Area Forecast Discussion
Issued by NWS Los Angeles, CA

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657
FXUS66 KLOX 260607
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1107 PM PDT MON JUL 25 2016

...Aviation discussion updated...

.SYNOPSIS...

Slightly above normal temperatures will continue this week, with
some warming for inland areas as high pressure strengthens. A
shallow marine layer will likely continue to result in night
through morning low clouds and fog for some coastal areas.
Monsoonal moisture may bring isolated thunderstorms to some
mountain and desert locations from Friday into the weekend.

&&

.UPDATE...

The latest infrared imagery shows high pressure building into the
Four Corners Region this evening. Southeast flow aloft is
developing over the area around the clockwise circulation of the
high pressure center. Humidity will tick up slightly over the
coming days, but a very warm air mass will remain in place through
the remainder of the week. Heat index values will creep up to
near critical levels on Tuesday across the valleys of Los Angeles
and Ventura Counties and into the Los Angeles and Ventura County
Mountains. In addition, monsoonal moisture moving north, primarily
in the form of middle and high level moisture could bring an
isolated shower and/or thunderstorm to the highest elevations of
the San Gabriel Range on Tuesday afternoon and evening. PoPs were
increased slightly for the San Gabriel Range for Tuesday.

The marine layer will remain rather shallow and any stratus clouds
will remain confined to the coast and lower valleys. Lower
confidence exists in stratus coverage south of Point Conception
as middle and high level cloudiness spreads over the area, but at
least patchy coverage should occur. Patchy dense fog has been
reported along the Central Coast this evening, which is not
surprising due to shallow marine layer in place. The highest
likelihood of dense fog exists for areas north of Point
Conception. Any issuance of a dense fog advisory will be tabled
until more information arrives.

&&

.SHORT TERM...(TDY-WED)

Main short term weather concern continues to be the gusty onshore
winds, low humidities, and hot temperatures across interior
sections where elevated fire danger exists. The greatest fire
weather danger continues to be across the Santa Clarita Valley
and Los Angeles County mountains (including the Sand Fire) as well
as the Antelope Valley where brief critical fire weather
conditions are expected this afternoon. Onshore gradients from
LAX- Daggett are expected to peak between +6 and + 7 mb later this
afternoon which is slightly stronger than yesterday. As a result,
expecting wind gusts up to 30 mph in the Santa Clarita Valley, up
to 35 mph in the Los Angeles county mountains, and up to 40 mph in
the Antelope Valley (except isolated gusts up to 50 mph in the
foothills). As of 1 pm, already seeing wind gusts up to 43 mph at
Lake Palmdale. Similar gusty onshore winds and low humidities
expected across these same areas each afternoon and evening during
the next few days. The south-southwest flow in the atmosphere
will continue to transport most of the smoke from the Sand Fire
into the mountains and Antelope Valley this afternoon, but could
once again settle more over the valleys by tonight.

The marine layer depth this morning ranged from around 800 feet
across the LA Basin to around 1200 feet on the 12z Vandenberg
sounding. Low clouds pulled away early for areas south of Point
Conception, allowing for a good warmup across Los Angeles and
Ventura counties. Meanwhile, low clouds and fog continue to hug
the Central Coast this afternoon. As the upper level ridge of high
pressure from four corners area expands westward over Southern
California, look for marine layer to remain rather shallow the
next few days. This will likely keep low clouds and fog confined
to coastal areas and bring warming across inland areas.

As the upper level ridge of high pressure builds over the area
on Tuesday and Wednesday, the upper level flow will become more
easterly. This will translate into triple digit heat becoming
more widespread across the warmest valleys, lower mountains, and
Antelope Valley. While it is unlikely that heat index levels will
reach critical warning thresholds, the prolonged period of
hotter temperatures and slight increase in humidity  will likely
bring some heat impacts during the next few days for inland
areas. Of greatest concern will be the Antelope Valley where high
temperatures between 104 and 108 degrees are expected Tuesday
through Thursday.

Some mid level monsoonal moisture continues to transport
northwestward from Baja Mexico into San Diego county this
afternoon. Earlier this morning, a weak upper level wave
associated with this moisture produced scattered showers and
isolated thunderstorms but that has dissipated in the past few
hours. The NAM model is still showing some southeast upper level
flow between 10,000 and 20,000 feet that will bring some mid and
high level clouds across Los Angeles county today. While not
likely, there is a very slight chance of showers or thunderstorms
this afternoon and evening across eastern Los Angeles county and
the adjacent coastal waters (10 percent probability). If any
storms were to develop...the main threats would be dry lightning
strikes and gusty downdraft winds due to the drier low levels of
the atmosphere. While the upper level pattern remains favorable
for monsoonal moisture into Southwest California due to the east
to southeast upper level flow, the models are not picking up much
in the way of lower level moisture or instability. As a result,
will just keep with 10 percent chance of afternoon/evening
thunderstorms for mountains and deserts next few afternoons and
evenings. One interesting feature to monitor is the NAM picking up
some vorticity energy that gets embedded into the easterly flow on
Tuesday night. This is the type of feature that can trigger
nocturnal elevated convection, but will just keep with 10 percent
probability for now until we see a better feed of moisture down to
at least the 600 mb level.

.LONG TERM...(THU-SUN)

There are still some discrpencies in how the long range models
are handling the monsoon moistsure in the extended period. For now
will keep with a slight chance of afternoon/evening tstms for
LA/Ventura county mountains and Antelope Valley. Otherwise
slightly above normal temperatures will persist for inland areas.
The coastal night through morning cloud pattern will also
continue.

&&

.AVIATION...26/06Z.

At 04z at KLAX... the inversion was about 2100 feet deep. The top
of the inversion was located near 2700 feet with a temperature
around 29 degrees Celsius.

Overall... Moderate confidence in the 06z TAFs. The reduced
confidence is primarily due to the uncertainty in the timing...
location... and intensity of the marine intrusion. LIFR/IFR
conditions have already moved into many coastal locations
especially along the Central Coast. Low clouds and fog should get
into most coastal locations by 12z then diminish in the 16z-18z
time frame. There is a twenty percent chance of the marine
intrusion affecting KBUR and/or KVNY. There is a forty percent
chance of IFR/MVFR conditions due to smoke in the forecast period
at KPMD and/or KWJF. Otherwise and elsewhere VFR conditions will
prevail.

KLAX... Moderate confidence in the 06z TAF. The reduced confidence
is primarily due to the uncertainty in the timing and intensity
of the marine intrusion. LIFR/IFR conditions will diminish in the
16z-18z time frame. Otherwise VFR conditions will prevail.

KBUR... Moderate confidence in the 06z TAF. The reduced
confidence is primarily due to the uncertainty in the timing and
intensity of the marine intrusion. There is a twenty percent
chance of the marine intrusion affecting KBUR. Smoke from the Sand
Fire will likely remain north of the airport. Otherwise VFR
conditions will prevail.

&&

.MARINE...25/130 PM.

High confidence in winds and seas remaining below Small Craft
Advisory through at least Tuesday. West to northwest winds will
start increasing some on Wednesday and Thursday with Small Craft
Advisory Likely in some areas. Moderate confidence in dense fog
with visibility of 1 NM or less will affect much of the coastal
waters through through Tuesday or Wednesday.

&&

.FIRE WEATHER...25/355 PM.

Widespread relative humidities currently in the teens with
locally gusty onshore winds have led to some locations across the
LA Mountains into the Antelope Valley including the Sand Fire to
experience several hours of critical fire weather conditions.
However, the duration criteria for red flag conditions is not
expected to be met. The elevated fire weather conditions across
these areas as well as other interior mountains and valleys will
persist each afternoon and early evening through Thursday across
due to low relative humidities and locally gusty southwest to
northwest winds. Elevated fire weather concerns extend into the
upcoming week weekend due to the possibility of afternoon
thunderstorms. Monsoon moisture aloft is expected to gradually
deepen over the area through the week with increased moisture
leading to an higher relative humidities at the surface beginning
Friday. The increasing moisture will bring the slight chance of
thunderstorms over the Los Angeles and Ventura county mountains
and Antelope Valley each afternoon Friday through Sunday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Marine Weather Statement in effect until 10 AM PDT this morning.
      (See LAXMWSLOX).

&&
$$

PUBLIC...Hall/Gomberg
FIRE...Munroe
AVIATION...KJ
MARINE...Smith
SYNOPSIS...Sukup

weather.gov/losangeles



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