Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 181004

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
304 AM PDT Sun Mar 18 2018


17/311 PM

Below normal temperatures will continue into Sunday, then the air
mass should warm to normal through Tuesday. Showers are possible
through tonight, mainly north of Santa Barbara. A drying trend is
expected Sunday through Monday. A significant storm system could
bring widespread moderate to heavy rainfall to the region between
Tuesday and Thursday night, with a potential debris flow threat.



8/302 AM

Skies will clear over most the forecast area this morning save for
a grip of north slope clouds that also cover the Cuyama Vly and
some low clouds near Paso Robles. Skies will turn partly cloudy
this afternoon and some high clouds move overhead. A weak 563 DM
ridge will move overhead as well. It will be dry today but max
temps will be 8 to 10 degrees blo normal across most of the coasts
and vly locations.

A weak trof ripples across the top of the state. It will usher in
a fair amount of mid and high clouds. Skies will be at a minimum
partly cloudy and will be mostly cloudy at times also.

Monday will be the warmest day of the next 7 as a 570 DM ridge
quickly slides from west to east over the state. A 1030 MB sfc
high will move into NV and set up weak offshore flow over the
state. The combination of increased hgts...offshore flow and sunny
skies will produce a 5 to 10 degree warm up across most locations
and will bring max temps to about normal.

At dawn on Tuesday the western portion of the ridge will be over
the state. More importantly a 536 DM upper low will be 1000 miles
to the west of the Bay Area. This location is much further west
and south than usual. As a result a long westerly flow pattern
sets up from under the low to srn CA. The flow will entrain a
large amount of subtropical moisture and will set the stage for
the season`s first atmospheric river event. The moist WSW flow
will approach the state.

All the mdls handle the details of this storm differently. The GFS
is by far the fastest and it brings rain to the entire forecast
area save the Antelope Vly by 18Z. The EC is slower but keeps the
moisture plume as far south as the brings rain to the
Central Coast by 18Z. The NAM is both slower than either spectral
mdl and also further north with its track of the 18Z it
only brings rain to the NW tip of SLO county. By 00Z the GFS is
going great forecasts rain over the entire forecast area
along with heavy rain over most of SLO and SBA counties. The EC
only brings rain as far south as VTA county and has no heavy rain
an barely any mdt rain. The NAM brings rain only to SLO and SBA
counties but does bring mdt/hvy rain to the western portion of
SLO county.

Clouds will will overspread the area and max temps will fall (a
great deal if the GFS is correct but only 2 or 3 degrees if the
NAM is correct)


18/302 AM


Both the EC and GFS agree that there will be a significant
hydrological event driven by an atmospheric river moving into SRn
CA. The EC and GFS do not agree on the details. In 24 hours time
the the NAM will provide a forecast for the first 36 hours of the
storm and hopefully at that time the large amount of forecast
uncertainty will be reduced.

The GFS is much faster and also much harder hitting than the EC.
The GFS wraps up the event Wednesday evening but produces
significantly more rainfall during the EC does from Tue Night to
Wed Evening. The EC keep the river over SRN CA a full 24 hours
longer than the GFS but does not forecast as strong rainfall
rates. The NAM does not extend far enough into the event to
provide meaningful guidance but it does seem to be leaning towards
a slower solution.

Very early rainfall estimates for the Tue-late Thu time from are:
1.50 to 4.00 inches for the coast and valleys, and 3-6 inches in
the foothills and mtns. Peak rainfall rates have the potential to
be 0.50-0.75 inch per hour, with local rates up to 1.00 inch per
hour, especially on S facing slopes. During the heaviest rains, it
will be possible to see 3-hour rainfall up to 1 to 2 inches in
some areas.

This storm will bring the potential for major mud and debris flows on
the recent burn areas, including the Thomas, Whittier, La Tuna
and Creek burn areas. Widespread urban and small stream flooding,
and rockslides are likely at times during this event.

The upper low will move east and open up into a trof before
moving over the state sometime late Thursday (GFS) or Friday (EC)
the trof will usher in dry NW flow. The rain will stop and skies
will clear. Max temps will warm3 to 6 degrees but will still be
blo normal.




At 0900Z, there was no marine inversion at KLAX.

Overall, high confidence in 12Z TAF package. Other than VLIFR
conditions this morning at KPRB, VFR conditions are anticipated
for all sites through the period.

KLAX...high confidence in 12Z TAF.

KBUR...high confidence in 12Z TAF.



18/158 AM

For the Outer Waters, good confidence in current forecast. Today
through Monday night, high confidence in winds and seas remaining
below Small Craft Advisory (SCA) levels. For Tuesday and
Wednesday, 60% chance of SCA level southeast to south winds
across PZZ670/673 and a 30% chance across PZZ676. On Thursday,
60% chance of SCA level west to northwest winds across the entire
Outer Waters.

For the Inner Waters, good confidence in current forecast. For the
waters north of Point Sal, there is a 60% chance of SCA level
southeast to south winds Tuesday and Wednesday then a 50% chance
of SCA level west winds on Thursday. For the waters south of Point
Conception, there is a 50% chance of SCA level southerly winds on
Wednesday and westerly winds on Thursday.





Periods of moderate to heavy rain are possible Tuesday night
through Thursday night. Rainfall rates during this time will
likely exceed USGS thresholds for debris flows for recent burn



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