Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 231124

424 AM PDT Sun Oct 23 2016

A cooling trend is expected through Monday as a trough of low
pressure deepens off the West Coast. Skies will be mostly cloudy
and there is a chance of light rain or showers, especially across
southern areas later today through Monday. High pressure will bring
dry and warmer weather Tuesday and Wednesday, then a stronger
storm may bring significant rain to the area late next week.



Areas of low clouds and fog have pushed into coastal sections of
SLO and SBA Counties. The marine inversion was very shallow, so
there was some dense fog, enough to warrant advisories. Low clouds
have also pushed into many coastal sections of L.A. and VTU
Counties, but any dense fog was localized and confined to VTU
County. Otherwise, plenty of mid and high clouds were streaming
into the region in the southwest flow aloft ahead of an
approaching upper trough. In general, skies should be partly to
mostly cloudy across the region today, even once any low clouds
break up, with the thickest cloud cover south of Pt Conception.
Lowering heights and a good amount of cloud cover should knock
several degrees off max temps today, perhaps as much as 10 degrees
in some areas.

A large upper low will approach the Pacific Northwest later today
and tonight, and a trough extending southward will sharpen as it
approaches the forecast area. Moisture will gradually increase
throughout the atmosphere above the forecast areas later today
into tonight as southwesterly flow aloft increase. A weak
short wave embedded into the swly flow will induce increasing
lift later today and tonight, especially across Los Angeles
County. All models show some light amounts of measurable rain
across much of Los Angeles County tonight into Monday morning,
and all the models have flip-flopped from run to run as to the
extent to which and rain would occur north and west of L.A.
County. Will go with a slight chance of light rain or showers as
early as this afternoon across L.A. County, with a better chance
of showers, or probably more appropriately light rain tonight and
Monday morning, with a slight chance of showers across the
remainder of the region. Have kept pops in the chance category
since there is such a deep layer of dry air in place, but if this
layer moistens, later shifts may have to increase pops, especially
in the mtns and foothills.

Any shower activity should end Monday afternoon in most areas,
but skies will generally remain at least partly to mostly cloudy
in the afternoon. Max temps should be down a couple more degrees
in most areas Monday. Winds in the mountains and higher valleys
will be gusty at times through Monday, but should remain below
advisory levels.

The upper low off the Pac NW will lift northeastward into British
Columbia Monday night and Tuesday, and a flat ridge will build
into the forecast area. Skies will be partly cloudy, and rising
heights and thicknesses should lead to a few degrees of warming in
most areas on Tuesday.


A strong short wave will wrap southward around the upper low off
British Columbia and a rather strong upper low will form well
to the west of the northern CA coast Wed. A sharp trough will
extend southward through the eastern Pacific. As this occurs, it
will pump up an upper ridge to its east, and heights/thicknesses
across the region will cont to rise. Skies will be partly cloudy
on Wed, and max temps will be up several degrees, to several
degrees above normal levels in most areas.

The confidence in the forecast beyond Wednesday decreases quite a
bit as the models begin to diverge. The EC takes the upper
low toward the northern CA coast late Thu, then show it opening
up and minoring out as it moves into Oregon and Idaho Thu night
and Fri. The GFS shows the upper low dropping southward to a
latitude south of Pt Conception late Thu, and is slower and
farther south with its track, showing it reach the nrn CA coast
during Fri afternoon. Not surprisingly, the EC solution sends a
fast moving and weakening front through the region Thu afternoon
into Thu night, while the GFS shows a slower and longer duration
frontal system, mainly affecting the region Thu night and Fri.
With so much variability, will just keep chance pops in most
areas Thu night and Fri, since exact timing is still in question.

There is still the potential for significant rain of an inch or
more across parts of the forecast area from this system, with
the threat of flash flooding and debris flows in recent burn
areas. This situation will be monitored very closely in the
coming days.

It is looking less and less likely that the threat of rain will
continue beyond Fri evening, so pops may be able to be trimmed
back or taken out of the forecast for most areas Fri night and
early Sat. Max temps should be down significantly Thu and Fri due
to the clouds and precip, but if the GFS is correct, temps may
still be above normal on Thu. Dry weather is expected Sat
afternoon (and probably as mentioned all day Sat), with temps
near normal in most areas.




At 1000Z, there was a weak 400 foot deep marine layer. The top of
the inversion was near 1500 feet with a temp of 21 degrees C.

Low confidence in coastal TAFs. LIFR to low IFR conds will move
into all sites before dawn but timing could well be off by a a
couple hours. Further low confidence in clearing there will be an
LIFR to IFR bump around mid morning but timing could be off by 90
minutes. Clearing to VFR will occur between 17Z and 19Z.

There is a 30 percent chc of light rain over KLAX and KLGB from
02Z to 12Z.

KLAX...Low confidence in TAF thru 18Z. flight cat transitions
could be off by as much as 90 minutes. There is a 30 percent chc
of an easterly wind component greater than 8 kt. There is a 30
percent chc of light rain from 02Z to 12Z.

KBUR...High confidence VFR conditions through Sunday. There is a
20 percent chc of light rain from 06Z to 12Z.



23/300 AM

High confidence that dense fog with visibility 1 mile or less
will be prevalent over much of the coastal waters through late
morning...though visibilities might be slightly above 1 mile
south of Point Mugu.

Good confidence that no small craft conds will exist through



22/400 PM.

An approaching upper level trough of low pressure will bring
increasing southeast to southwest flow across the region this
weekend. The strongest winds will continue to be focused across
the mountains, santa clarita valley, and Antelope Valley where
gusts between 25 and 40 mph will be common. These areas will also
generally see humidities in the 7 to 15 percent range during the
day with poor recoveries at night. As a result, elevated fire
danger will continue across the above mentioned areas through the
weekend, with brief critical fire weather conditions in the LA
county mountains. Already seeing a few stations in the LA county
mountains reaching brief critical fire conditions this afternoon
as single digit humidities have combined with southerly winds
gusting up to 35 mph. At lower elevations, the increasing onshore
flow will bring a slight cooling trend with increasing humidities
through the weekend.

Mid level subtropical moisture will bring a slight chance of
showers to areas south of Point Conception late Sunday into Monday
morning. Any rainfall that does fall is expected to be light,
generally less than a tenth of an inch. More significant
precipitation is possible later next week, sometime in the
Thursday to Friday time frame but there is considerable
uncertainty in timing and strength with this system.

CA...Dense Fog Advisory in effect until 9 AM PDT this morning For
      zones 34-35-39. (See LAXNPWLOX).
     Beach Hazards Statement in effect through this evening For
      zones 34-35. (See LAXCFWLOX).



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