Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 181323
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
523 AM PST Thu Jan 18 2018

.SYNOPSIS...
A cold and fast moving storm system will bring cooling
temperatures... gusty winds... and light rain and mountain snow
showers to the region by Friday. Snow showers will linger into
Saturday morning across the northern slopes. Warmer conditions
return next week, but there is a slight threat of a few showers
across the northern portion of the forecast area early next week.

&&

.SHORT TERM...(TDY-SAT)

Latest GOES-16 Satellite imagery indicated plenty of mid to high
level high clouds moving across the region ahead of an approaching
upper trough to the NW. Some patchy low clouds with dense fog were
across portions of the Central Coast this morning. The upper level
ridge will start to deteriorate later today with weak offshore
flow turning weakly onshore by this afternoon. This will
translate into cooler conditions across much of the forecast area
today. Generally 3-8 degrees cooler for coast and valleys.
Otherwise a few degrees of warming expected across the Antelope
Valley this afternoon.

Synoptically, a positive tilted upper trough will begin to move
towards California. Clouds will be on the increase today. The
upper trough will then dig further south over Southern California
on Friday. Low clouds should deepen for LA/VTU coast and valleys
with some patchy drizzle will be possible late tonight into early
Friday morning. Models now keep the trough further off the coast
which usually means a better chance to get more precip into the
region. However, wind profiles still keep much of the 850 mb
winds out of the NW with this system. There will be some cyclonic
flow aloft as the front associated with the trough moves into
southern L.A. County Friday. There have been some subtle changes
within the forecast. First, it is quite dynamic in respect to
bringing plenty of cold air behind the front. Also, a 120 kt jet
will approach the forecast area from the SW to NE direction early
Friday evening. The combination of the divergence aloft, MU Cape
around 500 j/kg near the upper trough and Lifted Index indicating
some instability moving across the coastal waters will justify
adding a slight chance for thunderstorms over the coastal waters
starting late this afternoon from the north, moving south
overnight. Not much moisture to tap into with this quick moving
system. Generally with NW flow, areas S of Point Conception will
not see much in the way of precip, especially coastal areas of SBA
S. Coast and Ventura County coast and valleys.

One item of concern, and it is VERY low confidence, but there
will be the slightest of chances that a rogue thunderstorm could
move over the Montecito area Friday evening with NW steering flow
aloft over the coastal waters when the atmosphere will become
briefly unstable. If a thunderstorm does develop,  it will be
quick moving, but could bring brief heavy showers over the recent
debris flow area. Again, not expecting this to occur, but it is
worth mentioning.

Overall, rainfall amounts will be light across much of the
forecast area, with the exception of the northern slopes of the
LA/VTU County Mountains. Snow levels will initially above 7500 ft
Thursday night into early Friday, when the colder air behind the
front will move into the region.

The big story with this event will be the low snow levels and
strong winds behind the front Friday afternoon into Saturday
morning. Snow levels could fall to between 2500 and 3500 feet
Friday night into Saturday morning, with the lowest snow levels
on the north facing slopes. Many mountain areas could see between
1 and 3 inches of snow accumulation with this event, with higher
elevation north slopes seeing potentially 3 to 6 inches.
Interstate 5 near the Grapevine could see significant travel
impacts from Friday afternoon through Saturday morning, with
potentially 1 to 3 inches of snow accumulation and strong winds
gusting up to 50 mph. Given the ongoing closures of Highways 101 and
33 forcing additional vehicle traffic through the Grapevine this
could have significant impacts to drivers trying to go north and
south. A Winter Weather Advisory will be issued later this
morning valid from 10 AM Friday through Saturday afternoon for the
LA/VTU/SBA County Mtns. Also for the foothills of the Cuyama
Valley in SBA County where an inch of snow is expected.

As far as winds go, besides the very windy conditions across the
local mountains, the Antelope Valley will also experience gusty
cold winds and a dusting of snow will be possible across the
foothills from LLano to Pearblossom and also around Neenach to the
NW at the base of the Tehachapi Mtns Friday evening into early
Saturday morning. North winds will also filter into the Santa
Clarita Valley, San Fernando Valley and into W. LA late Friday
night into early Saturday morning. A wind advisory will likely be
needed for the Antelope Valley, and to a lesser extent, the Santa
Clarita and San Fernando Valleys.

As far as temps, today will be the last warmer than normal day
for this time of year, then high temps will come crashing down
10-20 degrees across inland areas while coastal areas will be 5-10
degrees cooler by Friday. Highs will range from the upper 50s to
lower 60s for most low-land areas, and much colder in the high
country. Not much change for Saturday as cold air behind the front
will persist. There will be friezing temps across inland areas
Fri night and sat night. Winds will be diminishing some by
Saturday night and the possibility for freeze watches to be issued
for Sat night.

.LONG TERM...(SUN-WED)

Temps will continue to slowly rebound Sunday/Monday and be fairly
close to seasonal norms. The tail end of a weak system will brush
northern SLO County Monday and there could be some very light
precip there, otherwise just some increasing mid and high clouds.
Dry conditions and slowly warming Tue/Wed as weak offshore flow
develops.

Next chance of rain looks to be next Thursday, but models are not
in agreement with how far south precip will get. Will continue
with light showers across the northern portion of the forecast
area.

&&

.AVIATION...18/1245Z...

At 11Z, the marine layer at KLAX was around 300 ft deep. The top
of the inversion was near 1100 ft with a temperature of 22 deg C.

Patchy low clouds and fog with LIFR to VLIFR conds on the Central
Coast and in the Santa Ynez Valley should clear by mid morning.
Then, VFR conds are expected through this evening, with abundant
mid and high clouds across the region. Expect widespread MVFR cigs
in coastal areas this evening, spreading into the valleys tonight.
There is a chance of showers north of Point Conception after 06Z,
and a chance of drizzle across coastal and valley areas of L.A.
and VTU Counties late tonight.

KLAX...Moderate to high confidence in the 12Z TAF. There is a
20% chance that cigs will arrive as early as 03Z this evening.
There is a 20-30% chance that cigs will be in the IFR category.

KBUR...Moderate to high confidence in the 12Z TAF. There is a
20-30% chance that cigs tonight will be in the IFR category.
There a 20% chance that conds will remain VFR through the period.

&&

.MARINE...18/300 AM...

For the Outer Waters, moderate to high confidence in current
forecast. High confidence in Small Craft Advisory (SCA) level
conditions through Friday morning due to hazardous seas, with SCA
level winds in many areas as well.

Very large NW swell overspread the waters today, and peak tonight
and Fri morning, with combined seas of 15-19 feet. NW winds will
increase Fri. There is a 50% chance of Gale force winds Fri
afternoon thru Sat evening across the southern two zones (PZZ673,
PZZ676), and a 20% chance of gales across the northern zone (PZZ670).

For the Inner Waters, moderate to high confidence in current
forecast. For the waters north of Point Sal, high confidence in
Small Craft Advisory (SCA) level conditions due to hazardous seas
through tonight, then winds and seas Fri thru Sat evening. There
is a 20% chance of Gale force winds Fri afternoon and Fri night.

Across the SBA Channel and the southern inner Waters, west swell
will build tonight and Friday. There is a 40-50% chance of gale
force winds Fri afternoon thru Sat. Gale Watches are in effect,
but confidence is not high enough to upgrade to warnings at this
time. There is a good chance that a SCA will be needed as early as
late tonight or Fri morning, due to hazardous seas and/or winds.

Dangerous breaking waves are possible near harbor entrances,
including Morro Beach Harbor and Ventura Harbor through Saturday

&&

.BEACHES...18/450 AM...

An extended period of large surf is expected along west and
northwest facing beaches through Saturday evening as very
large westerly swells move through the coastal waters.

For west and northwest facing beaches along the Central Coast,
surf will build today, peak at 15 to 20 feet with local sets to
25 feet this afternoon or evening through Saturday before slowly
subsiding. A HIGH SURF WARNING remains in effect from this
afternoon through 900 PM PST Saturday evening. The high surf may
cause beach erosion and will produce life-threatening rip
currents. Minor coastal flooding is possible in low lying areas
such as parking lots, especially near the times of high tide.

For west and northwest facing beaches south of Point Conception,
surf will build today, then peak at 8 to 12 feet by late tonight
and Friday. Local sets to 15 feet are possible on exposed west
facing locations such as Ventura Harbor. High surf will continue
through Saturday evening before slowly subsiding. A HIGH SURF
ADVISORY remains in effect through 900 PM PST Saturday evening.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until noon PST today for zones
      34-35. (See LAXCFWLOX).
     High Surf Warning in effect from noon today to 9 PM PST
      Saturday for zones 34-35. (See LAXCFWLOX).
     Winter Weather Advisory in effect from 10 AM Friday to 4 PM
      PST Saturday for zones 38-52>54. (See LAXWSWLOX).
     Beach Hazards Statement in effect through Saturday evening
      for zone 39. (See LAXCFWLOX).
     High Surf Advisory in effect until 9 PM PST Saturday for
      zones 40-41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect until 9 PM PST Saturday for
      zones 645-670. (See LAXMWWLOX).
     Gale Watch in effect from Friday afternoon through Saturday
      evening for zones 650-655-673-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 1 PM PST Friday for
      zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
Gusty winds, cold temperatures and accumulating snow to low
elevations, likely including Interstate 5 over the Grapevine late
Friday into early Saturday. Frost advisories and/or freeze
warnings possible for parts of the area over the weekend due to
near or below freezing overnight temperatures.

&&

$$

PUBLIC...Kaplan
AVIATION...DB
MARINE...DB
BEACHES...DB
SYNOPSIS...Kaplan

weather.gov/losangeles


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