Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 230006

506 PM PDT Sat Jul 22 2017

For the next few days a typical summer weather pattern will bring
night through morning low clouds and fog to the coasts and some of
the valleys. Temperatures will be near normal. There is a slight
chance of thunderstorms in the afternoons and evenings for the
deserts and mountains on Monday and Tuesday.



High pressure aloft centered over Oklahoma will continue to weaken
and recenter over the Southwest United States through early next
week. 500 mb heights will fall into Monday as a weak trough of
low pressure near 37N and 130W approaches the North Coast of
California. Onshore flow will increase later today and the marine
layer should deepen slightly tonight and into Sunday. Night
through morning stratus clouds should penetrate a little farther
into the valley areas late tonight or early Sunday morning. As a
result, a cooling trend will take shape into Sunday, best along
the coastal areas.

The forecast gets a little more complicated on late Sunday as
monsoonal moisture moves northwest into Southern California. Model
solutions are in good agreement that a decent monsoonal surge will
take place over Southern California through Monday night, then
start to dry out between late Monday night and Tuesday night. The
marine layer stratus forecast will get difficult and low
confidence should placed in the cloud cover forecast between
Sunday night and Tuesday night as the moisture moves into the
region. PoPs have been bumped up accordingly as convective showers
and thunderstorms could spill over into the coastal and valley
areas on Monday and Monday night. Precipitable water values
increase to near 1.50-2.00 inches by Monday afternoon. 850-700 mb
dewpoints and mixing ratios remain favorable supporting PoPs at
this time across the area.

The air mass should start to dry out as drier southwest flow
entrains into the air mass on Tuesday. Convective shower activity
should become confined to the mountains and desert on Tuesday and
Tuesday night. PoPs have been pretty much untouched for this


Ridging aloft to the east of the region will wobble west through
Thursday, then strengthen across the Desert Southwest. A warming
trend should take place for the latter half of next week. The
marine layer depth will shrink and onshore flow will weaken as
subsidence aloft presses down on top of the marine layer. 500 mb
heights increase and 1000-500 mb thickness values climb into late
week. Monsoonal activity will likely remain east of the area for
Wednesday through Friday.

The developing story could be tropical depression Nine-E currently
off the Mexican Coast near 10N and 99W. This tropical system is
expected to become more developed over the next several days,
possibly becoming a hurricane on Monday. While this system has a
moderately-high probability of producing high surf and strong rip
currents for Southern California beaches as early as next Friday,
there is still the possibility, albeit lower that remnant
moisture associated with the tropical system could move north into
the Southwest United States next weekend and into the following
week. Stay tuned as this developing system could be the main
weather story.




At 2345Z, the marine inversion at KLAX was based around 1400
feet. The top of the inversion was 2300 feet with a temperature
of 28 degrees Celsius.

High confidence in return of stratus/fog to all coastal sites,
but only moderate confidence in timing which could be off by as
much as 2 hours. There is a 30 percent chc that cigs could be 1
flight cat lower than fcst. Clearing times could be an hour
earlier than fcst.

Low confidence in KBUR and KVNY with a 40 percent chc of IFR cigs

KLAX...Moderate confidence in TAF. Arrival of MVFR CIGs could be
+/- 2 hours of current 09Z forecast. There is a 30 percent chc
of IFR cigs through 16Z.

KBUR...Low confidence in TAF. There is a 40% chance of IFR CIGs



22/100 PM

For the Outer Waters, good confidence in current forecast. Winds
and seas are expected to remain below Small Craft Advisory (SCA)
levels through Thursday.

For the Inner Waters, good confidence in current forecast. Winds
and seas are expected to remain below SCA levels through Thursday.

There will be a chance of thunderstorms Monday afternoon and
evening across the entire coastal waters.



22/230 PM

Significant surf conditions may impact the Southern California
beaches late next week. A tropical cyclone developing off the
Mexican coast will generate a long period southeasterly swell
across the coastal waters beginning next Friday and continuing
through the weekend. This swell will likely generate high surf and
strong rip currents on south facing beaches.


CA...Beach Hazards Statement in effect through Sunday evening for
      zones 40-41-87. (See LAXCFWLOX).


A developing monsoonal flow pattern will continue chances of
showers and thunderstorm across the mountains and desert Tuesday.

A significant surf event could develop as early as Friday as
swells from what could become Tropical Cyclone Hilary arrive at
Southern California beaches. H.igh surf and strong rip currents
could develop Friday, possibly peaking over the weekend.



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