Southwestern California Discussion

 
000
FXUS66 KLOX 190411
AFDLOX

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
811 PM PST Sun Feb 18 2018

.SYNOPSIS...18/345 PM.

Light rain and mountain snow will fall across much of the region
tonight and Monday with snow falling as low as 1500 feet Monday
morning. Gusty winds and a blast of cold air will create very cold
wind chill temperatures through Monday with frost or freezing
conditions Tuesday and Wednesday mornings. Conditions will be dry
but cool for the remainder of the week.

&&

.UPDATE...

18/811 PM

Very difficult forecast this evening as a weak and fairly dry
but very cold system moves in from the north. There are a fair
amount of showers over MRY county and these will move over SLO and
western SBA counties in a few hours. The forecasted eddy and
convergent flow for SE LA county really did not materilize so the
odd of that area seeing a shower tonight are much reduced.

This system will very likely produce highly changable skies but
not much rain. There is a pretty good chc that SLO and most of SBA
county (but not the SBA south coast) will see some light showers.
The best chc of showers will occur after midnight with the cold
north flow impinges on the north faceing slopes near the Kern
County line.

Current suite of winter...wind and freeze products looks good.

Updated the forecast to raise pops in the advisory areas as well
as SLO and western SBA counties. Al;so play with the cloud wording
a little to reflect current conditions.

&&

.SHORT TERM (SUN-WED)...

18/400 PM

Skies were mostly clear early this afternoon with some partly
cloudy skies across portions of the Central Coast down to Ventura
County coastal areas. Surface gradients between LAX and DAG have
trended over +8 mb in the last 24 hours. A wind advisory is now in
effect for the Antelope Valley due to the strong onshore
gradients causing SW winds to increase to around 30 to 35 mph with
gusts expected to be around 45 to 50- mph through this evening.
There will be locally gusty winds across the Highway 14 corridor
as well, including the Soledad Pass through this evening as well.
Motorists driving high profile vehicles should slow down if
encountering gusty cross winds. There will be gusty NW winds
developing this afternoon across the Central Coast this afternoon
but remain below advisory thresholds. Temps were trending down
6-10 degrees in most areas today compared to yesterday at this
time. Highs today are expected to be around normal for this time
of year.

For this evening, NW winds will expand across the Santa Barabara
County Mtns and adjacent wind prone south coast as well as the
Los Angeles and Ventura County Mountains. Wind advisories will be
in effect by 8 pm this evening in these locations.

Synoptically, an upper level low located in the PAC NW will begin
to dive south into the Great Basin tonight while the base of the
low continues to dig into southern California. The trajectory of
the trough will remain mainly over land, therefore not expecting
much precip with this system. However, the combination of strong
height falls and cold temperatures expected with this system will
help squeeze out what little moisture there is to tap into to
tonight through Monday evening. The best chance for light showers
will be across L.A. County, especially the southern and eastern
areas where the cyclonic flow aloft will be most most favorable.
Otherwise, broad NW to N flow will bring a chance of rain and snow
to the north facing slopes of SBA/Ventura and the NW portion of
the LA County Mtns near the Grapevine and I-5 Corridor. The NW
portion of the Antelope Valley could see a few showers as well
through Monday eve.

Snow levels will plummet tonight, dropping from between 5000 and
5500 feet this evening, to between 2500 and 3000 feet late
tonight, and to 1500 to 2000 feet Monday morning. This could
bring snow showers into foothills areas Monday morning. Little or no
snow accumulation is expected in the foothills, but in the mountains
above 2500 feet, snow accumulations of 2 to 4 inches are possible.
1-2 inches possible for the Santa Barbara County Mountains, and
around an inch of snow for portions of the Cuyama Valley including
highway 166. A winter weather advisory is in effect for the mtns
of L.A. and VTU Counties as well as the Santa Barbara mountains
and the Cuyam Valley this evening through Monday night for the
snow and advisory level northwest winds. There could be a dusting
of snow across the Antelope Valley foothills as well. Especially
across the western portion, but not expecting valley floor
accumulations.

Gusty northwest winds will begin to shift out of the north and
filter into the Santa Clarita and San Fernando Valleys and likely
into West L.A. monday night into Tuesday morning. Additional wind
advisories might be needed during that time. It will be very cool
on Monday, with highs only in the 50s in most coastal and lower
valley areas, in the mid to upper 40s in the interior valleys, in
the upper 30s to mid 40s in the lower mountains and the Antelope
Valley and in the mid 20s to mid 30s at higher mountain locations.

With the cold conditions and added wind, it will feel much
colder. In fact, wind chill values could drop to between 10
degrees and -10 degrees in the higher elevation of the mountains
late tonight and Monday. This is unusually cold for Southern
California. People planning to hike, camp, ski or snowboard in the
mountains on the holiday are urged to plan for the unusual cold
and low wind chill values.

Winds are expected to decrease rather suddenly Monday night in
most areas. With light winds and clear skies expected Monday night
into Tuesday morning, radiational cooling will help temps drop
significantly. The Hard Freeze Watches have been converted to Hard
Freeze Warnings for both Monday night into Tue and Tuesday night
into Wednesday morning for the Central Coast, the Santa Ynez and
Cuyama Valleys, and the interior valleys of Ventura County,
especially for the Ojai Valley. In these areas, temperatures will
drop to 28 degrees or lower for at least two hours Monday night.
Some areas within the Hard Freeze Warning will experience lows in
the lower 20s. A Freeze Watch has also been converted to a Freeze
Warning for the same time periods across South Coast of SBA
County, coastal sections of VTU County, and the VTU County coastal
valleys. Temperatures in these areas could bottom out between 29
and 32 degrees for at least two hours Monday night. It will also
be very cold in the interior valleys of SLO County and the
Antelope Valley, with temps between 17 and 27 degrees there, but
those locations have already experienced two hard freezes this
season. Frost is likely in the colder locations in the valleys of
L.A. County Monday night, and a Frost Advisory will likely be
needed for those areas. Tuesday will be mostly sunny but it will
continue to be very cool across the region.

.LONG TERM (THU-SUN)...

18/154 PM

The EC and GFS are coming into better agreement with large scale
features from Wednesday through Saturday. Temperatures will remain
quite cool through the extended period as a broad upper trough
across the west will persist. Skies should be mostly sunny Tuesday
through Thursday before clouds increase Thursday night.

Another trough will drop southward through the Pac NW Thursday.
The GFS shows this trough moving eastward into Arizona Thu night
and Fri, while the EC is farther west and slower with it, tracking
just to the northeast of the area Fri morning. Slight chance POPs
have been added across most of the forecast area except the
Antelope Valley for Thursday night into Fri at this time. High
temps will remain quite cool and there could be more issues for
anyone going across the I-5 Corridor Thurday night and Friday.
Again, this looks like a light precip maker at this point, but
will have to continue to monitor.

Models were hinting at a more significant and another cold system
the following Monday and Tuesday. So expect the cooler than normal
pattern to continue for a while.

&&

.AVIATION...

19/0013Z

At 23Z over KLAX, a weak inversion bottom was at 2000 ft and 9
degrees Celsius, and a top at 2600 ft and 11 degrees Celsius.

Challenging next 24 hour forecast. Low confidence on timing and
heights of ceilings. 30 percent chance of occasional -SHRA and
BKN015-025 through 16Z KPRB KSBP KSMX KSMO KLAX KLGB KBUR KVNY.
Moderate confidence on timing and strength of winds, with fairly
widespread gusty westerly winds. Occasional LLWS and turbulence
anywhere, especially over KSBA tonight.

KLAX...Low confidence on ceilings, moderate confidence in winds.
Periods of BKN015-025 with -SHRA possible through around 12Z.
North winds likely over airport 12-16Z Monday, either surfacing
or creating LLWS issues.

KBUR...Low confidence on ceilings, moderate confidence in winds.
Periods of BKN015-025 with -SHRA possible through around 12Z.
10KT LLWS possible from North winds over airport 12-16Z Monday.

&&

.MARINE...

18/709 PM

Updated marine forecast, with significant larger sea heights for
the next 48 hours, and expanded Gale Warnings to the Santa Barbara
Channel and western Santa Monica Basin.

High confidence in Gale Force winds (around 35 KT) everywhere by
3 AM tonight, except for the eastern Santa Barbara Channel and the
nearshore waters of Los Angeles and Ventura Counties. With the
strong winds however, all areas will see dangerous and large short
period seas developing tonight. High confidence in the winds
expanding and peaking Monday afternoon, with gusts of 30 to 40 KT
over all waters. The winds will continue through Monday evening,
then weaken through Tuesday, but Small Craft Advisory (SCA) are
likely from the Central Coast to San Nicolas Island. Improved
conditions are likely on Wednesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect from 6 AM Monday to 2 PM PST
      Tuesday for zones 34-35. (See LAXCFWLOX).
     Hard Freeze Warning in effect from 1 AM to 9 AM PST Tuesday
      for zones 34>36-38-44. (See LAXNPWLOX).
     Hard Freeze Warning in effect from 1 AM to 9 AM PST Wednesday
      for zones 34>36-38-44. (See LAXNPWLOX).
     Winter Weather Advisory in effect until 3 AM PST Tuesday for
      zones 38-52>54. (See LAXWSWLOX).
     Wind Advisory in effect until 10 PM PST Monday for zones
      39-52-59. (See LAXNPWLOX).
     Freeze Warning in effect from 1 AM to 9 AM PST Tuesday for
      zones 39-40-45. (See LAXNPWLOX).
     Freeze Warning in effect from 1 AM to 9 AM PST Wednesday for
      zones 39-40-45. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 9 AM PST Monday for zone
      645. (See LAXMWWLOX).
     Gale Warning in effect from 9 AM to 9 PM PST Monday for zone
      645. (See LAXMWWLOX).
     Gale Warning in effect until 9 PM PST Monday for zone 650. (See LAXMWWLOX).
     Small Craft Advisory in effect until 1 AM PST Monday for zone
      655. (See LAXMWWLOX).
     Gale Warning in effect from 1 AM to 9 PM PST Monday for zone
      655. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PST Tuesday for zones
      670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Widespread temperatures around or below freezing is expected
early Wednesday morning away from the beaches, posing a risk for
hypothermia and plant damage.

&&

$$

PUBLIC...Kaplan/Kaplan/Bruno
AVIATION...Kittell
MARINE...Kittell
SYNOPSIS...Kj

weather.gov/losangeles

NWS LOX Office Area Forecast Discussion