661
FXUS66 KLOX 171804
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1104 AM PDT Mon Jun 17 2024

updated aviation discussion

.SYNOPSIS...17/345 AM.

Temperatures will gradually trend downwards in most areas this
upcoming week, following the warm to hot conditions this weekend.
Strong northwest to north winds will impact the Central Coast
through interior sections of Santa Barbara County to the Antelope
Valley through Monday night, with fire weather concerns.

&&

.SHORT TERM (TDY-WED)...17/1006 AM.

***UPDATE***

The Red Flag Warning for gusty northwest to north winds and low
relative humidity for the Interstate-5 corridor in northwest Los
Angeles County and the Ventura County mountains is now in effect
until 6 PM PDT Tuesday, and includes an expected wind shift to
the northeasterly direction between midnight and 6 AM PDT Tuesday.
In addition, the Red Flag Warning has been extended in areal
coverage to include the western San Gabriel Mountains and Highway
14 corridor from Midnight tonight to 6 PM PDT Tuesday, where gusty
northeast winds and relative humidity falling to the single digit
readings are expected. Please reference the Fire Weather section
below for additional information.

A Wind Advisory has also been issued in conjunction with the Red
Flag Warning for the western San Gabriel Mountains and Highway 14
corridor. Smoke from the Post Fire complex will affect a large
portion of LA County and Ventura County and will bring air-
quality impacts to many areas. The forecast has been updated to
account for the addition of smoke. Please reference the Air
Quality Alert Message for additional information.

For additional details regarding the forecast, please reference
the discussion sections below.

***From Previous Discussion***

Good eddy circulation in progress across the inner waters. The
marine layer has deepened to about 2000 ft, and clouds have surged
northward from coastal L.A. County into the valleys and into VTU
County as well. By daybreak, expect low clouds in all coastal and
most valley areas of L.A./VTU Counties with the possible exception
of the Santa Clarita Valley and some of the interior valley
locations of VTU County. There is even a chance that low clouds
will push into southeastern coastal sections of SBA County, but
that remains in doubt given the strong offshore N-S gradients
across SBA County. N of Pt. Conception, skies were clear. Some
patchy low clouds and fog may form in southern portions of the
Central Coast and the Santa Ynez Valley by morning.

Any low clouds N of Pt Conception should dissipate quickly this
morning. Across VTU/L.A. Counties, clouds will clear more slowly
today, generally by mid to late morning in the valleys and by
early afternoon across the coastal plain. High res models suggest
that clouds could linger at the beaches today, which is entirely
possible given the strength of the onshore gradient between KLAX
and KDAG both currently, and during this afternoon. Early this
morning, strong N-S gradients continued to produce strong NW-N
winds across much of southwestern SBA County, the mountains of
interior SBA County eastward thru the mtns of L.A. County,
including the I-5 Corridor, in the Antelope Valley foothills, and
in the Santa Clarita Valley. There were still some local High Wind
Warning levels gusts in the mtns and thru the I-5 Corridor, but
winds were mostly at advisory levels in these areas. Winds had
dropped below advisory levels on the Central Coast and in much of
the Antelope Valley.

An unseasonably strong and cold upper low was moving thru eastern
Oregon early this morning and it will push into Idaho and western
Montana later today. A sharpening trough axis extending S from
this low will push across the area today. Behind this trough, NW
flow aloft will increase and there will be increasing subsidence
across the area. In addition, there will be some weak cold
advection. This should translate to an increase in northwesterly
winds across the forecast area this afternoon/evening. Given this
expected bump up in winds again this afternoon/evening, will keep
the warnings and advisories as is. With lowering heights and
thicknesses, increasing onshore gradients between KLAX and KDAG,
and lots of low clouds this morning, expect several degrees of
cooling in most areas today. The exception will be across southern
SBA County, where gusty north winds may bring a few degrees of
warming late this afternoon and early this evening.

Low level flow will turn more northeasterly across L.A./VTU
Counties late tonight, with gradients between KLAX and KDAG
actually turning weakly offshore. N-S gradients across SBA County
will begin to weaken tonight. This should cause the NW winds to
drop below advisory/warning levels in most areas by late tonight.
NE flow at 850 mb will actually briefly increase to 40 to 45 kt
across portions of L.A. County/eastern VTU County Tue morning,
with a corresponding increase in surface winds, similar to a Santa
Ana wind event. Expect a period of strong to possibly advisory
level NE winds in the mtns of L.A. County and possibly eastern
Ventura County Tue morning.

In coastal and valley areas, the forecast is more challenging, and
it all depends on the marine layer tonight. At this point, it is
expected that low clouds will push into coastal and at least the
lower valley areas of L.A./VTU Counties. Where there are low
clouds, the winds would have to erode the stubborn marine layer to
allow them to surface, which is no small task. Therefore, it will
be difficult for the northeast winds to surface in coastal and
lower valley areas of L.A./VTU Counties. This will be most likely
to occur in the valleys of Ventura County and northern/western
L.A. County for a few hours Tue morning. That is far from a sure
thing, given how unusual this would be for mid June. But there is
still a chance that it will happen, and it will have to be
watched closely. Given the offshore gradients, do expect much
faster clearing of any low clouds Tue. Max temps should jump up
several degrees in most areas Tue, especially in the valleys of
L.A./VTU Counties. If winds do actually surface, it will likely
be warmer than currently forecast on Tue in coastal and valley
areas of L.A./VTU Counties. Expect little change in max temps
across SLO and SBA Counties.

A broad trough will sag southward thru the West Coast Tue night
and Wed. N-S gradients should still be strong enough for some
gusty winds during the late afternoon thru late evening hours
across SBA County Tue/Tue night, but much there should be much
less in the way of wind across L.A./VTU Counties. Expect more
widespread night thru morning low clouds and fog in coastal and
valley areas Tue night/Wed morning, with low clouds likely
returning to areas N of Pt. Conception as well. Max temps should
be down several degrees in most coastal and valley areas south of
Pt Conception Wed, with somewhat less cooling elsewhere.

.LONG TERM (THU-SUN)...17/507 AM.

The broad upper level trough will linger across the region into
Thu. Expect widespread night thru morning low clouds and fog in
most coastal/valley areas Wed night/Thu. There may be slight
warming Thu, mainly across the interior, with max temps within a
few degrees of normal in most areas.

Heights will gradually rise Fri and Sat as a large upper high over
the south central U.S. expands westward into the forecast area.
The marine layer will shrink in depth, with night thru morning
low clouds and fog likely squeezed out of most valley areas by
Fri, or at the latest Sat. Max temps should rise a few degrees
both Fri and Sat, especially away from the immediate coast, and
should be several degrees above normal in most areas by Sat. Temps
could approach 105 degrees in the Antelope Valley, and rise well
into the 90s to near 100 degrees in the valleys west of the mtns.

As the marine layer becomes more shallow late this week into the
weekend, the marine inversion will become stronger, which may make
for slower clearing near the coast, minimizing any warming there.

Expect mostly minor changes in the marine layer depth Sat night/Sun
as the upper level pattern remains rather stagnant. An increase
in onshore flow may bring slight cooling to coastal areas Sun, but
otherwise, expect little change.

&&

.AVIATION...17/1803Z.

At 17Z at KLAX, the marine layer was 2500 feet deep. The top of
the inversion was at 5400 ft with a temperature of 18 deg C.

Overall, moderate to high confidence in the 18Z TAF package. For
most sites, VFR conds are anticipated through the period. However
for KSMO, KLAX, and KLGB, cig restrictions to MVFR will be
possible until around 20Z today for KSMO and KLGB, then sometime
between 05Z-18Z tonight and Tue for all three airfields. However,
there is also a 20% chance of MVFR cigs developing tonight at
KBUR. The timing of the onset and any dissipation of the low
clouds may be off +/- an hour or two.

Gusty W to NW winds are expected this afternoon into this evening
for KSBP, KSMX, KPMD and KWJF, strongest in the deserts where
gusts over 40 knots are possible along with a 20% chance of
restricted vsbys due to blowing dust.

KLAX...Moderate to hi confidence in the 18Z TAF. VFR conds will
prevail into this evening. For tonight, moderate to hi confidence
in return of MVFR cigs between about 06Z and 20Z. The timing of
the onset and dissipation of the low clouds may be off +/- an
hour or two. An east wind component tonight up to 6 knots is
expected from about 06Z-17Z.

KBUR...Moderate to high confidence in the 18Z TAF. VFR conds are
expected thru the TAF period, except for a 20% chance of MVFR cigs
developing tonight aft about 07Z and linger into early Tue
morning.

&&

.MARINE...17/824 AM.

For the Outer Waters, high confidence in current forecast. Winds
are expected to remain at Gale force levels into late tonight.
Seas will peak today around 10 to 13 feet. For Tuesday through
Friday night, winds will subside, but remain at Small Craft
Advisory (SCA) levels. Seas drop below 10 feet Tuesday night.

For the Inner Waters north of Point Sal, moderate to high
confidence in current forecast. SCA level winds will persist
through tonight, and seas will peak today at around 9 to 12 feet.
Then Tuesday and Wednesday, there is a 60-80% chance of SCA level
winds, in the afternoons and evenings. For Thursday through
Friday night, winds and seas should remain below SCA levels.

For the Inner Waters south of Point Conception, moderate to high
confidence in current forecast. Noon today through this evening,
Gale Force winds are likely for the western Santa Barbara
Channel. Tuesday and Wednesday, there will be a 60-80% chance of
SCA level winds across the western half of the Santa Barbara
Channel, during the afternoons and evenings. South of the SBA
Channel, there is a 50-70% chance of SCA level winds for the
western and northern portions of the inner waters, this afternoon
into evening. Otherwise and elsewhere, SCA conditions are not
expected.

&&

.BEACHES...17/824 AM.

Short-period wind waves, due to gale force winds over the nearby
coastal waters, will generate elevated-to-high surf conditions and
dangerous rip currents across the beaches through early this
evening. West and northwest facing beaches will see the highest
surf.

&&

.FIRE WEATHER...17/1006 AM.

The Red Flag Warning for gusty northwest to north winds and low
relative humidity for the Interstate-5 corridor in northwest Los
Angeles County and the Ventura County mountains is now in effect
until 6 PM PDT Tuesday, and includes an expected wind shift to
the northeasterly direction between midnight and 6 AM PDT Tuesday.
In these areas -- after northwest to north winds reached the
45-60 mph range last night, with isolated locales to around 65
mph, northwest to north winds will gust to 35 to 55 mph for the
rest of today, increasing up to 60 mph tonight. Between Midnight
tonight and 6 AM PDT Tuesday, winds will shift to the north to
northeast and gust 40 to 60 mph and then gradually decrease to 25
to 45 mph on Tuesday. For today, minimum afternoon relative
humidity values are expected to range from 15 to 25 percent,
locally as low as 10 percent in downslope-flow favored areas. Only
poor to moderate overnight recovery to around 25 to 45 percent is
expected tonight. Relative humidity will fall to the single digit
readings through the day Tuesday, as a very dry air mass
overspreads the region.

In addition, the Red Flag Warning has been extended in areal
coverage to include the western San Gabriel Mountains and Highway
14 corridor from Midnight tonight to 6 PM PDT Tuesday, where gusty
northeast winds and relative humidity falling to the single digit
readings are expected. In these areas -- after occasionally gusty
northwest to north winds through this evening, winds will shift
to the northeast after midnight with speeds of 20 to 30 mph and
gusts to 45 mph, locally 55 mph. Relative humidity is expected to
be 20 to 45 percent tonight falling to 5 to 10 percent Tuesday.

While live fuel moisture readings are still quite high, ample
fuel loading of dead fuels is likely contributing to the extreme
fire behavior with the Post Fire complex, and increasingly
favorable meteorological conditions are expected to foster further
growth and intensification of this fire complex or any other
fires developing in the Red Flag Warning areas. After 6 PM PDT
Tuesday, decreasing winds and relative humidity recovery Tuesday
night will decrease the critical fire-weather risk.

Elsewhere across the region, the wind shift to the northeast
tonight into early Tuesday morning will come with the passage of
a midlevel trough to the north of the forecast area, and also
with the continued consolidation and deepening of a Catalina Eddy.
The same Catalina Eddy driving this Santa Ana wind-type pattern
will also draw a thickening marine layer northward across the
coasts and coastal valleys including much of the LA Basin. This
marine layer will greatly temper relative humidity reductions at
the lower elevations, possibly extending as far as the Santa
Monica and Santa Susana Mountains and especially the nearby
foothills, and the Santa Clarita Valley. As a result, there is a
lot of uncertainty regarding how far the northeasterly wind surge
will extend off the San Gabriels and the Ventura County Mountains.
Nevertheless, the potential will exist for this wind surge and
accompanying deep mixing with very low relative humidity to bring
critical fire-weather conditions much farther toward the coast, in
a Santa Ana wind-type pattern. Areas south of the western San
Gabriels and Ventura County Mountains toward the coast will be
closely monitored for possible expansions of Red Flag Warnings
(30% chance of further extension), and elevated to brief critical
conditions will be a definitive possibility in these areas
(60-80% chance).

Elsewhere across the region, the combination of gusty winds, warm
temperatures, and low relative humidity will continue to bring
elevated fire-weather conditions across the Santa Barbara County
interior mountains today and Tuesday.

Areawide on Wednesday, weakening winds will be lessening the
fire-weather risk while dry conditions persist across the
interior.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect until 7 PM PDT this evening
      for zones 87-340-346-362-366. (See LAXCFWLOX).
     Wind Advisory remains in effect until 3 AM PDT Tuesday for
      zones 88-340-341-346-347-352-353-356-383. (See LAXNPWLOX).
     High Wind Warning remains in effect until 3 AM PDT Tuesday
      for zones 349-351-376>378-381. (See LAXNPWLOX).
     High Surf Advisory in effect until 7 PM PDT this evening for
      zone 354. (See LAXCFWLOX).
     Wind Advisory in effect from midnight tonight to 6 PM PDT
      Tuesday for zone 379. (See LAXNPWLOX).
     Red Flag Warning in effect until 6 PM PDT Tuesday for zones
      376>378. (See LAXRFWLOX).
     Red Flag Warning in effect from midnight tonight to 6 PM PDT
      Tuesday for zone 379. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Tuesday for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until noon PDT today for zone
      650. (See LAXMWWLOX).
     Gale Warning in effect until 9 PM PDT this evening for zone
      650. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 PM this afternoon to 9
      PM PDT this evening for zone 655. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PDT Tuesday for zones
      670-673-676. (See LAXMWWLOX).

&&

$$

PUBLIC...Cohen/DB
AVIATION...Sirard
MARINE...Sirard/Schoenfeld
BEACHES...Sirard/Schoenfeld
FIRE...Cohen/Gomberg
SYNOPSIS...Cohen/RS

weather.gov/losangeles

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