Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 271254

554 AM PDT Thu Jul 27 2017

Night and morning low clouds and fog will affect coastal areas
through early next week. Otherwise, upper level high pressure
will keep mostly clear skies and seasonably warm temperatures over
southwestern California through Saturday, then monsoonal moisture
will move in for partly cloudy skies Sunday through early next
week. There will also be a slight chance of mountain and desert
thunderstorms each day Sunday through Wednesday.



Latest fog product imagery indicated widespread stratus across the
Central Coast, working its way into the Santa Ynez Valley. Areas S
of Point Conception showed los clouds less organized with mainly
patchy low clouds across coastal areas of LA/Ventura/SBA Coast
this morning. Not expecting low clouds to fill in across inland
coastal areas valleys should remain mostly sunny today. A few high
clouds will move across LA/VTU counties this morning, but not
enough to justify partly cloudy skies today.

Synoptically, an elongated upper ridge will persist across much of
the southwestern states with a weak upper trough well to the NW
across northern California today. Earlier model runs were
advertising a strong area of high pressure settling over Southern
California through the weekend, but the upper trough to the NW
will deepen, yet H5 heights will remain around 590 DM over the
next few days. Surface onshore gradients will be moderately strong
through Saturday. This should keep coastal areas fairly mild
through the short term forecast period. Today should be the
warmest day through Saturday with warmest valley locations
reaching the upper 90s while most valleys will be in the upper 80s
to mid 90s. There will be a degree or two of cooling across the
SBA South Coast Fri and LA/VTU County Valleys, otherwise expect
similar high temps on Friday, with a few more degrees of
additionalcooling for areas N of Point Conception on Saturday,
due to the lower heights from the weak trough and lower boundary
layer temps. There will be a few flat cumulus buildups across the
Ventura/SBA County Mtns on Friday with additional cumulus buildups
across the LA Mtns on Saturday. Not expecting any monsoonal
moisture to move into the region until next week. Expect night
through morning low clouds and patchy fog to continue across
coastal areas through the period.


Should be an interesting long term forecast with some
uncertainty in respect to monsoonal moisture. The dry SW flow
aloft will begin to shift to the south to southeast by Sunday. At
this point there will be no remnants from Hurricane Hilary or from
what is now Tropical Storm Irwin. However, the GFS and ECMWF are
in agreement of the upper ridge will strengthen and set up across
the Four Corners region allowing south to southeasterly flow to
advect some mid level moisture out of nrn Mexico and southern
Arizona into the far southern portion of Calif Sunday afternoon.
Most of the activity will remain to the south and east of LA
County, but there will be enough mid level moisture and some
limited instability across the San Gabriel Mtns to justify a
slight chance for showers for the LA County Mtns and eastern
Antelope Valley. High to mid level clouds should increase across
LA County Sunday night into Monday. By Monday, there will be
increasing clouds and more widespread mid level moisture moving
into the mountains and deserts. Slight chance for thunderstorms
for the LA/VTU County Mtns and Antelope Valley. By Monday evening,
an easterly wave sets up over LA County and areas to the south.
In coordinating with the San Diego NWS office, have added slight
chance of thunder for the LA County Valleys for now, but see no
reason based on latest models why showers wont reach the coastal
waters Monday evening through the overnight hours. If models do
not change over the next 24 hours, expect that showers or slight
chance thunderstorms will be added to the forecast. This easterly
wave will continue into Tuesday and should bring additional
chances for showers or slight chance thunderstorms for the same
areas and possibly into the Ventura County valleys and Coastal
areas as well Tue evening. As we get closer to next Mon/Tue, POPs
might have to be increased for the interior areas including the
LA/VTU Mtns and Antelope Valley.

As far as temps go, the stronger ridge aloft will allow for some
additional warming, although with the added humidity and cloud
cover temps might be a few degrees too warm for LA/VTU inland
areas including the valleys. The added humidity will make it quite
uncomfortable at night as well for the lower mountain elevations,
Antelope Valley and possibly the elevated valley locations. It is
hard to pin down when and where showers will occur, so blanketing
LA/VTU counties with slight chance makes the most sense at this



At 12Z, the marine layer depth was around 1300 ft. The top of the
marine inversion was around 2100 ft with a temperature of 23C.

Widespread low clouds with LIFR to IFR conditions across coastal
sections from VTU County northward and in the Santa Ynez Valley.
Skies should become mostly clear during the mid to late morning

Skies were mostly clear across L.A. County, but low clouds were
very nearby off the coast and may affect beach areas for a few
hours this morning.

Low clouds should be widespread in coastal areas and the Santa
Ynez Valley tonight, with generally IFR to LIFR conds expected.

KLAX...Low confidence in the 12Z TAF. There is a 40% chance that
skies will remain clear and conds VFR this morning. There is a
30% chance that skies will remain clear tonight/Fri morning.

KBUR...High confidence in the 12z TAF with VFR conditions
expected thru the period.


.MARINE...27/300 AM.

For the Outer Waters, moderate confidence in current forecast.
There is a 20-30% chance that NW wind gusts to SCA levels could
become widespread enough and frequent enough to require a SCA at
times late this afternoon through Sat evening, especially across
the northern two thirds of the outer waters. Otherwise, winds and
seas expected to remain below SCA levels through Mon.

For the Inner Waters, good confidence in current forecast. For all
the waters, winds and seas are expected to remain below SCA levels
through Mon.

Long period south swell will spread into the waters Fri, then
moderate to occasionally large swell with a rather long period
will affect the waters Fri night through Mon. The exception will
likely be across most of the Santa Barbara Channel where the
Channel Islands will block much of the swell.


.BEACHES...26/200 PM.

Hurricane Hilary has seen a weakening trend today, but is still
expected to generate a southeasterly swell that will impact
Southern California beaches this weekend. The southeast swell
will begin to move into the coastal waters Friday into Saturday,
then peaking late Sunday through Monday or Tuesday. In addition, a
Southern Hemisphere swell will move into the waters Saturday. High
surf conditions will be possible, especially Sunday and Monday across
south-facing beaches this weekend. Strong Rip currents are very
likely through the event.


CA...Beach Hazards Statement in effect from Friday morning through
      Tuesday evening for zones 40-41-87. (See LAXCFWLOX).


A high surf event should peak over the weekend as a long-period
southerly swell arrives at the Southern California beaches. High
surf and strong rip currents will occur through the weekend and
probably into late next week.



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