Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 221400
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
700 AM PDT Sun Apr 22 2018

.SYNOPSIS...22/658 AM.

There will be overnight and morning coastal low clouds and fog
for several days this coming week otherwise conditions will be
warm and dry. Temperatures will cool a bit Friday and Saturday as
a weak cold front moves across the region.

&&

.SHORT TERM (TDY-TUE)...22/351 AM.

High clouds across the region have thinned out quite a bit this
morning. Plentiful high clouds were likely one reason that stratus
has not yet formed. There is a small chance that some patchy
stratus may develop by sunrise, mainly on the Central Coast or
across coastal sections of L.A. and VTU Counties. If any stratus
develops, expect it to clear by mid morning. Heights and
thicknesses will remain quite high across the region today, so
expect another rather warm day in most areas away from the
immediate coast. Slightly increased onshore flow should induce a
few to several degrees of cooling across the coastal plain, with
slight cooling in the valleys, at least in the lower valley
locations.

The WRF indicates that there should be a return of low clouds to
coastal areas tonight. The marine layer will be quite shallow, so
there could be patchy dense fog, at least early in the night. The
marine layer may deepen enough to allow low clouds to squeak into
the San Gabriel Valley late tonight or early Monday. Expect skies
to clear in most areas by mid to late morning on Monday. Heights
will not change much, or may even rise a bit, but onshore
gradients are forecast to increase, and 950 mb temps will be at
least 2-3 degrees Celsius lower west of the mountains than those
today, so expect a few degrees of cooling west of the mtns. Expect
little chance in the mtns and deserts, and there could even be a
touch of warming.

A weak upper low will approach the forecast area from the
southwest Monday night and Tue. WRF cross sections indicate some
deepening of the marine layer, so expect more in the way of night
through morning low clouds in the valleys. Still expect skies to
clear well in most areas by afternoon Tue. Max temps may be down a
couple more degrees, especially across interior portions of the
coastal plain and in the valleys, thanks to a stronger marine
intrusion, later clearing, and some additional slight cooling
at 950 mb.

.LONG TERM (WED-SAT)...22/431 AM.

The EC and GFS continue to be generally in good agreement thru
Wed morning, then begin to disagree quite a bit. Both models show
a large upper low moving through the eastern Pacific. Early Wed
morning, both models show the center within 50 nm or so of each
other, about 700 nm west of Monterey. By late afternoon, the GFS
shows the upper low nearly 200 nm closer to the coast than does
the EC. The aforementioned weak upper low which will be to our
southwest Tue will open up and basically flatten out altogether by
early Wed. Its main affect on our weather may be to deepen the
marine layer a bit more Tue night, sending more widespread low
clouds into the valleys. For Wed across the region, the GFS shows
lowering heights and a good onshore push, with some cooling. The
EC, with a ridge popping up ahead of the large upper low, actually
shows some height rises. For now, will keep the forecast for
slight cooling across the region.

Both models show the upper low moving northward Thu, with the GFS
continuing to be farther east. The GFS also shows a much more
pronounced trough extending southward from the main low, with the
axis a few hundred miles west of Pt Conception. The trough axis
is forecast to push into the forecast at Thu night and Fri, which
would likely bring a very deep marine layer, possible drizzle, and
fairly cool weather. The EC, with the upper low farther nw and
without the pronounced southward-extending trough, keeps heights
much higher across the region, with less cooling, especially
inland. Both models show the upper low translating eastward Fri,
then the EC shows it hooking northward toward the far northern CA
coast Sat, while the GFS shows it continuing to move eastward,
passing just north of the forecast area early Sat, then moving
into the Great Basin late Sat. The GFS solution would bring a
chance of showers to at least portions of the forecast area late
Fri through Sat morning. At this point, have kept pops out of the
forecast since there is so little confidence and the end of the
rainy season is approaching, but this will continue to have to be
watched closely.

&&

.AVIATION...22/1022Z.

At 1030Z, the marine inversion at KLAX was based around 400 feet.
The top of the inversion was 2300 feet with a temperature of
23 degrees Celsius.

Overall, moderate confidence in coastal TAFs and high confidence
in valley/desert TAFs. There is a 20% chance of IFR/LIFR
conditions developing 12Z-17Z this morning at coastal sites. For
tonight, moderate confidence in development of stratus, but low
confidence in timing/flight category.

KLAX...overall, moderate confidence in 12Z TAF. There is a 20%
chance of IFR/LIFR conditions 12Z-17Z this morning. For tonight,
moderate confidence in return of stratus, but low confidence in
timing (+/- 4 hours of current 09Z forecast) and flight category
(could range from MVFR to LIFR).

KBUR...high confidence in 12Z TAF as VFR conditions will prevail.

&&

.MARINE...22/322 AM.

For the Outer Waters, moderate to high confidence in current
forecast. There is a 70% chance of Small Craft Advisory (SCA)
level winds across PZZ670 today and tonight. Otherwise, high
confidence in conditions remaining below SCA levels through
Wednesday. On Thursday, there is a 50% chance of SCA level winds
across the entire Outer Waters, especially across PZZ670/673.

For the Inner Waters, high confidence in current forecast. For the
waters north of Point Sal, there is a 30% chance of SCA level
winds Thursday afternoon. For the waters south of Point
Conception, high confidence in winds/seas remaining below SCA
levels through Thursday.

A shallow marine inversion may generate some patchy dense fog
tonight and Monday morning.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
      for zone 670. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (TUE-SAT).

No significant hazards expected.

&&
$$

PUBLIC...DB
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Kj

weather.gov/losangeles


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