Winter Storm Warning 4, NWS Los Angeles, CA

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
217 PM PST Mon Nov 25 2019

.SYNOPSIS...25/151 PM.

Cool, windy and dry conditions will continue through Tuesday.
Then, an unseasonably cold storm system will bring rain and
snow showers as well as a slight chance of thunderstorms Tuesday
night through Saturday. Below normal temperatures are expected
Wednesday through Friday. Another system could bring additional
rain through the weekend.

&&

.SHORT TERM (TDY-THU)...25/202 PM.

Strong north to northwest winds have arrived on schedule and
reducing visibilities in the Antelope Valley due to blowing dust
and sand. Winds expected to peak later this afternoon and evening,
likely surfacing in the LA valleys and coast before weakening and
shifting to the northeast overnight into Tuesday morning. These
winds are expected to remain below advisory levels for the most
part though there could be some local gusts to around 40 in the
higher hills. Otherwise a pretty quiet weather day for most areas
Tuesday.

Made very few changes to the forecast after that as models look
very similar in timing and strength, though possibly a few hours
faster with the onset Tuesday night into Wednesday morning. The
NAM and higher res models continue to show more orthogonal low
level flow creating slightly higher rainfall rates and the
forecast strongly leans towards these numbers. Snow levels during
this time are expected to be around 6000'. The main front should
be through LA County by late morning Wednesday with plenty of
shower activity behind it. May get a rumble of thunder or two with
the front but models are not showing much instability at that
time. May even see some peeks of sunshine between the clouds
Wednesday after the front passes but generally cool and showery.

A secondary impulse expected to arrive Wednesday night into
Thursday morning with not as much upslope enhancement but with
much colder air aloft. We'll see increased shower coverage with
much lower snow levels, dropping rapidly Wednesday night to around
3000' and locally to around 2500'. It's during this period when
we expect the biggest impacts to travel through the mountains as
accumulating snowfall is expected over the major passes and
mountain roads in LA/Ventura Counties. Air mass is a little more
unstable during this time as well so chances for convection are
higher, though still pretty marginal so thunderstorms should be
isolated at most. With the lower freezing levels expect we'll get
lots of graupel or small hail reports at lower elevations during
this time with or without thunderstorms.

Rain rates in general are not expected to pose widespread issues,
though the usual urban flooding problems are likely with the
frontal passage and with any convection. Burn areas will need to
be monitored but even with thunderstorms it's unlikely we'll see
rainfall rates high enough to generate impactful flows. The
biggest impacts are expected to be in the mountains due to the
lower snow levels and possible road closures before and during the
holiday.

By Thursday morning most areas should have received at least 90%
of their expected storm total, though scattered showers and even
an isolated thunderstorm or two are still expected through
Thanksgiving day and evening.

Overall rain totals pretty much the same, 1-2" coast/valleys
1.5-3" in the mountains with locally higher amounts in the eastern
San Gabriels and coastal foothills. Snowfall amounts will be
highest in the eastern San Gabriels as well with 1-2' possible
there. Elsewhere mostly 6-12" above 5000 and 3-6" around the
Grapevine and in the foothills of the Antelope Valley. Possibly a
dusting up to an inch on the valley floor. Will be monitoring snow
levels closely as we get closer to determine if a winter weather
advisory will be needed in the AV.

.LONG TERM (FRI-MON)...25/213 PM.

Models in better agreement now showing the system exiting the area
by Thursday night will only minimal rain chances as only a few
ensemble members show any precip lingering into Friday. Could be a
very cold morning Friday if clouds and precip do clear out faster
and many areas may need frost/freeze headlines. And a chilly day
Friday despite the increased sunshine with highs still mostly in
the 50s at lower elevations and 30s in the mountains with plenty
of snow on the ground.

Forecast gets tricky again over the weekend as models show another
system dropping out of the north and over the eastern Pacific.
Weak ridging will develop Saturday ahead of it helping to warm
temps a few degrees but still very cold in the morning. By
Saturday afternoon both models and some of their ensemble members
are showing some warm frontal precip developing along the Central
Coast as early as Saturday afternoon. How far east this spreads is
still very much uncertain as the GFS and its ensembles are
trending wetter while the EC is holding onto the ridge longer and
preventing any further eastward progression. For now agree with
the small chances of rain area-wide Sun/Mon until models come into
better agreement.

&&

.AVIATION...25/1852Z.

At 1747Z around KLAX, the marine layer depth was 2200 feet with
the top of the inversion at 3500 feet and 13 degrees C.

High confidence in 18z TAFs except 20% chance for VLIFR-IFR CIGs
across L.A County Coast from 15z-20z.

KLAX...High confidence in 18z CAVU TAF. There is a 30 percent chc
of an north wind component greater than 8 kt tonight.

KBUR...High confidence in 18z CAVU TAF.

&&

.MARINE...25/134 PM.

Outer Waters...A GALE WARNING is in effect for the outer water
zones through tonight. After that, there is good confidence that
SCA level winds and seas will continue for the remainder of the
outer waters through at least midweek.

Inner Waters N of Point Sal...There is a 80% chance of SCA
conditions developing this afternoon through tonight and again at
times Tuesday night through Wednesday night.

Inner Waters S of Point Conception...There is a 60% chance for Gale
force winds across the western portion of the Santa Barbara
Channel from this afternoon through late tonight, with SCA
conditions by mid morning leading up to the GALE WARNING valid
from 3 PM this afternoon through late tonight. Also, there is a
50% chance for a SCA for gusty North- Northeast winds tonight
across the Santa Monica Basin to Point Mugu. There will then be a
50% chance of SCA winds Wednesday and Wednesday night.

There will be an increasing NW swell approaching 12-18 feet
from late today through Tuesday night affecting the outer waters.

&&

.BEACHES...25/134 PM.

A High Surf Advisory remains in effect as a seconday, larger northwest
swell moves in across the Central Coast this morning. High surf is
expected to continue through Wednesday at 10-15 feet.

Furthermore, the area affected by high surf is expected to expand into
areas south of Point Conception, specifically west facing beaches of
Ventura County with surf 4 to 7 feet this afternoon through Tuesday.
Los Angeles County beaches will experience elevated surf between
3 to 6 feet which will bring a high risk for strong rip currents,
but remain below High Surf Advisory thresholds.

Finally, astronomical high tides will peak near 7 feet early this
week during the morning hours between 7-8AM, and combined with
high surf will result in the issuance of a Coastal Flood Advisory
for minor tidal overflow during the time of peak tide, mainly for
Los Angeles and Ventura counties. Minor flooding will be confined
to the lowest areas of coastline, including possibly some exposed
beach parking lots.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until 3 PM PST Wednesday for
      zones 34-35. (See LAXCFWLOX).
     Wind Advisory in effect until 9 PM PST this evening for zones
      34-35. (See LAXNPWLOX).
     Wind Advisory in effect until 3 AM PST Tuesday for zones
      39-59. (See LAXNPWLOX).
     Coastal Flood Advisory in effect until 10 AM PST Tuesday for
      zones 40-41. (See LAXCFWLOX).
     High Surf Advisory in effect until 7 AM PST Tuesday for zone
      40. (See LAXCFWLOX).
     Wind Advisory in effect from 6 PM this evening to 3 AM PST
      Tuesday for zones 44>46-88-547. (See LAXNPWLOX).
     High Wind Warning in effect until 3 AM PST Tuesday for zones
      52>54. (See LAXNPWLOX).
     Winter Storm Warning in effect from 4 AM Wednesday to 4 AM
      PST Friday for zones 52>54. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Tuesday for
      zone 645. (See LAXMWWLOX).
     Gale Warning in effect until 3 AM PST Tuesday for zones
      650-670-673-676. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM this evening to 6 AM
      PST Tuesday for zone 655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (THU-MON).

Significant impacts to travel expected Thursday especially in the
mountains due to low snow levels. Locally heavy rain, small hail,
and impacts to burn areas are possible elsewhere. Cold
temperatures are forecast for Friday and Saturday mornings with
widespread frost or freeze conditions likely across the interior,
possibly extending into some coastal valley locations.

&&

PUBLIC...MW
AVIATION...Sweet/Kaplan
MARINE...Sweet
BEACHES...Sweet
SYNOPSIS...Delerme

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