Walnut, California Weather Forecast Discussion

734
FXUS66 KLOX 090642
AFDLOX
Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1142 PM PDT Mon Sep 8 2025
.SYNOPSIS...08/724 PM.
A significant cooling trend will continue through Thursday as a
low pressure system approaches northern California. This will
result in much cooler temperatures across Southwest California.
In addition, marine layer clouds will expand across coastal areas
and well inland over the valleys with morning drizzle possible
through Friday. A small warming trend is then expected Friday into
the weekend.
&&
.SHORT TERM (MON-THU)...08/838 PM.
***UPDATE***
A 569 MB upper level low is making landfall along the northern
California/Oregon border this evening, with a trough extending
southward along the California coast. The upper low center will
slowly migrate to the southeast through Thursday before shifting
further east late Thursday into Friday. The main effect over
Southwest California will be a significant cooling trend, with
highs on Tuesday lowering by 5 to 10 degrees in most areas, and
even as much as 12 to 15 degrees cooler in some areas.
As heights aloft lower, a deepening marine layer and increasing
onshore flow are expected. In addition, the NAM12 shows a fairly
good eddy forming over the San Pedro Channel and LA Basin
tonight into Tuesday morning. With these factors in place, expect
to see low clouds moving over all coastal areas (except the SBA
South Coast) and into the coastal valleys, especially the San
Gabriel and San Fernando Valleys, and to a lesser extent into the
Ventura Valleys (and the Salinas and Santa Ynez Valleys).
Over southwestern Santa Barbara County, Advisory level north to
northwest winds are gusting between 40 to 45 mph this evening,
over the South Coast and western Santa Ynez foothills. These winds
are expected to persist through 6 AM Tuesday.
Additionally, the onshore surface pressure gradient from LAX to
Daggett will trend more strongly onshore through the next 24
hours, from 6.4 MB to 7.6 MB by 00Z (5 PM). As a result, gusty
southwest winds are expected to rise to strong Advisory levels
Tuesday afternoon and evening across the interior passes and
canyons of the San Gabriel Mountains and the Antelope Valley
Foothills. Wind gusts of 45 to 50 mph are expected, with isolated
gusts up to 55 mph possible.
***From Previous Discussion***
With lowering heights and a deepening marine layer already
starting, high temperatures have already seen cooling compared to
yesterday. With onshore flow strengthening to 7mb on Tuesday,
further cooling is expected across the region, with highs dropping
by 2 to 4 degrees across the coast and 5 to 10 locally 12 degrees
across the valleys. On Wednesday, another 2 to 5 degrees of
cooling will be possible as well. With the temperatures dropping
each day, the valleys (who have high temps reaching the upper 80s
to mid 90s today) will end up in the upper 70s to mid 80s by
Wednesday.
The marine layer will deepen Tuesday and up to 3000 ft on Wednesday
thanks to the falling heights and continued onshore flow (for the
most part). Drizzle will be possible during the late night to
morning periods as early as Tuesday and continuing through Friday.
However, additional lift provided by the trough (vort lobe moving
through) will allow the marine layer to further deepen on
Wednesday, meaning the best chances for drizzle will be Wednesday
morning and some measurable light rain might even occur across the
coasts to the coastal slopes/foothills. Low clouds and fog will
likely push into the valleys overnight toward dawn (though the
lack of clouds south of Point Conception the last few nights does
introduce uncertainty in the forecast for those areas, and there`s
a 30% chance Ventura and Los Angeles remain cloud free tonight
into tomorrow). However for southern Santa Barbara, gusty
advisory level sundowner winds will affect the SW portion of Santa
Barbara County during the evening and overnight hours again Tuesday
night, which will help keep the clouds away from that area.
In addition to the sundowner winds, gusty SW winds during the
afternoon and evening hours for the San Gabriel Mountains and
Antelope Valley this week. These winds will peak on Tuesday, and
a wind advisory was issued for the San Gabriel Mountains and the
entire Antelope Valley foothill locations Tuesday morning through
mid-evening. These areas could see wind gusts up to 50 mph, and
locally up to 55 mph across higher elevations. Otherwise, most
coastal locations will see breezier onshore winds than a typical
day, but still remain well under advisory levels.
On Thursday, once again there will be plenty of morning low
clouds and maybe some more drizzle. Max temps will continue to
run 3 to 6 degrees below normal at the coasts and 5 to 10 degrees
below normal in the valleys.
.LONG TERM (FRI-MON)...08/256 PM.
The upper low will move out on Friday, with a weak ridge building
in behind the trough over SW California on Saturday. Like a
broken record repeating itself, Friday will start out cloudy
across the coasts and valleys, but the clearing will be a little
faster. Slightly higher heights and slightly faster clearing will
bring 1 or 2 degrees of warming to the area.
The weak ridge on Saturday will bring 500 mb heights up to 585
dam, and while there will not be much change in the onshore flow,
the higher heights may shrink the marine layer enough to keep it
out of most of the valleys with clearing of clouds noticeably
faster across the region compared to earlier in the week. Max
temps will respond with 2 to 4 degrees of warming, bringing a few
readings in the lower 90s to the warmest valley locations.
Despite the 2 days of warming, max temps will still be around 2
to 4 locally 6 degrees below normal.
Not much change slated for Sunday as another weak trough starts to
move into the region, but the heights really don`t change much.
&&
.AVIATION...09/0627Z.
At 0423Z at KLAX, the marine layer was 800 feet deep. The top of
the marine inversion was near 1800 ft with a temperature 23 C.
Good confidence in TAFs for KPMD and KWJF.
Moderate confidence in the remaining TAFs. Timing of flight cat
changes could be off by as much as two hours. Cig hgt could be off
by +/- 300 ft. There is a 30 percent chc of 1/4SM FG conds at KSMX
and KSBP 09Z-15Z. There is a 30 percent chc of no low clouds at
KBUR and KVNY.
KLAX...Moderate confidence in TAF. Cigs could arrive as late as
10Z. There is a 30 percent chc of VFR conds arriving at 1830Z.
Good confidence that any east wind component will be under 6 kt.
KBUR...Moderate confidence in TAF. There is a 30 percent chc of no
low clouds.
&&
.MARINE...08/852 PM.
For the Outer Waters south of Point Sal, Small Craft Advisory
(SCA) northwest winds are expected through at least Saturday,
with brief lulls possible during the morning hours. North of
Point Sal (including the nearshore waters) there is a slight
chance of SCA winds Tuesday afternoon/evening, becoming likely by
late Wednesay evening, lasting much of the time through Saturday.
Seas will peak at around 6 to 8 feet Thursday through the weekend.
South of Point Conception, areas of SCA level west winds are
likely each afternoon and evening through Wednesday night,
especially across the Santa Barbara Channel, near Point Dume, and
across the San Pedro Channel. There may only be a brief break in
SCA winds Tuesday morning, so will consider extending the current
SCA through Tuesday night. Gusts could approach 30 kt for the
Santa Barbara Channel on Tuesday/Tuesday night. Choppy wind
driven seas are likely to peak Wednesday night at around 5 to 7
feet. Chances for SCA winds (50 percent) will remain through
Saturday.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory remains in effect until 5 AM PDT Tuesday for
      zones 349-351. (See LAXNPWLOX).
     Wind Advisory in effect from 10 AM to 9 PM PDT Tuesday for
      zone 379. (See LAXNPWLOX).
     Wind Advisory remains in effect from 10 AM to 9 PM PDT
      Tuesday for zones 380>382. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 5 AM PDT Tuesday for
      zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Wednesday for
      zones 673-676. (See LAXMWWLOX).
&&
$$
PUBLIC...Lund/Ciliberti
AVIATION...Rorke
MARINE...Phillips/Schoenfeld
SYNOPSIS...CC/KL
weather.gov/losangeles
Experimental Graphical Hazardous Weather Outlook at:
https://www.weather.gov/erh/ghwo?wfo=lox

NWS LOX Office Area Forecast Discussion