Weather Service Report


147 
FXUS66 KLOX 190500
AFDLOX

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

.SYNOPSIS...

A series of storms will bring rain and mountain snow, heavy at
times, a slight chance of thunderstorms, and strong, gusty
southerly winds to southwestern California through early next
week. The first storm will affect the area tonight into Thursday.
The second weather system can be expected Friday into early
Saturday. The third storm is expected late Saturday night into
early Monday, which should bring the heaviest precipitation
amounts. Temperatures will be below normal across the area into
next week.

&&

.SHORT TERM...(WED-SAT)

A weak warm frontal boundary brought some light rain to the area
this afternoon and this evening. The warm frontal band put around
a tenth of an inch in the gage for most areas with some higher
totals in the San Luis Obispo County foothills. The cold front
sits north of the area this evening, stretching from the Bay Area
southwest out to near 30N and 130W. The front will push southeast
quickly tonight. Models solutions indicate the best instability
moving quickly from northwest to southeast between 10 pm this
evening and 8 am early Thursday morning. Rainfall will increase in
areal coverage and intensity later tonight as the stronger
dynamics arrive with the system and interact with the orographic
lift. A 500 mb cold pocket near -25 degrees Celsius brushes the
area, bringing the potential for isolated thunderstorms on late
tonight and Thursday morning. 700 mb temperatures cool to near -8
degrees Celsius in the latest model solutions which should drop
the snow level to between 4500 and 5000 feet on Thursday. Locally
the snow level could drop to around 4000 feet in heavier snow
showers. For those planning to travel on the Grapevine Thursday,
the Tejon Pass could be impacted heavier snow showers at times.
The forecast will update to reflect the change in snow levels,
otherwise snow totals between 5 and 10 inches of snow seems
reasonable above 7000 feet.

*** FROM PREVIOUS DISCUSSION ***

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, tapering 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...19/0228Z.

Moderate confidence in the current forecast. Periods of MVFR
conditions will develop in rain through 06Z, then MVFR conditions
will spread from northwest to southeast through 10Z as a cold
front sags into the area. There is a chance of IFR conditions
in/near heavier rainfall or thunderstorms. There is a slight
chance of thunderstorms very late tonight through early Thursday
morning. Periods of moderate to strong turbulence could impact
Antelope Valley terminals and terminals north of Point Conception
through 16Z.

KLAX AND KBUR...Periods of MVFR conditions will develop in rain,
then MVFR conditions will spread in between 06Z and 10Z. Southeast
winds should develop between 04Z and 05Z, turning to the south
around the timing of the frontal arrival between 13Z and 17Z.
There is a 30 percent chance of southerly cross greater than 20
knots impacting KLAX operations between 13Z and 17Z. Between 13Z
and 17Z, there is a 50 percent chance of IFR conditions in/near
heavier rainfall or thunderstorms. There is a 20 percent chance of
thunderstorms between 13Z and 17Z.

&&

.MARINE...18/900 PM.

Southeast to south winds will continue to increase tonight across all
coastal waters ahead of a cold front. Gales are forecast for the
coastal waters northwest of the Channel Islands through early
Thursday morning. Winds will turn to the west and diminish some
between early Thursday morning and Thursday evening. There is a 40
percent chance that small craft advisory headlines may need to be
extended into Thursday night. Winds will increase again early
Friday across the entire coastal waters and remain elevated
through at least Saturday. A gale watch remains in effect for the
entire coastal waters from early Friday morning through Saturday
night. There is a slight chance of thunderstorms tonight into
Thursday and again on Friday.

A large long period swell will build across the coastal waters
through Thursday, then continue to build to very large levels
Friday night into Saturday. The swell will likely reach heights
of more than 20 feet over the northern outer waters late Friday
night into Saturday. West-to-northwest facing bays and harbors
may be impacted by the swell over the next coming days,
especially Morro Bay and Ventura Harbors.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Wind Advisory in effect until 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 until 9 AM PST Saturday for
      zones 53-54. (See LAXWSWLOX).
     Flash Flood Watch in effect from 2 AM PST Thursday through
      Thursday morning for zones 54-88. (See LAXFFALOX).
     Wind Advisory in effect until 10 AM PST Thursday for zone 59.
      (See LAXNPWLOX).
PZ...Gale Warning in effect until 3 AM PST Thursday for zones
      645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 PM PST Thursday for
      zone 650. (See LAXMWWLOX).
     Gale Watch in effect from Friday morning through Saturday
      morning for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect from midnight tonight to 9 AM
      PST Thursday for zone 655. (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)

For Saturday, very large and potentially damaging surf will likely
impact west-facing beaches. Minor coastal flooding is also
possible Saturday morning. There could also be some lingering
mountain snow showers on Saturday morning, with potential impacts
on the Interstate 5 corridor going over the Grapevine.

Another storm system will impact the region late Saturday night
through Monday. Potential impacts include flash flooding and
debris flows for the burn areas, urban and small stream flooding,
rock slides along canyons roads, downed trees and isolated power
outages, significant travel delays, and dangerous winter driving
conditions in the mountains.

&&

$$

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

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