Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 222352

452 PM PDT Fri Sep 22 2017

Updated Aviation Section

A low will bring cold air this week with breezy winds and major
temperature drops inland. Next week, a high, an offshore flow
should bring a significant warming trend into Friday.



Lingering moist air under a weak inversion leading to some
stratocumulus development this afternoon but otherwise lots of
sunshine and slightly warmer temps. Low clouds will again struggle
to form overnight south of Pt Conception as the inversion remains
weak. However there`s a better chance for solid marine lyr
formation north of Pt Conception where the inversion is stronger
and it`s still quite moist in the boundary layer. Highs Saturday
will continue to trend warmer but still 4-8 degrees below normal.

The warming trend will continue through early next week as
offshore flow increases each day. The 12z NAM boosted forecast
offshore gradients by about 1mb from previous runs and upper
support is better too. Not surprisingly the GFS is weaker on both
accounts but either way most areas west and south of the mountains
should see a generous warm up Sunday and Monday with valleys
mostly in the 90s by Monday and coastal areas in the 80s. One
caveat to this is that the NAM is still showing some lingering
low lvl moisture under a much stronger inversion early Sunday so
there is the potential for some low clouds and dense fog across
coastal areas from Ventura County south which could also impact
coastal temperatures. Depending on how strong the offshore flow
ends up being these clouds could possibly linger through the
afternoon and into Monday morning as well but that`s always a
tricky forecast this far out.


Models in pretty good agreement now through at least next Friday
showing some additional upper level energy dropping south through
the Great Basin Tuesday into Wednesday reinforcing the northeast
flow aloft and likely being the period of strongest winds, though
it`s still not looking advisory level at least based on the GFS
where 850 winds top out between 20-25kt. However, very often as
we get closer to an event models start to increase the gradients
and upper support so advisory level Santa Ana`s are still
possible with the best chances across the LA/Ventura valleys and
lower mountains. Temperatures Tue/Wed will continue to warm up at
lower elevations with highs jumping into the lower 90s for some
inland coastal areas and high 90s for the warmer valleys.

After Wed models show gradients remaining offshore but weaker and
with less upper support. However, the air mass will be warming as
high pressure tries to move in from the west. There is some
disagreement between the GFS and ECMWF with regard to how quickly
the trough will shift east with the GFS a little faster and thus
warmer. But either way temps will remain 10-15 degrees above
normal through the week and likely continuing into next weekend.



At 23z at KLAX...There was no marine layer or inversion.

Good confidence in the current TAF suite through Saturday. About a
20 percent chance of sct-bkn LIFR CIGS overnight near KSBP and
KSMX, but quickly SKC by 14Z as drier northerly flow shown on
model data. Patchy moisture could form over the LA Basin before
14Z bringing sct-bkn MVFR CIGS to coastal sites.

KLAX...High confidence in VFR conditions through 10Z, then 20
percent chance of sct-bkn MVFR CIGS 12-19Z. Low confidence of
this layer occurring, then back to high confidence for SKC-SCT050
after 19Z. SE winds likely after 10Z tonight, with a 20 percent
chance of exceeding 8KT.

KBUR...High confidence in VFR conditions through Saturday. Less
than 10 percent chance of sct 020-030 from 12-18Z.


.MARINE...22/100 PM.

Confident that the gusty northwest winds that peaked last night
will slowly weaken and shrink in coverage through the weekend.
They should be strong enough to warrant a Small Craft Advisory
(SCA) for the outer waters tonight (especially between Point
Conception and San Nicolas Island)...with a 60 percent chance they
persist through Saturday night. Over the inner waters (SBA
Channel, Santa Monica Basin, and immediate Central Coast), 60
percent chance of a low-end SCA through this evening, likely
staying under SCA after that. Short period choppy sea conditions
will persist everywhere through at least Saturday.


PZ...Small Craft Advisory in effect until 11 PM PDT this evening
      for zones 645-650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Sunday for
      zones 670-673-676. (See LAXMWWLOX).


It will be hot Monday through Thursday, especially away from the
coast. There will be gusty northeast winds at times, especially
across Los Angeles and Ventura Counties Monday and Tuesday. The
combination of heat, gusty winds and low relative humidity values
will bring heightened fire danger, especially to the mountains and



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