Beach Hazards Statement for the
beaches of Los Angeles and Ventura Counties.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from Tuesday morning
      through Thursday evening for zones 354-362-366. (See LAXCFWLOX).
PZ...NONE.

&&

$$

PUBLIC...Cohen/Rorke/jld
AVIATION...Cohen
MARINE...DB/Cohen
BEACHES...Cohen
SYNOPSIS...MW

weather.gov/losangeles

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