Acton Weather

000
FXUS66 KLOX 210903
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
203 AM PDT Mon Jun 21 2021

.SYNOPSIS...20/830 PM.

Temperatures across the region will cool to near normal by the
middle of next week as low clouds and fog push farther inland with
gusty onshore winds. Some warming is forecast by late next week.

&&

.SHORT TERM (TDY-WED)...20/813 PM.

***UPDATE***

The marine inversion early this evening ranged from around 1200
ft deep at VBG to near 1400 ft deep at LAX. Strong onshore
gradients to the N and E has kept low clouds along many coastal
areas into early this evening, with slow inland intrusion noted by
sunset. The low clouds are expected to continue expanding inland
overnight eventually to some of the adjacent vlys by late tonight.
Areas of fog will likely accompany the low clouds as well.
Otherwise and elsewhere, clear skies are expected overnight. Gusty
SW winds across the Antelope Vly this evening will diminish some
overnight.

***From Previous Discussion***

Overall, 12Z models in good synoptic agreement through the short
term period. At upper levels, a cutoff low develops off the
Central CA coast. Near the surface, onshore flow will continue
with some slight weakening of the gradients each day.

Forecast-wise, nothing too significant anticipated through the
period. The combination of lowering H5 heights, in association
with the upper low, and continued onshore flow will allow for the
marine inversion to gradually deepen and stratus/fog should be
able to push into the coastal valleys each night. Each day, the
stratus should dissipate by late morning although some beach areas
will likely remain cloudy through the afternoon hours. Other than
the stratus, skies should remain mostly clear although some partly
cloudy skies are not out of the question, given the southwesterly
flow around the upper low.

As for temperatures, will expect some widespread cooling on Monday
with minor day-to-day fluctuations Tuesday/Wednesday (based on
the whims of the marine influence). Temperatures by
Tuesday/Wednesday will be a couple degrees above seasonal normals
for most areas. However, interior San Luis Obispo county will have
high temperatures a few degrees below seasonal normals.

As for winds, the continued onshore gradients will generate gusty
southwesterly winds across interior sections each afternoon and
evening. The strongest winds will be this afternoon and evening
(with local gusts up to 50 MPH possible across the Antelope Valley
foothills) with weaker winds anticipated Tuesday/Wednesday

.LONG TERM (THU-SUN)...20/149 PM.

For the extended, models start to diverge synoptically, leading to
only moderate confidence in the forecast. The GFS builds an
impressive ridge over the state Thursday through Saturday before
weakening the ridge on Sunday. The ECMWF keeps the upper low
spinning along the Central CA coast Thursday through Saturday
before building a ridge over the area on Sunday. As a third model
possibly, the Canadian follows along the ECMWF idea.

Given the above model difference, moderate confidence levels
exist. The GFS solution would indicate another round significant
inland heat for the weekend with limited coastal stratus/fog.
Conversely, the ECMWF/Canadian solutions would indicate somewhat
cooler conditions for the weekend with more extensive marine layer
stratus/fog. With the afternoon forecast, will toe a middle ground
between the possibilities indicating warming trend through the
period with stratus/fog remaining in play for the coastal plain
and coastal valleys. Hopefully, future model runs will bring more
consensus with the weekend forecast.

&&

.AVIATION...21/0607Z.

At 04Z, the marine layer depth was around 1550 feet deep at KLAX.
The top of the inversion was near 4700 feet with a temperature of
around 26 degrees Celsius.

Moderate confidence in the current forecast for coastal and
valley terminals. High confidence in the current forecast for
desert terminals. IFR to MVFR conditions will spread into coastal
and valley terminals through 13Z. There is a moderate-to-high (50-70
percent) chance of LIFR conditions for terminals north of KNTD
between 08Z and 16Z. There is a moderate (30-50 percent) chance
of slightly better clearing on Monday afternoon. An early arrival
of IFR conditions should be expected on Monday evening.

KLAX...Higher confidence in flight categories. Less confidence in
timing. IFR conditions could arrive as soon as 08Z, or as late as
11Z. There is a 50 percent chance of VFR conditions developing as
soon as 19Z, or as late 21Z. On Monday evening, IFR conditions
could arrive as soon as 02Z, or as late 05Z.

KBUR...There is a 50 percent chance LIFR condition as soon as 10Z,
or as late as 13Z. VFR conditions should develop as soon as 15Z,
or as late as 17Z.

&&

.MARINE...21/152 AM.

Patchy dense fog at times will continue across the coastal waters
through at least mid week, and possibly through late this week.
The poorest visibility and most widely spread conditions will
occur in the overnight and morning hours.

Across the Outer Waters... Small Craft Advisory (SCA) level winds
from near Point Conception southward are likely across the outer
waters Thursday through Saturday. Otherwise, conditions will
remain below SCA level.

Across the Inner Waters north of Point Sal... SCA level winds are
likely on Thursday. Otherwise, conditions will remain below SCA
levels.

Across the Inner Waters south of Point Conception... SCA level
winds through the Santa Barbara Channel are likely Thursday and
Friday. Otherwise there is a 30% chance for SCA level gusts across
the western portion Tue/Wed evenings. Elsewhere, conditions will
remain below SCA level.

&&

.BEACHES...20/827 PM.

Elevated surf and high astronomical tides associated with a full
moon will combine to create minor tidal flow during the time of
high tide, beginning Monday and continuing through next Saturday.
The highest daily tide between 7.0 and 7.5 feet will occur during
the evening hours. A south swell of 3 feet with a period of 18
seconds will arrive on Monday.

&&

.FIRE WEATHER...20/459 PM.

The strong upper level high pressure system that brought the long
duration heat wave last week is beginning to break down today,
with an approaching upper level low pressure system off the coast
that will draw closer to the region Monday through at least
Wednesday. Despite a few degrees of cooling today, most interior
areas still remain hot with highs ranging between 95 and 106
degrees. The cooling trend will become more pronounced across
interior areas Monday through Wednesday as the upper low draws
closer to the region. On Monday, highs will be in the 90s across
the interior, lowering to the mid 80s to lower 90s by Wednesday.
Minimum humidities between 7 and 15 percent will be common across
the interior today and Monday. Onshore winds will gusts between 25
and 40 mph across the interior through Monday, strongest during
the afternoon and evening hours. Isolated gusts of 40 to 50 mph
will be likely in the foothills of the Antelope Valley. The
combination of these gusty onshore winds, persistent hot and dry
conditions, and very dry fuels, will maintain elevated to brief
critical fire weather conditions through Monday across the
interior.

Mid level moisture is expected to be drawn into Los Angeles
county and points south and east Tuesday through Wednesday. With
limited instability, it will most likely just bring some mid level
clouds to Los Angeles county, but will continue to monitor
closely for any convective threat. Another warming and drying
trend is possible Friday through Sunday, with a potential return
of elevated fire weather conditions across the interior.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from Tuesday evening
      through Thursday evening for zones 40-41. (See LAXCFWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Hot temperatures are expected to return, and especially for inland
areas, starting Saturday and will likely continue for at least
several days beyond that. This will likely bring heat-related
illness concerns to inland areas.

&&

$$

PUBLIC...Thompson/Sirard
AVIATION...Hall
MARINE...CK
BEACHES...Sweet/Kj
FIRE...Gomberg
SYNOPSIS...Kj

weather.gov/losangeles