Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 211558
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
858 AM PDT Thu Sep 21 2017

.SYNOPSIS...
An upper low over the Pacific Northwest will drop southward into
Nevada today, bringing plenty of clouds, and cool and breezy
weather to the forecast area. There is a chance of showers across
Ventura and Los Angeles Counties. The upper low will linger over
Nevada into Saturday, keeping temperatures below normal. An upper
level ridge will move eastward from the eastern Pacific into the
West Coast Sunday through Tuesday, then linger into Wednesday.
Significant warming is expected Sunday and Monday, with well
above normal temperatures persisting into wednesday.

&&

.SHORT TERM...(TDY-SAT)

Drizzle/light rain scenario playing out almost exactly as expected
this morning across LA County and parts of Ventura County. Have
had reports of a tenth to a quarter inch of rain along some of the
south facing foothills of the San Gabriels while most other areas
range from a trace to a few hundredths. All this a result of a
cold upper level trough dropping south along the west coast
pushing the marine layer up to 5000` or higher. Starting to see
significant clearing north of Pt Conception this morning and this
should work its way south through the afternoon, though likely
staying pretty cloudy across LA County and at least eastern
Ventura County with some lingering showers as well, especially in
the foothills.

We`ll see increasing west to northwest winds this afternoon and
evening, especially western SB County and the Antelope Valley.
Models continue to advertise sub advisory level winds in these
areas but could see some isolated gusts near 35 mph in some
favored areas of western SB County and 40-45 in the foothills of
the Antelope Valley (ie. Lake Palmdale area).

***From previous discussion***

There could be some lingering showers across Los Angeles County
and on the northern slopes this evening. Otherwise, clouds will be
on the decrease, although it will probably stay partly cloudy.
There should be enough cooling to wipe out any marine inversion,
so stratus is not expected to form tonight/Fri morning. A large
elongated upper low will extent from western Montana across Idaho
and Nevada during the day Friday. Skies should be mostly sunny to
partly cloudy across the region. There should be a few degrees of
warming due to the extra sunshine, but max temps will remain well
below normal for this time of year. There will likely be another
round of sundowner winds across southern SBA County Fri evening,
and that is probably the only place that winds will get close
to advisory levels. Fri night will be clear, and it will likely
be the coolest night in quite a while across the region. Models
are hinting that there might be some low clouds across coastal
sections of L.A. County Fri night/Sat morning, but confidence that
the marine inversion will become established that quickly is low.

A strong vort rotating southward through the western Great Basin
will keep the upper low from moving eastward very much on Sat.
This should keep max temps well below normal across the region,
through possibly a bit higher than those on Fri.

.LONG TERM...(SUN-WED)

The upper low will begin to move eastward Sat night and Sunday,
reaching western Colorado late Sunday. At the same time, an
amplifying upper ridge in the Pacific will move eastward toward
the West Coast. Low level gradients are forecast to turn weakly
offshore, possibly strong enough for some locally gusty northeast
winds in the mtns and valleys of L.A. and VTU Counties Sun morning.
Heights will rise on Sunday, so expect significant warming across
the region, especially west of the mountains thanks to the
offshore flow. Max temps will likely get close to 90 degrees
across the warmer locations in the valleys on Sunday.

The upper ridge will continue to amplify on Monday. The 06Z run of
the GFS again shows some shortwave energy dropping southwestward
through the Great Basin, with a closed low developing over
northern Arizona Monday. As a result, it shows lower heights over
the region than do either the EC or the its own 00Z run. Once
again will ignore this outlier, and will go with the scenarios
showing continued height rises and offshore flow, with a couple
of more degrees of warming in most areas Mon.

The ridge axis will move into northern and central CA on Tue.
Heights will cont to rise, and low level flow will be offshore.
Tue should be a hot day across most of the region, with highs in
the 90s in most valley locations, and at least lower 90s on the
coastal plain away from the immediate beaches. It is not out of
the question that high temps could approach 100 degrees in the
warmest valley locations on Tue.

There will likely be some gusty northeast winds Mon and Tue,
especially in the mountains of L.A. and VTU Counties, and in
conjunction with the heat and lowering relative humidities, fire
weather danger will likely become heightened.

Heights may actually rise a bit more on Wed as the ridge axis
moves right over the region, so there may be a bit more warming in
the mountains and deserts. A return to onshore flow may bring a
few degrees of cooling to the coastal plain, with little change in
max temps expected in the valleys.

&&

.AVIATION...21/12z.

At 02z at KLAX... the inversion was around 6000 feet. The top of
the inversion was around 6900 feet with a temperature of 11
degrees Celsius.

Overall... Low to moderate confidence in the current TAFs.
There are widespread IFR/MVFR conditions across the coastal and
adjacent valley terminals though with a lot of wafting in and out
of the lower values. VFR conditions are expected to return by 17z
though could linger into the afternoon hours particularly in Los
Angeles County. Otherwise and elsewhere VFR conditions will
prevail.

KLAX... Low to moderate confidence in the current TAF. Primarily MVFR
conditions through 17z with a ten percent chance of periodic IFR
conditions wafting in and out. Thirty percent chance of MVFR
conditions lingering into the afternoon hours. Otherwise VFR
conditions will prevail. A fifteen percent chance of east winds
near ten knots 22/06z-16z.

KBUR... Low to moderate confidence in the current TAF. Primarily
MVFR conditions through 16z. Thirty percent chance of MVFR
conditions lingering into the afternoon hours. Otherwise VFR
conditions will prevail.

&&

.MARINE...21/900 AM.

Northwest winds across the outer waters will be at Small Craft
Advisory (SCA) levels until mid morning then increase to Gale
Warning levels through late tonight. SCAs will likely be issued
for Friday as the winds begin to diminish on Friday.

A fairly low confidence forecast today as wind speeds are falling
right on the criteria cutoff point between categories. Northwest
winds will reach SCA level across all of the inner waters by late
morning and will continue into this evening or overnight.
Nearshore winds along Central Coast and across the western Santa
Barbara Channel may increase to Gale Warning level by late
afternoon.

Steep short-period seas are likely on this afternoon and evening
across much of the waters.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from noon today to 9 PM PDT this
      evening for zones 34-35. (See LAXNPWLOX).
     Wind Advisory in effect until 4 AM PDT Friday for zones
      39-52. (See LAXNPWLOX).
     Wind Advisory in effect until 10 PM PDT this evening for zone
      59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 PM PDT this afternoon
      for zones 645-650. (See LAXMWWLOX).
     Gale Watch in effect from 3 PM PDT this afternoon through
      this evening for zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for zone
      655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT this morning
      for zones 670-673-676. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PDT Friday for zones
      670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
It will be hot Monday through Wednesday, especially away
from the coast. There will be gusty northeast winds at times,
especially across Los Angeles and Ventura Counties. The
combination of heat, gusty winds and low relative humidity
values will bring heightened fire danger, especially to the
mountains and valleys.

&&

$$

PUBLIC...MW/DB
AVIATION...Kj
MARINE...Sweet
SYNOPSIS...DB

weather.gov/losangeles



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