425
FXUS66 KLOX 142039
AFDLOX

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

.SYNOPSIS...14/100 PM.

There is a chance of thunderstorms this afternoon over the Los
Angeles and Ventura County mountains, otherwise skies will be
clear except for morning low clouds across the coasts. The
cooling trend will continue through at Tuesday, but extreme heat
is possible across the interior by next weekend.

&&

.SHORT TERM (TDY-WED)...14/132 PM.

Convective chances this afternoon and early evening across the
local area remain 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
will eventually yield thunderstorms. 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 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).

The active system that moved through Los Angeles County this
morning produced over 20 positive cloud to ground strikes over
0.1" of rain in some areas. For the weather the rest of today -
see the mesoananalysis section above.

Synoptically, to our east there is a strong (598 dam) upper level
High centered along the WY/NW border, and to our west there is a
588 dam Low with a weak trough. The high drops to 594 dam Monday
and Tuesday and the ridge slowly wanders westward. By Wednesday
the area will be under 591-594 heights with the High over the
four-corners area. By Saturday the High will have rebuilt to
around 598 dam and have moved a little further west driving
temperatures back up for the weekend.

Benign weather is on tap for Monday. SoCal will be in between an
upper high to the east and weak troffing to the northwest,
resulting in dry southwesterly flow aloft. Heights will be near
592 dam. There will be weak onshore flow on Monday, and due to
the offshore trends areas like SLO and SBA Counties may warm
slightly on Monday. The SBA south coast may see the biggest warm
up as local northerly flow develops across the county. Night
through morning low clouds will persist across all of the coasts
and may penetrate into the lower valleys.

On Tuesday the onshore flow will have a little stronger push to
the east. It should be the coolest day of the next seven with
afternoon high temperatures approaching seasonal normals. Night through
morning low clouds will again persist across all of the coasts
and may penetrate into the lower valleys. Look for 1 to 3 degrees
of cooling with most areas (except the Antelope Valley) coming in
2-4 degrees below normal.

By Wednesday, model solutions continue to show the upper high
strengthening and pushing to the west. This will push up heights
and kick off a warming trend which will persist into next
weekend, especially for interior areas.

.LONG TERM (THU-SUN)...14/138 PM.

Ensemble solutions show the upper high strengthening and pushing
to the west and establishing along the NV/UT border area Thursday
through Sunday. Heights will push up to near 596 dam again. The
warming trend will be most pronounced in the interior, as during
the last heat event.

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

&&

.AVIATION...14/2022Z.

At 20Z at KLAX, the marine layer was 1000 feet deep. The top of
the inversion was at 4000 feet with a temperature of 25 deg C.

High confidence in CAVU TAFs for KPRB and KVNY.

Moderate confidence for all other TAFs.

Isolated to scattered thunderstorms this afternoon and early
evening 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...14/122 PM.

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 each late 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 early to middle parts
of this week.

&&

.BEACHES...14/122 PM.

A series of Southern Hemispheric storm systems will send a long-
period, southerly to southwesterly swell (around 18-second wave
periods) to the Southern California coastal waters during early
to middle parts 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 likely (over 80% chance) from Tuesday through Thursday.
The area of greatest concern for these impacts will be those
beaches with highest 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/jld
AVIATION...Cohen
MARINE...DB/Cohen
BEACHES...Cohen
SYNOPSIS...jld

weather.gov/losangeles

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