Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 212351 AAA

451 PM PDT Fri Jul 21 2017

...updated aviation discussion...

Marine layer clouds will spread into the coast and locally into
the valleys during the night and morning hours into next week. An
upper level ridge of high pressure will bring temperatures around
normal to slightly above normal through the weekend, with a
slight cooling for next week. A moist monsoonal flow will bring
possible showers and thunderstorms in the afternoons and evenings
for the deserts and mountains Monday and Tuesday.



Strong high pressure aloft centered over the Plains States will
continue to assert an influence on the weather pattern across the
region through Saturday. Despite model solutions indicating a
slight increase of onshore flow tonight and into Saturday, 500 mb
heights increase by a decameter and 950 mb temperatures warm some.
The air mass will warm into Saturday. The marine layer depth
should thin to around 900 feet or so.

Southerly flow aloft will develop on Sunday as a shortwave trough
of low pressure near 34N and 137W deepens as it approaches the
North Coast of California. Model solutions strengthen onshore
pressure gradients, which would suggest a cooling trend; however,
a battle will be staged between a southwest flow aloft/onshore
flow regime and a monsoonal flow pattern. Confidence is growing
for a monsoonal flow pattern to win out. The forecast remains
consistent for the current time, but significant changes could be
made for early next week.


Model solutions are continuing to develop a monsoonal flow regime
for the region between Sunday night and Monday. NAM-WRF solutions
continue to be aggressive relative to GFS solutions in the
possibility of shower and thunderstorm activity spilling over into
the coastal and valley areas. If this trend continues, future
shifts will need to increase PoPs and add mentions of shower
activity for Monday and Monday night for the coast and valleys.
850-700 mb mixing ratios increase in excess 10 g/kg across the
area and NAM-WRF precipitable water values increase above 2
inches across portions of the area. A warm, humid and sticky
weather pattern looks to be developing for early next week. PoPs
have been increased across the area. Though the forecast averages
out to a slight chance of showers and thunderstorms for the
Mountains and Desert, the Eastern San Gabriel Mountains and the
eastern portion of the Antelope Valley (Pearblossom, Littlerock,
and Lake Los Angeles) have a better chance of showers and
thunderstorms. Stay tuned as this will be a fluid situation for
early next week.

Southwest flow aloft should start to reestablish on Tuesday.
Slight chance PoPs have been left in the forecast for now as
isolated showers or thunderstorms cannot be ruled out. Certainly,
drier southwest flow aloft could prevail and convective showers
could not develop.

Stronger onshore flow should develop for mid-to-late next week
keeping near-to-slightly above normal temperatures in place across
the region. A persistent marine layer will likely allow for night
through morning low clouds and fog for the coast and possibly the
lower valley areas.



At 2307Z, the marine inversion at KLAX was around 900 ft deep. The
top of the inversion was at 2500 ft with a temperature of 24 deg C.

Moderate confidence overall in the 00Z TAFS at all coastal airfields
due to uncertainties in the timing of the low clouds and
LIFR/IFR/MVFR conditions. The low clouds are expected to move into
the airfields from around 08Z at KSMX to around 15Z at KSBA. The low
clouds should clear to VFR between 16Z and 17Z Sat then continue VFR
thru Sat afternoon. The timing of the onset and dissipation of the
low clouds may be off +/- an hour or two.

Otherwise there is generally hi confidence in the 00Z TAFs at KPRB,
KBUR, KVNY, KWJF and KPMD with VFR conditions expected thru Sat
afternoon, altho there is a 20 percent chance of MVFR cigs
developing at KBUR late tonight into early Sat morning. Gusty sw to
w afternoon and evening winds will persist at KWJF and KPMD through
the period.

KLAX...Moderate confidence overall in the 00Z TAF due to
uncertainties in the timing of low clouds and IFR/MVFR conditions
later tonight thru Sat morning. The low clouds are expected to move
into the airfield around 09Z then clear to VFR around 17Z Sat and
continue VFR thru 04Z Sat evening before low clouds and IFR cigs
move back in. The timing of the onset and dissipation of the low
clouds may be off +/- an hour or two.

KBUR...Generally hi confidence in the 00Z TAF with VFR conditions
expected thru Sat afternoon, although there is a 20 percent chance
of MVFR cigs developing late tonight into Sat morning.


.MARINE...21/100 PM...

For the Outer Waters, good confidence in current forecast. Small
Craft Advisory (SCA) level winds will continue through Saturday
morning then will slowly diminish by Saturday evening. So the SCA
may need to be cancelled early. There will be a 30% chance of
some gusts reaching Gale force levels this afternoon and evening.
For Sunday through Wednesday, winds and seas are expected to
remain below SCA levels.

For the Inner Waters, good confidence in current forecast. For the
waters north of Point Sal, SCA levels are likely this afternoon
and evening then winds and seas will remain below SCA levels
Saturday through Tuesday. For the waters south of Point
Conception, winds and seas will remain below SCA levels through


.BEACHES...21/1245 PM.

A potential exists for significant surf to impact Southern
California beaches late next week and into the following week.
Model solutions have been consistent with developing a tropical
cyclone off the Mexican Coast and pushing it into an area
consistent with higher surf for the region. Model solutions are
consistent in developing swells between 25 and 35 feet off the
Baja California Coast. Early guidance suggests a moderately long
period swell impacting the Southern California Coastline as early
as next Friday, building into next weekend.


CA...Beach Hazards Statement in effect through Sunday evening for
      zones 40-41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
      for zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PDT Saturday for
      zones 670-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Sunday for zone
      673. (See LAXMWWLOX).


A monsoonal flow pattern developing on Sunday will transport
subtropical moisture northwest into Southern California. Shower
and thunderstorm chances are in the forecast for Monday and
Tuesday, but there is a possibility that shower activity could
spill over into the coastal and valley areas.



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