Southwestern California Discussion

FXUS66 KLOX 271610

910 AM PDT Thu Jul 27 2017

Partly cloudy skies will persist into next week, with an overnight
marine layer along the coast. Early next week, monsoonal moisture
should bring possible thunderstorms and showers in the afternoon and
evening for the Ventura mountains and Los Angeles valleys and
eastward. The temperatures are forecast to be around normal this
week, with a warming trend next week.



Overall, a quiet Thursday morning across Southwestern California.
Latest sounding data indicates marine inversion based around 900
feet. Satellite shows areas of stratus/fog across the coastal
plain with some cirrus clouds floating overhead across Los Angeles
and Ventura counties. Stratus should dissipate by late morning
with mostly sunny skies for all areas this afternoon. As for
temperatures, expect some slight warming most areas with slightly
warmer boundary layer.

Forecast-wise, initial look at 12Z model data indicates no major
changes to previous thinking. At upper levels, weak trough will
approach the West Coast the next couple of days while onshore flow
continues near the surface. High resolution models indicate weak
eddy circulations the next few mornings. So, with slightly
lowering H5 heights and eddy, marine inversion should deepen with
more widespread night/morning stratus pattern for the coastal
plain with some localized intrusion into the coastal valleys. With
this pattern, afternoon temperatures will exhibit minor
fluctuations from day-to-day with temperatures remaining around
seasonal normals.


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...27/900 AM.

For the Outer Waters, moderate-to-high confidence in current
forecast. A small craft advisory was added for the waters near
Point Arguello and Point Conception from late this afternoon
through late this evening. There is a 30-40 percent chance that
Northwest winds could reach small craft advisory levels each
afternoon through Saturday evening. Otherwise, winds and seas
expected to remain below advisory levels through Monday.

For the Inner Waters, high confidence in current forecast. For
all the waters, winds and seas are expected to remain below SCA
levels through Monday, except for local small craft advisory level
gusts across the Inner Waters each afternoon and evening.

A long-period southerly swell will spread into the coastal waters
on Friday, then a moderate-to-occasionally large swell with a
rather long period will affect the waters from Friday night
through weekend. The Santa Barbara Channel will likely be blocked
from the swell energy due to the Channel Islands.


CA...Beach Hazards Statement in effect from Friday morning through
      Tuesday evening for zones 40-41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Friday for zone 673. (See LAXMWWLOX).


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.




NWS LOX Office Area Forecast Discussion