296
FXUS66 KLOX 300657
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1157 PM PDT Sun Sep 29 2024

.SYNOPSIS...29/923 PM.

A warming trend will occur through at least Wednesday as a ridge
of high pressure builds over the region. Dangerous hot
temperatures could develop in some valley and mountains
locales on Tuesday and Wednesday when the hottest weather is
forecast. A slight cool down is expected for late this week and
into the weekend, but temperatures will remain above normal away
from the coast. Except for some coastal low clouds and fog at
times during the night and morning hours, skies will continue to
be mostly clear for much of the forecast period.

&&

.SHORT TERM (SUN-WED)...29/956 PM.

The latest fog product shows low clouds and fog less entrenched
than the past several nights. The low cloud field is likely
starting to feel some of the effects of the trough being nudged
to the south and west by the upper-level ridge building in. The
latest AMDAR soundings and earlier balloon soundings from KVBG
indicate a shallow marine layer depth in place. Any low clouds and
fog that develop tonight could turn dense. The latest high-
resolution indicate a moderate to high chance of dense fog
overnight tonight and into Monday morning, but the main question
is if the modeled extent of the low cloud field will match up with
reality.

An upper-ridge of high pressure over the eastern Pacific Ocean
will merge with another ridge over the Sonoran Desert, then the
larger high pressure system force the trough vacate the region by
Tuesday. A warming trend will take shape through Monday, then turn
significantly warmer, especially away from the coast for Tuesday
and Wednesday. The latest deterministic solutions indicate
onshore flow weakening and turning offshore for through the first
half of the week, but a bias correction of the modeled pressure
gradients would suggest weak onshore flow remaining in place on
Tuesday. It will be a tough forecast at the coast for middle
portion of the week depending upon what unfolds as there is some
offshore low-level component. Higher confidence exists in the
hottest temperatures developing inland and well away from the
coast. Dangerously hot conditions could develop between Tuesday
and Wednesday with the highest likelihood across the valleys,
foothills, and mountains. A few tweaks were made to temperatures
for Tuesday and Wednesday, mainly to warm overnight low
temperatures slightly across the region.

The latest update has been issued to add some tweaks to low clouds
and fog and tweak temperatures over the next several days.

***From Previous Discussion***

A weak H5 upper level low (584 dam) was centered over nrn L.A.
County early this afternoon. This upper level low is forecast to
drift S of the area tonight thru Mon then largely dissipate Mon
night. Upper level ridging is expected to build into central CA
Mon thru Tue then expand some into srn CA for Wed. The forecast
area will be on the southern periphery of this upper level ridging
on Tue with H5 heights increasing to around 591 dam then H5
heights should lower slightly to 588-589 dam on Wed. A broad
easterly flow aloft can be expected over the region Tue thru Wed.

There will be increasing offshore pressure gradients tonight thru
Mon with little change thru Tue. A slight onshore trend is
expected Tue night and Wed, but pressure gradients will remain
rather weak. In addition, significant warming is expected in the
boundary layer up to at least 950 mb thru Mon, also helping to
shrink the marine inversion to about 600 ft or less. There will
likely be a surface-based inversion along the Central Coast by
Mon morning.

Low clouds and fog are expected again tonight into Mon morning
but there will be much less coverage, with just the immediate
Central Coast and some coastal areas S of Point Conception being
affected. The night and morning low clouds and fog should then be
confined to the immediate L.A. County coast Mon night through Wed
morning. Any low clouds will continue to be accompanied by patchy
dense fog thanks to the shallow marine inversion.

Otherwise, mostly clear skies will cover the forecast area
tonight through Wed, except some hi level clouds may move into the
area from the E by Tue. Breezy onshore flow is expected afternoon
and evening hours, with breezy offshore flow night and morning
hours Mon and Tue especially for the foothills including the Santa
Lucia Mtns in SLO County. There will also be gusty NW to N
sundowner winds in SW SBA County this evening, with local gusts
close to Advisory levels possible.

The offshore pressure gradients will help to bring significant
warming to the forecast area for Mon thru Wed. By Tue afternoon
temps will be 10-20 deg above seasonal norms with little change
expected Wed. This combined with overnight lows quite a bit above
normal will bring heat concerns to the area especially Tue and
Wed. As a result, Heat Advisories and Excessive Heat Watches will
be issued with the afternoon zone package for the vlys, mtns and
Antelope Vly foothills. The hottest temps Tue and Wed should be in
the 95-105 degree range for much of this area, altho isolated max
temps could be a degree or two warmer.

.LONG TERM (THU-SUN)...29/221 PM.

The EC and GFS mean ensembles were is somewhat good agreement for
the extended period, with the deterministic showing more
differences especially later in the fcst period. Overall, the mean
ensembles indicate that upper level ridging will prevail over the
forecast area Thu thru Sun, altho the EC continues to be much
more pronounced with the ridging.

Pressure gradients should turn more onshore for Thu with slightly
cooler temps expected altho remaining quite a bit above normal.
It may be warm enough to extend Heat Advisories for some inland
areas. For Fri thru Sun, temps will again cool slightly but
continue to be 5-15 deg above normal away from the immediate
coast. However, the EC ensembles, with significant upper level
ridging next weekend along with persistent offshore pressure
gradients, indicates temps could be much warmer than is currently
forecast.

The marine inversion should remain rather shallow thru the
extended period. As far as any night and morning low clouds and
fog, they should be minimal Thu and Fri, then possibly affect
mainly portions of the Central Coast and VTU/L.A. County coasts
Sat and Sun. Otherwise, mostly clear skies will continue except
for the chance of some high clouds moving in at times.

&&

.AVIATION...30/0656Z.

At 0505Z at KLAX, the marine layer was 1100 feet deep. The top of
the inversion was at 2000 feet with a temperature near of 25 C.

High confidence in desert TAFs and KPRB.

Moderate confidence in TAFs for KBUR and KVNY with a 25 percent
chc of LIFR conds 11Z-15Z.

Moderate confidence in coastal TAFs. Flight cat changes could be
off by +/- 90 minutes. Cigs could be off by +/- 200 ft. Low
confidence in timing of return of low clouds Monday evening.

KLAX...Moderate confidence in TAF. There is a 25 percent chc that
vis will remain AOB 3SM. Good confidence in VFR conds arriving
between 17Z and 19Z with a better chc of it occuring between 17Z
and 18Z. Low confidence in low cloud arrival timing Monday
evening.

KBUR...Moderate confidence in TAF through 15Z then high
confidence. There is a 25 percent chc of 2SM BR OVC003 conds
11Z-15Z.

&&

.MARINE...29/932 PM.

In the Outer Waters (Central Coast to San Nicolas Island), current
Small Craft Advisory (SCA) looks on track with SCA level winds
lasting into early tomorrow morning. Then, moderate confidence
in sub advisory winds Monday through Wednesday morning. There is a
10% chance for SCA level winds for the waters north of Point Sal
Monday night, and a 20-30% chance Tuesday night into Wednesday
focused on the waters south of Point Conception. There is a
better chance (40%) for SCA level winds during the afternoon thru
late evening hours Wed and Thurs, and another chance Friday
afternoon thru evening for the entire Outer Waters.

For the inner waters north of Pt Sal, high confidence in SCA winds
ending tonight. There is a 20-30% chance of SCA level winds
Wed/Thur afternoon and evening, and possibly also Friday.

For the Santa Barbara Channel, and inner waters off the LA and OC
coasts, conditions are expected to be below SCA levels through
Tuesday morning. There is a 20-50% chance of SCA level winds in
the Santa Barbara Channel and southern inner waters Tuesday
afternoon and evening. Wednesday may also be windy for the waters
south of Point Mugu, but will more than likely remain below SCA
levels.

&&

.FIRE WEATHER...29/1000 PM.

Warm and dry conditions are expected to continue away from the
coast through Wednesday. High temperatures between 90 and 100
degrees will be common through Monday, then increasing to 93 to
106 degrees on Tuesday and Wednesday. The warmer temperatures are
expected to spread into many of the valleys and coastal foothills
Monday through Wednesday as a weak offshore wind influence
develops. Dry air with humidity levels between 5 and 20 percent
over the interior valleys, mountains, and deserts earlier today will
expand into the coastal valleys and foothills through Wednesday.
By Monday and Tuesday, the winds will become more northeast to
southeast over the interior, likely expanding into other areas
such as the Santa Lucias, Santa Susana mountains, and portions of
the LA/Ventura county valleys. On Monday, southeast winds gusting
20 to 30 mph will be common across the San Gabriels and Ventura
county mountains, with isolated gusts as high as 35 mph across
ridgetops. Mixing heights are expected to peak between 8,000 and
13,000 feet this afternoon across the interior, then increase to
between 10,000 and 17,000 feet Monday through Wednesday due to
increasing temperatures and instability. As a result, there will
be an increased risk of large vertical plume growth of new fires
across the interior Monday through Wednesday. mainly in the
mountains and foothills. The combination of hot temperatures, very
low humidities, locally gusty winds and potential for large
vertical plume growth will result in elevated to brief critical
fire weather conditions today in the mountains, expanding into the
Santa Lucias, Santa Susana mountains, and portions of the valleys
Monday into Wednesday. A slight cooling trend could develop
Thursday into Friday, then a possible offshore wind influence may
reinforce very warm and dry conditions next weekend.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory in effect from 11 AM Tuesday to 8 PM PDT
      Wednesday for zones
      38-342>345-348-351-356>358-369>371-374>377-380-382-548. (See LAXNPWLOX).
     Excessive Heat Watch in effect from Tuesday morning through
      Wednesday evening for zones 88-352-353-372-373-378-379-381. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until midnight PDT tonight for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT Monday for
      zones 670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Hall/Sirard
AVIATION...Rorke
MARINE...Phillips
FIRE...Gomberg/Hall
SYNOPSIS...Hall

weather.gov/losangeles

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