Flash Flood Watch 11, NWS Los Angeles, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
144 PM PST Fri Dec 30 2016

.SYNOPSIS...

Two storms will bring showers and slight chance of thunderstorms
to the area Today and Saturday. It will be dry with much below
normal temperatures Sunday into early next week.

&&

.SHORT TERM...(TDY-MON)

Upper-level low pressure is currently near Pt Conception and is
expected to slowly drift eastward tonight. Most of the steady rain
has lifted north and east of the region, with some scattered to
isolated showers across the area. Still a slight chance of
t-storms south of SLO County through this afternoon, but the
threat is very minimal. There is currently a cluster of showers
near the Lompoc area that looks a little convective, but the 1-hr
amounts with this have been below 0.10 in/hr.

Scattered to isolated showers will continue through this evening,
but will continue to diminish overnight. A much colder system
quickly approaches from the NW tomorrow morning and precip chances
will increase through the day and into NYE. This system has yet
to fully develop, but a small swirl can be seen in the water vapor
imagery near Oregon. The best chance for precip with this system
will be tomorrow afternoon through NYE in most places, with
precip continuing into early Sunday morning for the AV and north-
facing mountain slopes.

The main story with the NYE storm will be the low snow levels and
significant snow accumulations in the mountains. A Winter Storm
Warning will be issued for the San Gabriel Mountains. The most
dangerous period in the San Gabriel Mountains will likely occur
after about 4pm on NYE, when heavy wrap around precip and
lowering snow levels are expected. Up to a foot of snow is
expected by New Year's Day for the highest elevations in the San
Gabriels. Snow levels are expected to lower to the Grapevine by
NYE as well, with a good chance for light accumulations on the
Grapevine. This could cause some major travel headaches for people
planning to travel over the Tejon Pass. Winter Weather Advisories
remain in effect for the Ventura and SBA County mountains. The
foothills of the Antelope Valley may need to be added to the
advisory as well. Please see WSWLOX for additional details on the
expected snow.

As far as rainfall amounts with the NYE storm, most populated
coastal and valley areas will receive 0.10-0.50", but the amounts
will be highly variable as this will be a very showery system,
with no well defined surface cold front to focus the rain along.
At the moment, the forecast includes a slight chance of
thunderstorms for Ventura and LA County tomorrow afternoon and
NYE. T-storms may need to be added for SLO and SBA counties too.
There is a very small threat for flash flooding and debris flows
near the burn areas, but not large enough to warrant a Flash Flood
Watch at this time.

Precip will quickly come to an by New Years Day, with some
lingering snow showers possible on the north-facing mountain
slopes in the morning. Cool and breezy conditions are expected
through Monday with highs generally in the 50s for coastal and
valley areas and gusty W-NW winds.

.LONG TERM...(TUE-FRI)

Somewhat low confidence in the long term, as long range models are
now showing much different conditions for Wed than they were
yesterday. The overall upper-level pattern looks about the same
for Tue-Thu, with a broad long-wave trough slowly shifting east.
However, both the GFS and ECMWF now have a fairly significant rain
event for Wednesday. Yesterday, they were both showing a fairly
strong offshore flow event developing by Wednesday. Tuesday should
stay mostly dry, except for maybe some light precip on the Central
Coast with moist onshore flow. For Tuesday night through
Wednesday night, just broad brushed the area with slight chance
PoPs to account for the low confidence in the GFS and ECMWF. Dry
conditions and offshore flow are more likely by Thu-Fri.
Regardless of whether or not it rains next week, it will be a cool
pattern, with below normal temperatures likely through at least
Thursday.

&&

.AVIATION...30/1620Z...

At 1600Z, there was a surface-based inversion at KLAX. The top of
the inversion was 1100 feet with a temperature of 15 degrees
Celsius.

Overall, moderate confidence in 18Z TAF package. Through this
evening, expect VFR ceilings as storm moves out of the area with
light showers gradually diminishing (although there will be a 30%
chance of MVFR ceilings developing through this afternoon). There
will be a slight chance of TSTMs all areas this morning then
across just LA county this afternoon. For tonight, moderate
confidence in VFR conditions continuing (there will continue to be
a 30% chance of MVFR ceilings developing). Showers could begin to
impact the area again after 12Z.

KLAX...moderate confidence in 18Z TAF as VFR conditions are
forecast to continue. There is a 30% chance of MVFR ceilings
developing through the period. A slight chance of TSTMs will
continue through this afternoon.

KBUR...moderate confidence in 18Z TAF as VFR conditions are
forecast to continue. There is a 30% chance of MVFR ceilings
developing through the period. A slight chance of TSTMs will
continue through this afternoon.

&&

.MARINE...30/130 PM...

Isolated showers will continue across the coastal waters tonight.
On Saturday and Saturday night, another storm will move across the
coastal waters. This storm will bring another round of showers and
the slight chance of thunderstorms.

For the Outer Waters, moderate confidence in current forecast.
Northwest winds will be on the increase on Saturday behind the
passing cold front. Small Craft Advisory (SCA) level winds will be
possible Saturday afternoon/night. For Sunday through Wednesday,
the gusty northwest winds will continue with a good chance of SCA
level winds developing.

For the Inner Waters, moderate confidence in current forecast. For
the waters north of Point Sal, winds and seas are expected to
remain below SCA levels through Wednesday (although there is a 30%
chance of SCA level winds developing on Sunday and Sunday night).
For the waters south of Point Conception, there is a 30% chance of
SCA level winds developing on Saturday and Sunday across western
sections of the waters. After a lull Monday/Tuesday, there is a
good chance of winds reaching SCA levels on Wednesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory in effect from 8 AM Saturday to
      midnight PST Saturday night for zone 52. (See LAXWSWLOX).
     Winter Weather Advisory in effect from 8 AM Saturday to 6 AM
      PST Sunday for zone 53. (See LAXWSWLOX).
     Winter Storm Warning in effect from 8 AM Saturday to 6 AM PST
      Sunday for zone 54. (See LAXWSWLOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK...(MON-FRI)

There is a small chance for a significant rain event around the
middle of next week.

&&

PUBLIC...Sukup
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Sweet

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