Flash Flood Watch 9, NWS Los Angeles, CA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1020 PM PST Tue Dec 13 2016

.SYNOPSIS...
Dry conditions with near-normal temperatures can be expected
through Wednesday, with areas of coastal low clouds and fog during
the overnight and morning hours. A Pacific storm system will bring
widespread rain, possible mountain snow and gusty winds to the
region Thursday and Thursday night. A chance of showers and
mountain snow showers will linger Friday. Dry and cool conditions
will then prevail through the weekend.

&&

.UPDATE...

Ridging aloft remains over the area this evening from an upper-
level high pressure system off the Baja California Coast. A
slight warm-up should continue into Wednesday as 500 mb heights
climb and 1000-500 mb thickness values increase above weak onshore
flow. The ridge axis will slowly slide over the area through
Wednesday, then the air mass should start to moisten at the lower
levels ahead of the impending storm system.

Onshore flow will keep some semblance of a marine layer stratus
deck in place through Wednesday morning, but middle and high level
moisture streaming over the area will play tricks on the marine
inversion. Moderate confidence exists in the stratus forecast as
the stratus deck could disorganized.

Winds have been increased across the area, especially for the
mountains and desert on Wednesday afternoon and night. Breezy
southwest winds should develop, especially across the foothills of
the Antelope Valley.

&&

.SHORT TERM...(TUE-FRI)

No big changes to the forecast for Thu/Fri. Models didn't offer up
too much in the way of changes, though GFS was a touch lower on
the precip amounts. Nothing significant though and likely just
typical model wobble. In terms of the onset time we should be
seeing rain along the Central Coast Thursday morning and reaching
Ventura and possibly the western portion of LA County late in the
afternoon. The big burst though will be overnight, particularly
between 10pm and 6 am.

Overall amounts and impacts still the same. Lower elevations
should average around an inch plus or minus and foothills/mtns in
the 1-3 range. Though models are still very unimpressive with the
convective parameters the high pwats (around 1.5" just offshore)
combined with a warm air mass and decent upslope flow should make
for an efficient rainfall producer. So even without thunderstorms
there should be pockets of heavy rain where rates are right up
there against the low end of the short term usgs debris flow
thresholds, particularly the 15/30 minute thresholds. So if
models still look good tonight we'll probably go with a burn area
flash flood watch for Thursday night and early Friday.

Still some decent rain for LA County during the Friday morning
rush hour commute but showers will taper off quickly by lunch
time.

Snow levels will be well over 9000' for a good portion of the
event. The cold front that will come through Friday morning will
lower them but not quickly enough to generate much accumulation
and am not expecting to need any winter headlines for this event.
Perhaps an inch or two at some of the higher elevations Friday
morning.

Breezy Friday afternoon behind the front, possibly close to
advisory levels for some of the coastal areas.

.LONG TERM...(SAT-TUE)

Mostly clear but cool weekend with some frosty mornings. Gradients
turn offshore Sunday and peak Monday, possibly requiring some low
end wind advisories for Santa Anas. Another cool offshore event
with highs no warmer than the low 70s at best. Lighter winds
Tuesday and slightly warmer.

&&

.AVIATION...

14/0600Z

At 0540Z at KLAX, there was a weak 1200 foot marine layer. The
top of the inversion was around 4100 ft with a temperature of 15 deg
C.

Low confidence in non desert TAFs. Marine layer low clouds will
likely develop haphazardly. Cigs will mostly be in the MVFR
category but there is a 30 percent chc of IFR cigs. Good
confidence in slow to no clearing for the afternoon.

KLAX...Low confidence in TAF. Low clouds have an equal chc of
forming from 07Z-11Z. There is a 30 percent chc of IFR conds from
10Z-16Z.

KBUR...Low confidence in TAF. Low clouds have an equal chc of
forming from 08Z-13Z. There is a 30 percent chc of IFR conds from
10Z-16Z.

&&

.MARINE...

13/900 PM.

Winds and seas will remain below small craft advisory criteria
through at least Wednesday night, then south winds should
increase between Wednesday night and Thursday ahead an approaching
frontal system. Small craft advisory levels winds will likely
develop at least across the outer and northern waters Thursday.

Winds will increase further and seas will build behind the
frontal system late Thursday night and Friday. Gale force west to
northwest winds with steep short-period hazardous seas are
possible across the entire waters late Thursday night through
Friday.

Patchy dense fog with visibility one nautical mile or less is
possible across the coastal waters tonight.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK...(FRI-TUE)
Moderate to heavy rain with possible debris flows in burn areas
Friday morning. Gusty advisory level Santa Ana winds possible
Sunday and Monday.

&&

PUBLIC...Hall/MW
AVIATION...Rorke
MARINE...Hall
SYNOPSIS...Sirard

weather.gov/losangeles