Debris Flow in California, December 6, 2018

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
512 AM PST Thu Dec 6 2018

.SYNOPSIS...06/511 AM.

Light to moderate rain and mountain snow will continue through
this evening, as low pressure system moves across the region.
Additional showers are possible through Friday as well as cool
temperatures. A high pressure will bring dry and warm conditions
over the weekend. Another storm system could move into the area
early next week, bringing chances of rain and cooler temperatures.

&&

.SHORT TERM (TDY-SAT)...06/304 AM.

Main weather concern over the next 24 hours will be shower
activity associated with upper low located off the coast of
Southwest California. Rainfall rates have increased across
portions of LA/Ventura counties as the upper low approaches and
turns the low level flow more southerly.

The is upper low is currently just to the west of Pt Conception
and is moving eastward. The current track is further north than
previously forecasted and this will bring better upper level
diffluence and colder temperatures aloft. This will result in more
destabilized atmosphere, especially near the core or the upper low
that has 500 mb temperatures near -22 Celsius. Perhaps more
significantly the low levels of the atmosphere have become more
more saturated which is allowing much better rainfall rates and
amounts despite the low lever offshore flow. LI values of -2 to
-4 will move into the southern waters and southern LA county today
as the cold core moves overhead. There is also enough veering in
the lower atmosphere to increase the risk of waterspouts.

Rainfall will be fairly light north of Pt Conception with amounts
mostly under a quarter inch. Rainfall amounts will ramp up from
west to east south of Pt Conception with about a quarter inch of
rain forecast for southern SBA county and western VTA county...a
quarter to a half inch over eastern VTA county. LA county (xcp for
the interior) will be the big winner today with a half inch to
three quarters of an inch falling. Rainfall rates will be near .5
inches per hour and could be higher this afternoon as the
atmosphere becomes more unstable.

There will be a decent amount of snow above 6500 feet with some
snow falling as low as 6000 feet. A winter weather advisory is in
effect now through midnight for 3 to 6 inches of snow with
isolated higher amounts above 7500 feet.

After 3 days of clouds and rain the upper low responsible for it
all will quickly shift to the east and ridge will build in in the
predawn hours of Friday. The rain will end and skies will clear.
Friday should dawn mostly sunny. Hgts will rise and will kick off
a warming trend but not enough to bring max temps back to normal.

Weak offshore flow and continued ridging will make Saturday quite
the nice day. Max temps will warm to near normal under sunny
skies.

.LONG TERM (SUN-WED)...06/252 AM.

The ridge will slowly slide to the east on Sunday. Offshore flow
will continue at the sfc. The hgts will only lower slightly and
not enough to offset the offshore flow. Max temps will rise
another 2 to 4 degrees and max temps will be above normal almost
everywhere. Sunday will be the warmest day of the next 7.

Both the EC and GFS agree that a trof will plunge into the state
from Oregon. Hgts will begin to fall, the offshore flow will
start to reverse and the clouds will increase. At one point it
looked like there might be some rain with this system but now with
the trof taking more of an inside track there is just a slight
chc of rain over SLO county. Max temps will drop a couple of
degrees esp at the coasts and across SLO county.

Both the EC and GFS now pinch off a small upper low at the base of
the trof and drop it through the center of the state on Tuesday.
Both advertise rain with this system (although not much) This is
a new development and for now only worth slight chc wording. Good
confidence however that max temps will drop as hgts fall to 562
DM and skies cloud over. Max temps will likely drop by 4 to 8
degrees.

Dry anticyclonic NW flow sets up on Wednesday. Skies will clear
out and max temps will warm some although will remain under
seasonal normals.

&&

.AVIATION...06/1302Z.

At 1112Z, there was no marine layer nor a sfc based inversion.

Moderate confidence in all TAFs. Cigs will frequently vary
between 015 and 025. There is a 30 percent chc of BKN015 and a 10
percent chance of bkn008. Scattered rain showers will persist
through the night and into the day. Rain showers will increase
later Thursday afternoon and increase in intensity over LA county.

KLAX...Moderate confidence in TAF. Cigs will frequently vary
between 025 and 035. There is a 30 percent chc of BKN015 and a 10
percent chance of bkn008. Moderate to high confidence of east
winds around 10 knots or greater through this evening, and again
on Thursday morning, with occasional gusts over 15 knots. East
winds will diminish towards sunset. Heavier showers are possible
after 00Z with vsby 2-3SM.

KBUR...Moderate confidence TAF. Cigs will mostly be between
015-025. There is a 20 chance of bkn008. Heavier showers are
possible after 00Z with vsby 2-3SM.

&&

.MARINE...06/455 AM.

For the Outer Waters, high confidence in current forecast. There
will be local gusts to 25 knots at times across PZZ670 through
this afternoon. Friday through Sunday, winds and seas will remain
below SCA levels. On Monday, there is a 60% chance of SCA level
winds developing again.
For the Inner Waters north of Point Sal, high confidence in
current forecast. Winds and seas will remain below SCA levels
through Sunday. On Monday, there is a 30% chance of SCA level
winds in the afternoon/evening hours.

For the Inner Waters south of Point Conception, moderate
confidence in current forecast. SCA level east to southeast winds
will continue across PZZ650 from the Ventura Coast through the
channel islands. There will be local gusts across the northern
portion of PZ655 through this afternoon. Otherwise, winds and seas
are expected to remain below SCA levels.

An upper level low pressure system will bring a slight chance of
thunderstorms to the coastal waters mainly west and south of Point
Conception today. Any thunderstorms that develop could produce
brief heavy downpours with reduced visibility, gusty winds, small
hail, and dangerous cloud to ground lightning. In addition,
conditions will be favorable for isolated waterspouts across the
coastal waters including areas near Huntington Beach to the Palos
Verdes peninsula.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Weather Advisory in effect until midnight PST tonight
      for zones 53-54. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect until 3 PM PST this afternoon
      for zone 650. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SAT-WED).

No significant hazards expected.

&&

PUBLIC...Rorke
AVIATION...Rorke
MARINE...Kaplan
SYNOPSIS...Delerme/Sirard

weather.gov/losangeles