Winter Storm Warning 1, NWS Los Angeles, CA

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
1125 AM PST Sun Jan 7 2018

.SYNOPSIS...

Dry and mild weather through tonight. A strong storm system Monday
and Tuesday will bring significant rain, mountain snow, gusty
winds, and a slight chance of thunderstorms to the region. Debris
flows across the recent burn areas are possible with the heavy
rain. Gusty northerly winds and a diminishing shower threat on
Wednesday will be followed by dry weather and a warming trend into
the weekend.

&&

.SHORT TERM...(TDY-TUE)

Increasing middle and high level cloudiness will start to occur
throughout the rest of today and into this evening as ridging
aloft centered near the southern tip of Baja California weakens.
The ridge axis currently near the Los Angeles-Riverside-San
Bernardino County borders will continue to exit the area today and
tonight allowing southwesterly flow to develop aloft.

The air mass will moisten up rapidly tonight as a warm front moves
over the region late tonight and into Monday. PoPs have been
nudged higher for mainly late tonight and into Monday morning as
model precipitation values reflect light widespread precipitation
developing across the area during this time. Small tweaks were
made to precipitation amounts for the storm, but not enough to
change the overall forecast.

Still, the bulk of the rainfall with this storm system will fall
between Monday night and Tuesday across the region. Storm total
amounts still look to range from 1 to 3 inches of rainfall is
expected for the coastal and valley areas, and 3 to 5 inches for
the foothills and mountains. South facing slopes are forecast to
receive local amounts up to 5 to 6 inches.

Rainfall will be heavy at times on Monday night and Tuesday,
especially across portions of Los Angeles, Ventura, and Santa
Barbara Counties. Peak rainfall rates still look to be in the 0.50
to 1.00 inch per hour thresholds. These thresholds could be
locally exceeded beneath any thunderstorms that develop. A Flash
Flood Watch remains in effect between late Monday afternoon
through Tuesday evening. While much of the focus has been placed
on the potential for flash flooding and debris flows near burn
areas, which is of greatest concern, some concern also needs to
be placed for urban flooding and roadway flooding. Rainfall rates
such as these will be sufficient enough to produce urban and
roadway flooding. The Flash Flood Watch may be expanded to include
this idea later today.

Snow levels above 8000 feet tonight through Monday evening will
fall rapidly late Monday night into Tuesday morning behind the
system's cold front. Snow levels will quickly fall to near 6000
feet before daybreak Tuesday morning, then to near 4000 feet by
Tuesday afternoon. It is possible that the Grapevine portion of
the Interstate 5 Corridor could see light snow accumulations and
icy conditions late Tuesday afternoon and evening. Generally,
snowfall amounts are forecast to be between 8 to 18 inches above
8000 feet, with between 3 and 6 inches falling between 6000 to
8000 feet. A Winter Storm Watch remains in effect as strong and
potentially damaging winds will likely develop ahead of and along
the cold front with moderate to heavy snowfall likely developing
along and behind the cold front. This watch may be upgraded later
today to a Winter Storm Warning.

Strong and potentially damaging southerly winds will also develop
ahead of the storm system. High Wind Warning level wind gusts
between 60 to 70 mph are likely with this system across the
mountains and interior valleys. A High Wind Watch remains in
effect from Monday afternoon and Tuesday evening and may be
upgraded to High Wind Warning later today. Advisory level wind
gusts are also possible across the across the remaining coastal
and valley areas.

*** FROM PREVIOUS DISCUSSION ***

The will be out of the area by Tuesday afternoon it will leave a
moist unstable airmass in its wake and there will be plenty of
showers and isolated TSTMs across the area. The winds will switch
from the south to the north and will decrease in speed. The
rainfall will quickly diminish late Tuesday afternoon and evening.

.LONG TERM...(WED-SAT)

Drier NW flow sets up behind the storm on Wednesday. The only chc
of a shower will be up against the north slopes of the mtns. But
overall just look for clearing skies. There will be a decent
amount of westerly winds across the area as there will be a strong
west to east sfc gradient and a decent amount of cold air
advection. Even though it will be about 6 degrees warmer than it
was on Tuesday max temps across the coasts and vlys will still
struggle to reach the mid 60s.

Upper level ridging and offshore flow sets up Thursday through
the weekend and this will produce mostly sunny skies along with a
warming trend. Afternoon high temperatures could be 10 degrees
above normal by Friday and perhaps 15 degrees above normal on
Saturday with some highs in the lower 80s.

&&

.AVIATION...07/1715Z.

At 1700Z, the marine inversion at KLAX was based at 1400 feet. The
top of the inversion was 3400 feet with a temperature of 15
degrees Celsius.

Overall, low to moderate confidence in 18Z TAF package. For
coastal zones, current VLIFR-IFR conditions is expected to
dissipate by 20Z. For tonight, MVFR CIGs should return this
evening, although confidence in timing is low. Overnight, light
rain is expected to develop across the Central Coast and work its
way down to the bight by Monday morning.

KLAX...low confidence in 18Z TAF. There is a 40% chance that
current IFR restrictions may not dissipate this afternoon. For
tonight, low confidence in timing of return of MVFR CIGs (+/- 3
hours of current 03Z forecast). There is a 30% chance of easterly
winds increasing to 8 knots after 14Z.

KBUR...moderate confidence in 18Z TAF. Current LIFR conditions
could dissipate before 20Z. For tonight, low confidence in timing
of return of MVFR CIGs (+/- 3 hours of current 07Z forecast).

&&

.MARINE...07/830 AM.

For the Outer Waters, good confidence in current forecast. High
confidence in winds/seas remaining below Small Craft Advisory
(SCA) levels through Sunday night, altho there is a 30 percent
chance of SCA wind gusts this evening around Point Conception. On
Monday and Monday night, southeast to south winds will increase
with Gale force winds possible (70% chance). A GALE WATCH remains
in effect for Monday and Monday night. Late Monday night and
Tuesday, the winds will shift to the southwest and west then to
the northwest Tuesday night through Thursday. Wind speeds from
Tuesday through Thurday will likely remain at SCA levels.
For the Inner Waters, good confidence in current forecast. For
the waters north of Point Sal, winds and seas will remain below
SCA levels through Monday morning. From Monday afternoon through
Monday night, southeast winds will be increasing with a 60%
chance of Gale force winds developing. Therefore, a GALE WATCH
remains in effect. By Tuesday, the winds will shift to the
southwest then the northwest on Wednesday, likely remaining at SCA
levels. For the waters south of Point Conception, southeast to
south winds will increase to strong SCA levels Monday and Monday
night with even the possibility of Gale force gusts across the
western SBA Channel (where a GALE WATCH remains in effect). By
Tuesday and Wednesday, the winds will shift to a westerly
direction with a chance of SCA level winds continuing.

There will be isolated thunderstorms across the coastal waters
with a threat of weak waterspouts from Monday evening through
Tuesday afternoon. Read the latest Marine Weather Statement for
more details.

A large westerly swell will move across the coastal waters
Tuesday and Wednesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch in effect from Monday afternoon through
      Tuesday evening for zones 36-39-40-44-52-53. (See LAXFFALOX).
     High Wind Watch in effect from Monday afternoon through
      Tuesday morning for zones 37-38-51-52-59. (See LAXNPWLOX).
     Flash Flood Watch in effect from Monday evening through
      Tuesday evening for zones 41-46-54-88-547. (See LAXFFALOX).
     Winter Storm Watch in effect from Monday afternoon through
      Tuesday evening for zones 53-54. (See LAXWSWLOX).
PZ...Gale Watch in effect from Monday morning through late Monday
      night for zones 645-670-673. (See LAXMWWLOX).
     Gale Watch in effect from Monday afternoon through Monday
      evening for zone 650. (See LAXMWWLOX).
     Gale Watch in effect from Monday afternoon through late
      Monday night for zone 676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)

A storm system will move out of the area Tuesday. Possible impacts
include...periods of heavy rain and possible flash flooding and
debris flows primarily near recent burn areas (including the
Thomas Fire), accumulating snow down to as low as 4000 feet,
strong and potentially damaging southerly winds, and high surf
with minor coastal flooding. Gusty northerly winds could develop
Tuesday night through Thursday bringing travel impacts to the
Interstate 5 Corrifor and across Southern Santa Barbara County.

&&

PUBLIC...Hall/Rorke
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Kj

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