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FXUS66 KLOX 220238

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
738 PM PDT Wed Mar 21 2018


21/600 PM.

A strong storm will continue to move over the region and bring
waves of significant precipitation to the region through early
Friday. This could result in debris flows at all recent burn areas
as well as periods of urban and small stream flooding. The threat
of heavy rainfall and a chance of isolated thunderstorms will
continue through Thursday afternoon. Notable snowfall will remain
above 9000 feet.



21/734 PM.


The atmospheric river (AR) which was oriented in a mostly W to E
direction is now in the process of shifting to a more SW to NW
direction. This shift was caused as the upper low which was near
30N 140W was absorbed into a larger upper low NW of Seattle and
switched the upper flow from W to SW. The rain lifted to the N an
W as the AR tilted and now rain is almost entirely confined to SLO
and SBA counties and will likely remain there overnight.

Satellite shows quite a bit of lightning activity in the plume and
this energy is going to move into SLO county after midnight...
probably more towards dawn and this will be the main area of
concern for tonight.

The jet is fairly parallel to the AR and it too will be bending up
into more of a SW to NE orientation. As the whole complex (trof,
AR, jet) moves through the area starting tonight across the
Central Coast the jet energy will be enough to bring advisory
level winds to the higher terrain and the interior sections. the
Advisory level winds will start later this evening across SLO and
SBA counties and then towards dawn across VTA and LA county.

The jet energy will also create a favorable environment for TSTMs
esp tomorrow afternoon. This area of instability/dynamics now
looks like it will hold together all the way through LA county by
late afternoon Thursday.

Will issue an update to lower VTA/LA pops, add wind advisories to
the mtns and interior, and xtnd TSTM threat into LA county.

***From Previous Discussion***

Thursday morning low level winds are expected to back to the south
and increase quite a bit as the trough and cold approaches. At the
same time the moisture plume will shift back south and the
combination of those factors will bring a return of very heavy
rain to SB and Ventura Counties and by early afternoon LA County.
In fact, rainfall rates during this time are expected to be higher
than they were today as the orographic effects will be much
stronger and there will be much better atmospheric forcing with
the cold front and possibly even some isolated thunderstorms,
though model soundings were decidedly unimpressive with the
stability parameters. Once again rain rates of an inch per hour or
higher are possible during this period and debris flows likely
near burn areas.

Rain expected to start decreasing from west to east Thursday
afternoon and finally exiting LA County early Friday morning.
Models show a weak trough moving into northern California Saturday
but seriously lacking moisture. Still the trough is deep enough
that a few very light showers can`t be ruled out north of Pt


21/142 PM.

The trough that moves through Saturday is now expected to evolve
into a cut off low near Las Vegas Tuesday with northerly flow
developing behind it. It should remain dry as the trajectory isn`t
favorable for moisture but still uncertain how much offshore flow
we`ll see. Models have definitely trended cooler as the low now is
farther west and we`ll likely see some gusty north to northeast
winds at times through mid week but below advisory levels at this
point. Temps will warm up at lower elevations but remain cool
farther inland.




At 23Z, there was no marine inversion at KLAX.

Moderate confidence in the current forecast. MVFR to VFR
conditions with ceilings below 5000 feet are likely throughout
the period. Periods of IFR conditions are likely in heavier
rainfall. There is a possibility of moderate wind shear and
turbulence after 14Z.

KLAX...VFR conditions with ceilings below 5000 feet will
deteriorate to MVFR between 04Z and 07Z. There is a 50 percent
chance of IFR conditions in heavier rainfall after 16Z,
increasing to 60 percent around 23Z. East winds greater than 7
knots are likely through 14Z, possibly increasing to greater 10
knots after 16Z. There is a 30 percent chance of southerly winds
between 10 and 20 knots between 18Z and 23Z.

KBUR...VFR conditions with ceilings below 5000 feet will
deteriorate to MVFR between 04Z and 07Z. There is a 50 percent
chance of IFR conditions in heavier rainfall after 16Z,
increasing to 60 percent around 23Z. There is a 10 percent chance
of moderate wind shear and turbulence around 23Z.



21/1235 PM

SCA gusty south to southeast winds will continue into Thursday for
the Outer Waters and Inner Waters north of Point Sal as well as a
SCA for the western portions of the Santa Barbara Channel through
tonight. There is a 20 percent chance of reaching Gale force
winds for the Outer Waters sometime this evening into Thursday
morning. Isolated thunderstorms are possible at times across the
waters through Thursday evening. Brief gusty and erratic winds
with an isolated water spout is possible near any thunderstorm.

SCA winds are likely again late Friday into this weekend across
the Outer Waters with a 30 percent chance of reaching the inner
waters at times. There is also a 20 percent chance of Gale force
winds across the Outer Waters by Sunday.


CA...Flash Flood Watch in effect through Thursday afternoon for
      zones 34-36-37-39-40-44-51>53. (See LAXFFALOX).
     Wind Advisory in effect until 11 AM PDT Thursday for zones
      37-38-51-52. (See LAXNPWLOX).
     Flash Flood Watch in effect from 11 PM PDT this evening
      through late Thursday night for zones 41-46-54-88-547. (See LAXFFALOX).
     Wind Advisory in effect from 5 AM to 8 PM PDT Thursday for
      zones 53-54-59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for
      zones 645-650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT Thursday for
      zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for zone
      673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 PM PDT Thursday for
      zone 676. (See LAXMWWLOX).



No significant hazards expected.




Previous Forecast Discussions may be found at
NWS , (LOX) Office Forecast Discussions.
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Some may differ only in time posted.)

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