Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 190327

827 PM PDT Fri Aug 18 2017

Possible showers and thunderstorms for the mountains of Ventura,
Santa Barbara, and Los Angeles counties and deserts into Monday. A
low will bring overnight low clouds and fog to the coast and some
valleys into next week, The temperatures will be near normal with
a warmup on Thursday and into the weekend as the low weakens.



Weak upper level low pressure system off the coast of Southern
California helping to bring some mid level moisture and
instability across the local mountains this afternoon. As a
result, there were some cloud buildups over the local mountains,
with one thunderstorm observed near Pine Mountain Club across the
border of the Ventura/Kern County mountains. Otherwise, satellite
imagery showing some low clouds returning to some coastal areas
this evening, especially LA County and the Central Coast. ACARS
data showing marine layer depth across the LA Basin around 1400
feet, so still deep enough for some low clouds to locally extend
into some of the coastal valleys overnight. A slightly stronger
onshore flow on Saturday with greater coverage of marine layer
clouds in the morning should allow for a couple of degrees of
cooling for most areas on Saturday.

The main weather focus for Saturday will be the thunderstorm
threat over the local mountains. Latest 00z NAM model showing
upper level low pressure system in a more favorable location for
convection tomorrow afternoon, with a vort max swinging into the
Ventura county mountains. Precipitable water values were
generally around 1 inch this afternoon, and are expected to
increase to between 1.2 and 1.4 inches by late Saturday afternoon
across Ventura and Santa Barbara counties. 00z NAM cross sections
focusing best moisture across the mountains of Ventura and Santa
Barbara Counties, where there is greater instability (with Lifted
Index values ranging between -4 and -8). Model soundings over the
Ventura/SBA County mountains also showing impressive surface based
CAPE values ranging between 1000 and 2000 J/kg. Based on the
better moisture/instability profile, have increased tstm threat
for Ventura/SBA county mountains for Saturday afternoon into early
evening in evening update. While moisture levels are still only
moderate, upper level steering winds are rather weak. As a result,
any thunderstorm that do develop will be slow moving, capable of
producing brief heavy rainfall with localized flash flooding
potential. If these convective parameters are anywhere close to
what the NAM are indicating, thunderstorms will also have the
potential for large vertical growth, increasing the potential for
gusty downdraft winds and small hail. With the mid level wind flow
being more northerly tomorrow afternoon, some of these storms
could drift towards the foothills of the Ventura county valleys as
well as the eastern portions of the Santa Ynez mountains.

*** From previous discussion ***

Today is another fairly seasonal day with moderate onshore flow
in place across the region. The upper level trough will remain
into next week, but weaken a bit Saturday as 500 mb heights climb
slightly. The 1000-foot marine layer depth this morning should
deepen through the weekend, possibly deepening to near 2000-2500
feet by Monday. Near persistence seems to be the best forecast for
the immediate coastal locales through the weekend and into early
next week. Away from the coast, temperatures will cool for early
next week, after a slight warm-up today and Saturday.

The focus of the forecast for the weekend will be on the instability
associated with the trough. East to southeast flow aloft could
draw in enough moisture to get rain out of the afternoon clouds. PoPs
with isolated showers and thunderstorms extended through Monday
but still only at the slight chance level.


By Tuesday the marine layer should deepen as the trough deepens
and slowly lifts out of the region through Wednesday. Night
through morning drizzle cannot be ruled out for the Southland
early next week as the marine layer depth will likely deepen
substantially between Monday and Wednesday. Current forecast keeps
marine layer depths inline with 2000-2500 feet, but a depth up to
near 4000 feet between Tuesday and Wednesday would not be
shocking. A general cooling trend should be expected into midweek
next week as the trough wobbles around and creates ebbs and flows
of the marine intrusion.

Drier southwest flow aloft looks to develop for the latter half of
next week as the trough lifts out. Another weak trough of low
pressure reforms off the Pacific Northwest Coast and a ridge of
high pressure remains centered over the Southeast States. GFS is
still liking a warm up late in the week next week.



At 2320Z, the marine inversion at KLAX was based at 1400 feet.
The top of the inversion was 5300 feet with a temperature of 21
degrees Celsius.

Timing of return of low cigs/vsbys may be off by 3 hours or a
category lower than forecasted (or higher as well for KSMX). There
is a 20 percent chance of IFR cigs/vsbys for KVNY and KPRB
between 12Z and 16Z. There is a 20 percent chance that VFR
cigs/vsbys prevail through the period at KSBA.

KLAX...Moderate confidence in 00Z TAF after 05Z. Return of low
clouds is possible between 05Z and 11Z with a 30 percent of IFR
cigs/vsbys, which would be most likely between 05Z and 10Z.

KBUR...Moderate confidence in 00Z TAF. Have included a brief
IFR cig for KBUR early Saturday morning, but there is a 30 percent
chance that no cig will arrive.


.MARINE...18/800 PM...

For the Outer Waters, moderate to high confidence in current
forecast. Winds and seas are expected to remain below Small Craft
Advisory (SCA) levels through at least Tuesday. However...there is
a 40% chance that local gusts to 25 knots near Point Conception become
more widespread this evening...requiring issuance of a SCA.

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


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


A high risk of rip currents exists at area beaches through the
weekend, and a beach hazard statement is in effect through Sunday
evening. Isolated showers and thunderstorms cannot be ruled out
for Sunday and Monday.



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