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Southwest California area forecast discussion. 
National Weather Service Los Angeles/Oxnard California
1048 am PDT Sat Apr 18 2015

Synopsis...
expect a steady cooling trend through the middle of next week with
low clouds and fog in the coastal and valley areas. There will be a
slight chance of thunderstorms over some of the mountains early in
the week and a more widespread chance for showers and isolated
thunderstorms by Wednesday.

&&

Short term (rest of today-mon)...
dense fog along the central coast is slowly lifting this morning per
visible imagery...webcams...and observations. Some localized areas
still dense but for the most part the clearing trend is enough to
let the dense fog advisory expire. Some central coast beach areas
may have a difficult time clearing today otherwise expect most skies
to clear through the rest of the morning.

Surface pressure gradients have fully switched to onshore to begin our
cooling trend that will continue into next week. Highs today will be
5 to 8 degrees cooler than yesterday's...the most cooling along the
central coast. As what typically happens on these offshore to
onshore transition days the av will warm a few degrees in addition
to getting gusty southwest winds this afternoon. Overnight stratus
accompanied by some possible dense fog will also spread across
coastal areas south of Point Conception tonight...the one area in
question being the sba County South Coast which will still have some
offshore gradients tonight and tomorrow morning.

The upcoming week of troughiness begins tomorrow when an upper
trough drops south out of the Pacific northwest and begins to close off to our
west...then sharpen on Monday. There may be just enough instability
Sunday afternoon to add a slight chance of thunder vtu County
mountains. Forecast already has slight chance thunderstorm- for Monday
afternoon and that looks reasonable. Cooling both Sunday and Monday
due to the influence of the upper level trough that remains close by
through the period. A repeat of overnight low clouds and fog likely
Sunday night into Monday morning in continued onshore gradients and
to be across nearly all coastal and even nearby valley areas.

***From previous discussion***

Long term (tue-fri)...both the GFS and European model (ecmwf) are in fairly good
agreement with continuing an upper level trough persisting over the
forecast area through all of next week. There will be another slight
chance of elevated thunderstorms over the Ventura/sba County
mountains on Tuesday afternoon and evening with partly cloudy skies
likely increasing over the region. Both models continue to show the
upper low digging farther south and cutoff over the Southern
California coastal waters on Wednesday and Thursday. As has been
typical most of this past winter...the upper low will make it very
difficult to Pin Point when and where the precipitation will occur.
With 500 mb temperatures expected to be around -25 degrees celsius near the
center of the low...there will be the possibility of isolated
thunderstorms over the region. At this point will hold off on adding
thunder. But as we get closer to this scenario...there is a good
chance that some mention of "slight chance thunderstorms" will be
added. Snow levels will also be lower for this time of year...around
5000 feet...Wednesday into Wednesday evening...then lowering to around
4000 feet near the Grapevine. Wednesday night into Thursday morning.
We are not expecting much in the way of precipitation with this
system...except there could be brief periods of heavy rain with any
thunderstorms.

&&

Aviation...18/1800z.

Middle/upper level trough of low pressure over the area will persist.
Upper level moderate west winds will become moderate west-southwest
after 19/13z and middle level light south winds will prevail through
the period over the area. Weak to moderate onshore pressure gradient
through 19/19z then moderate onshore gradient. Weak capping low
level inversion base was approximately .7kft with an overcast cloud
field west and north of The Channel islands and clear over the bight
this morning. The inversion base is expected to differ by plus .7kft
and cloud field will become broken to overcast over the bight Sunday
morning.

Marine layer at lax at 1550 is 784 feet deep and the inversion top
is at 2056 feet with a temperature of 18 degrees c.

Klax...chance ceilings 007 visibility 5sm br between 19/10-19/12z and chance
ceilings 005 visibility 1sm br between 19/12-19/17z.

Kbur...chance ceilings 005 visibility 3sm br between 19/11-19/13z and chance
ceilings 003 visibility 1sm br between 19/13-19-17z.

Virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less

&&

Marine...18/900 am.

Northwest winds in the vicinity of the northern Channel Islands with
occasional gusts to 25 knots will continue to diminish through the
remainder of the morning. The occasional gusts will likely return
this evening and then diminish around midnight. Otherwise Small
Craft Advisory conditions are not expected through Wednesday.

&&

Lox watches/warnings/advisories...
Small Craft Advisory (see laxmwwlox).
High surf advisory (see laxcfwlox).
Marine weather statement through 19z(see laxmwslox).

&&

$$

Public...Jackson/Kaplan
aviation/marine...30
synopsis...Sweet

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