774 
FXUS66 KLOX 212128
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 
228 PM PDT Fri Jul 21 2017

.SYNOPSIS...

Onshore flow will continue night through morning low clouds and 
fog will continue through the period. High pressure will dominate 
through Saturday, then a weak upper trough will bring some modest
cooling. Monsoonal moisture could bring an unstable weather
pattern to mountains and desert early next week.

&&

.SHORT TERM...(TDY-MON)

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.

.LONG TERM...(TUE-FRI)

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.

&&

.AVIATION...21/1545Z.

At 1545Z, the marine inversion at KLAX was based near 1700 feet.
The top of the inversion was 3700 feet with a temperature of 
22 degrees Celsius. 

Overall, moderate confidence in 18Z TAF package. High confidence
in return of MVFR/IFR conditions to coastal sites tonight, but
only moderate confidence in timing. 

KLAX...moderate confidence in 18Z TAF. Timing of return of
stratus/fog could be +/- 2 hours of current 08Z forecast. 

KBUR...high confidence in 18Z TAF.

&&

.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
Wednesday.

&&

.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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
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).

&&

.HAZARD POTENTIAL OUTLOOK...(MON-FRI)

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.

&&

$$

PUBLIC...Hall
AVIATION...Thompson
MARINE...Thompson
BEACHES...Hall
SYNOPSIS...B/Hall

weather.gov/losangeles