-- Discussion containing changed information from previous version are highlighted. --
350
FXUS66 KLOX 200637
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1137 PM PDT Wed Jun 19 2024

.SYNOPSIS...19/1013 PM.

Overnight low clouds and fog will continue along the coast and
valley areas through Thursday. A warming trend will take shape
over the coming day as high pressure aloft over the eastern half
of the country expands west. Above normal temperatures will
prevalent through late week and into early next week, especially
in the interior where dangerously hot weather is possible over the
weekend.

&&

.SHORT TERM (WED-SAT)...19/1011 PM.

The latest satellite imagery indicates broad troughing over the
Western States with a secondary trough carving out off the
California coast. Onshore flow is strengthening out ahead of the
trough as cyclonic flow aloft helps deepen the marine intrusion.
Surface pressure gradients already show some reflection of onshore
flow increasing, especially north of Point Conception. The marine
layer induced low clouds and fog are a little bit more
disorganized this evening, but the latest fog product is
indicating more organization moving down the Central Coast. Low
clouds should become more entrenched overnight along the Central
Coast, while a Catalina Eddy regenerates overnight to expand
clouds south of Point Conception. Low cloud coverage along the
South Coast of California should expand northward into the Ventura
County coastal areas by Thursday morning. The latest marine layer
depth at KLAX suggest a 1300 foot deep marine layer depth in
place, while an earlier sounding from KVBG indicated the marine
layer depth about 1500 feet deep. With the depth of the marine
layer, it should be deep enough to intrude into the Santa Ynez
Valley and the San Gabriel and San Fernando Valleys. Because of
the later start, it seems unlikely to return to the Ventura County
Valleys.

Some concern exists for patchy drizzle as weak dynamics move over
the area tonight through Thursday morning. There is a bit lower
confidence due to a thinner marine layer depth to work with, but
the marine layer could still lift and squeeze out precipitation.
PoPs have been nudged higher and mention of patchy morning drizzle
was added for Thursday morning.

Even with the trough moving into Point Conception, 500 mb heights
climb and 1000-500 mb thickness values increase as strong ridging
over the Eastern States starts expand westward. As a result, the
trough`s progress is halted through Thursday near Point Conception
and is then forced to lift out over the ridge. NAM-WRF 950 mb
temperatures warm by about 6 degrees Celsius over Los Angeles
County on Thursday. With this in mind, the forecast was warmed a
bit more for Thursday to put values closer to forecast guidance
and EPS ensemble means.

Gusty Sundowner winds continue this evening across southwestern
Santa Barbara County, and a wind advisory remains in effect
through 3 am PDT. Both Refugio and Gaviota RAWS stations indicate
northerly winds gusting to between 45 and 47 mph currently.

An update to the forecast was issued prior, and no further
updates are expected at this time.

***From Previous Discussion***

Synoptically, the area is under an upper-level trough that extends
over the western two thirds of the CONUS. By late Friday this
feature will have moved to the east and the area will be in a
broad area of higher pressure. By Sunday the peak of the high will
be located near the four-corners area, a position favorable to
transport moisture to the area during the monsoon season, and
models are indicating there is over one inch of perceptible water
heading our way with it. This will complicate the temperature
forecast and could possibly lead to some afternoon mountain
showers although they are not currently indicated. Higher heights
will remain in place throughout the week until another trough
drops down the California coast on Friday.

Temperatures are currently 2-4 degrees cooler along the coast
with stronger onshore flow along the south coast and some
lingering marine clouds. Heights are also a bit lower today. Onshore
flow ad the LAX-DAG gradient is up to 5.3 and this may keep some
south coast beaches cloudy all afternoon. Still expecting strong
NW winds coming off the waters of the Central Coast to filter
through the Western Santa Ynez range and produce low end advisory
level gusts across the western half of the SBA south coast this
afternoon and evening.

Two competing forces will be at work on the marine layer cloud
pattern on Thursday as increasing troffing aloft combines with the
eddy to promote cloud development. On the other hand sfc
gradients will be weaker than they are this morning. Short range
ensemble guidance shows that the eddy will likely be weaker and
this should tip the scales to a clearer forecast. The best chance
for low clouds will be over the southern portion of LA county and
the Central Coast. Added in more low clouds for Oxnard plain
through Burbank for tomorrow based on latest model guidance. The
offshore trends will delay and weaken the seabreeze and this will
bring 2 to 3 degrees of warming to the area with the exception of
the Central Coast which will cool due to a more robust marine
layer presence. The Summer Solstice will occur at 151 PM PDT.

Friday will kick off a fairly dramatic warm up as a large and hot
upper high over the middle of the county expands westward. The
eddy looks a little stronger so probably a little more morning
low clouds across the coasts and into some of the lower vlys. By
noon the whole area will be clear. Heights will climb to ~588 dam
and max temps will warm 5 to 10 degrees across the interior and 2
to 4 degrees across the vlys. The coasts will still be under the
marine influence and max temps will not change much. Antelope Vly
residents will see max temps range from 100 to 103 degrees or
about 8 degrees over normal, and The Paso Robles area will also
near the 100 degree mark.

Saturday should be the hottest day of the heat event as SoCal
will be covered by the western half of an upper high`s western
half. Heights will rise to ~591 dam, but the fairly strong
onshore flow will remain in place resulting in a strong
temperature gradient from the coasts to inland. The high hgts
will really smoosh the marine layer down and create a very strong
inversion. Such strong inversions are hard to break down and this
could keep clouds at the beach all day long. The marine layer
will be under 1000 ft and this will keep it out of most if not all
of the vlys. The very steep inversion will also keep min temps
much warmer than normal right at the top of the inversion so
locations near 1000 ft in elevation will see these very warm
overnight lows. Look for 3 to 6 degrees of warming on Saturday
away from the beaches. Max temps across the interior will end up 6
to 12 degrees above normal while the vlys will be 2 to 4 degrees
above normal. There is about a 10% chance of 1" PWAT reaching into
LA County by Saturday afternoon. This moisture could impact the
Saturday night to Sunday marine layer, and overnight min temps.

.LONG TERM (SUN-WED)...19/152 PM.

Blended ensemble guidance shows some cooling on Sunday which could
be a reflection of the PWAT which is up to a 60% chance of more
than 1" by Sunday afternoon. The NAEFS forecast values for PW are
in the 90th to 99th percentile over Los Angeles County basically
from Sunday through Tuesday. This moisture will affect the marine
layer and temperature forecasts. There is even a small (10-20%)
chance of afternoon showers over the mountains.

The higher heights persist over the area Monday through Wednesday
but the broader area moves to the west and this position will
turn off the moisture to the area by late Tuesday. The temperature
differences will continue with the coasts sitting near normal
with a stubborn night through morning low cloud pattern in place
and the interior seeing well above-normal temps.

&&

.AVIATION...
-- Changed Discussion --
20/0636Z.

At 0515Z at KLAX, the marine layer was 1800 feet deep. The top of
the inversion was at 4100 ft with a temperature of 20 deg C.

High confidence in TAFs for KPRB, KPMD and KWJF.

Good confidence in TAFs for KSBA, KOXR and KCMA with only a 20
percent chc of IFR cigs 11Z-17Z.

Moderate confidence in TAFs for KSMX and KSBP. There is a 25
percent chc of 1/2SM VV002 conds 10Z-15Z. VFR transition may be an
hour later than fcst.

Low confidence in TAFs for KVNY and KBUR where there is a 40
percent chc of no low clouds.

Moderate confidence in TAFs for KLAX, KSMO and KLGB. Low clouds
may arrive as early as 08Z. There is a 20 percent chc of IFR
conds. VFR transition may be an hour later than fcst.

KLAX...Moderate confidence in TAF. There is a 20 percent chance
of BKN008 cigs from 11Z-15Z. There is a 30 percent chc BKN015
conds hanging on until 18Z. Any east wind component should be less
than 5 knots.

KBUR...Low confidence in TAF. There is a 40 percent chance of no
low clouds. If low clouds do form there is a 20 percent chance of
BKN008 conds and clearing may occur as late as 18Z.
-- End Changed Discussion --

&&

.MARINE...19/956 PM.

For the Outer Waters, high confidence in current forecast. Small
Craft Advisory (SCA) winds are expected through Friday night.
Brief lulls in the winds during the mornings hours are possible.
Seas will be steep and choppy through Friday night. Conditions
are then expected to be below SCA levels Saturday through Monday
night.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. SCA winds and choppy seas will
persist through tonight, and again Thursday afternoon and
evening. For Friday through Monday night, winds and seas should
remain below SCA levels.

For the inner waters south of Point Conception, moderate
confidence in current forecast. SCA conditions were limited to far
western portions of the Santa Barbara Channel today, and the next
shift may need to end the current SCA early. Thursday, SCA winds
will likely again be confined to far western portions of the Santa
Barbara Channel. Winds should remain below SCA levels for the
waters south of Point Mugu, but will be stronger over western
portions of zone PZZ655. Local winds near SCA levels are possible
again Friday for western portions of the Santa Barbara Channel and
near Santa Cruz Island. Winds and seas should remain below SCA
levels over the weekend.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory remains in effect until 3 AM PDT Thursday for
      zones 349-351. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for zone
      650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Saturday for
      zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/Rorke/jld
AVIATION...Rorke
MARINE...Phillips/Sirard
SYNOPSIS...Hall

weather.gov/losangeles

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