Flash Flood Watch 4, NWS Los Angeles, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
158 PM PST Wed Jan 18 2017

.SYNOPSIS...
Several storms will bring precipitation and gusty winds through
early next week. There may be a chance of thunderstorms on Thursday
and Friday. The first storm will move in Wednesday and Thursday.
The second...from late Thursday into early Saturday and will have
heavy rain especially in the foothills...and lower snow levels.
The third will be on Sunday into Monday and have the highest
precipitation amounts. The temperatures will be below normal into
next week.

&&

.SHORT TERM...(TDY-SAT)

First of at least three storms to impact our area through early
next week is just now approaching the Central Coast this
afternoon. There have been a few light showers out ahead of it but
the main part of the system is still to the north. Rainfall will
increase in areal coverage and intensity this evening as southerly
flow intensifies and stronger dynamics arrive with the colder air
aloft. Models have been adverstising at least some minimal
instability and an isolated thunderstorm or two is certainly
possible, mainly with the cold front itself, which will be mainly
after midnight tonight into Thursday morning. Rain rates with this
system, especially in and around thunderstorms, could reach or
briefly exceed criteria in our more susceptible burn areas, mainly
the fish and the sand burns, so a flash flood watch has been
issued to cover those areas later tonight through Thursday
morning. Overall rain numbers still look good, mainly around an
inch for lower elevations and up to 2" in the upslope areas.

Snow levels will be lowering to around 6000' later tonight and as
low as 5000' Thursday morning as the cold front passes. 5-10" of
snow are possible at the higher elevations, mainly above 7000'.
Because of the relatively small gap between this storm and the
second and the likelihood that both will generate at least
marginal warning level snow amounts for higher elevations the
decision was made to convert the winter weather advisory into a
winter storm warning and extend it into Saturday morning to cover
the second storm.

Rain and snow should taper off pretty rapidly Thursday morning
with mainly just some lingering afternoon showers over the higher
terrain. The second system will start bringing rain to NW SLO
County late Thursday night and to the remainder of the area
Friday, tarpering off Friday afternoon and evening. The low lvl
southerly winds are a bit stronger with this second one and model
qpf is a quarter to half inch higher which makes sense. Once again
model instability wasn't super impressive but certainly enough to
warrant a slight chance of thunderstorms across the area Friday.
This will likely require another flash flood watch for the burn
areas. Rain and snow amounts expected to be slightly higher with
this second system.

Winds at low levels and aloft shift to the west Friday afternoon
and northwest Friday night which should help shower coverage
diminish, especially at lower elevations south of Pt Conception.
Additional drying and low lvl subsidence should keep most of the
area dry Saturday except for possibly some isolated showers over
the mountains and Central Coast. A cool and breezy day however
with lower elevations mostly in the 50s to low 60s.

.LONG TERM...(SUN-WED)

The biggest change in the models today came with the timing of the
third system, which had been pegged for a late Sunday through
early Monday arrival. However, today's runs have sped that up a
good 12 hours, so now most of Sunday looks wet with the heaviest
period in the morning up north and late afternoon and evening in
LA County. There's also a considerable spread in the models with
precip amounts. The operational GFS is by far the outlier, a good
2" higher than the ensemble mean as well as the ECMWF. So while
confidence is high in a decent rain event Sunday, not real
confident on the amounts. The ensemble mean and ECMWF would
support a rain event that is similar to the previous two storms.
But if the operational GFS prediction of 50-60kt south winds at
850 comes to fruition then certainly the higher amounts would be
more likely. The GFS is also showing higher pwats with this
system, around 1.4" Sunday, which is up a good half inch from the
two earlier storms.

This also leads to considerable uncertainty with snow levels as
the GFS is tapping into a much warmer subtropical air mass and
freezing levels rise to over 10000'. The other models keep the
snow level fairly similar as before but with less moisture to work
with. So low confidence on the details with storm #3.

Much colder air is expected to arrive Monday, though here again
significant model differences are leading to lower confidence in
amounts. Snow level should drop to at least 4000' Monday, perhaps
as low as 3000' as thicknesses drop to around 531dm. The ECMWF is
the more aggressive model here pushing through a 4th and colder
system Monday that could lead to some additional heavy rain and
snow into Monday evening.

Most areas should start drying out Tuesday as northerly flow takes
over. Still quite cool through about Wednesday then warming up
later in the week.

&&

.AVIATION...18/18Z.

At 17Z at KLAX... the inversion was around 2500 feet. The top of
the inversion was around 3100 feet with a temperature of about 8
degrees Celsius.

Overall... Low to moderate confidence in the TAFs. The reduced
confidence is primarily due to the uncertainty in the timing of
the weather. Light precipitation will be moving through the
Central Coast locations this morning. The primary band of
precipitation will move onshore north of Point Conception early
this afternoon and then south to Los Angeles County by late
afternoon. The heaviest precipitation comes overnight through
early morning. MVFR conditions will accompany the arrival of the
precipitation and then IFR conditions develop overnight.
Southwesterly winds will increase through the afternoon across the
region then turn southeasterly overnight. There is a slight chance
of thunderstorms overnight and Thursday morning.

KLAX... Low to moderate confidence in the TAF. The reduced
confidence is primarily due to the uncertainty in the timing of
the weather. Precipitation will arrive this afternoon but the
heaviest precipitation comes overnight through early morning. MVFR
conditions will accompany the arrival of the precipitation and
then IFR conditions develop overnight. Winds will be light and
variable this morning then southwesterly winds will increase
through the afternoon and turn southeasterly overnight. There is a
slight chance of thunderstorms Thursday morning.

KBUR... Low to moderate confidence in the TAF. The reduced
confidence is primarily due to the uncertainty in the timing of
the weather. Precipitation will arrive this afternoon but the
heaviest precipitation comes overnight through early morning. MVFR
conditions will accompany the arrival of the precipitation and
then IFR conditions develop overnight. Winds will be light and
variable this morning then southwesterly winds will increase
through the afternoon and turn southeasterly overnight. There is a
slight chance of thunderstorms Thursday morning.

&&

.MARINE...18/200 PM.

For all coastal waters north of Point Conception... Small Craft
Advisory (SCA) level winds will increase through the day and reach
Gale Warning level gusts tonight through early Thursday morning.

For the southern outer waters south of Point Conception... Winds
will increase to SCA level by late afternoon through early
Thursday morning.

For the inner waters south of Point Conception... Southerly
winds will increase to SCA level winds this evening across the
Santa Barbara Channel and then south to the waters off Los Angeles
County during the overnight period.

Across all of the waters... Winds will lessen Thursday night then
increase again early Friday to at least SCA levels and remain
elevated through at least Saturday. There is a slight chance of
thunderstorms tonight into Thursday and again on Friday.

A large long period swell will arrive in the coastal waters on
Thursday then another but larger swell will arrive Friday night
into Saturday. This second swell will likely reach heights of more
than 20 feet over the northern outer waters late Friday night into
Saturday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect from 7 PM this evening to 4 AM PST
      Thursday for zones 34>38-51-52. (See LAXNPWLOX).
     High Surf Advisory in effect from 4 AM Thursday to 9 PM PST
      Tuesday for zones 34-35. (See LAXCFWLOX).
     High Surf Advisory in effect from 4 PM Thursday to 9 PM PST
      Tuesday for zones 39>41-87. (See LAXCFWLOX).
     Winter Storm Warning in effect from 10 PM this evening to 9
      AM PST Saturday for zones 53-54. (See LAXWSWLOX).
     Flash Flood Watch in effect from late tonight through
      Thursday morning for zones 54-88-548. (See LAXFFALOX).
PZ...Small Craft Advisory in effect until 3 PM PST this afternoon
      for zones 645-670-673. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PST Thursday for zones
      645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM this evening to 9 PM
      PST Thursday for zone 650. (See LAXMWWLOX).
     Gale Watch in effect from Friday morning through Saturday
      morning for zone 650. (See LAXMWWLOX).
     Gale Watch in effect from Friday morning through Saturday
      afternoon for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Friday for zone
      676. (See LAXMWWLOX).
     Gale Watch in effect from late Thursday night through
      Saturday afternoon for zone 676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
Periods of heavy rain and mountain snow are expected Sunday and
Monday that could pose hazards to burn areas and drop significant
snow amounts in the mountains. There is a chance of damaging
warning level surf and coastal flooding issues Saturday,
especially along the Central Coast.

&&

PUBLIC...MW
AVIATION...Kj
MARINE...Kj
SYNOPSIS...STU

weather.gov/losangeles