Winter Storm Warning 2, NWS Los Angeles, CA

Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
427 AM PST Fri Feb 1 2019

.SYNOPSIS...01/334 AM.

Showers have moved out of our area and dry conditions are
expected for most of the day. A major winter storm is forecast to
impact the area tonight through Saturday with heavy rain and
mountain snow at higher elevations possible along with damaging
winds. Lingering showers are likely Sunday through Tuesday.

&&

.SHORT TERM (TDY-SUN)...01/332 AM.

There will be a brief reprieve from yesterday's storm before a
more powerful and larger storm system moves into the northern
portion of SLO County region by this afternoon, then will
overspread the entire forecast area by this evening. Initially,
the warm sector of this storm system will move into the region
with light rain and increasing southeast flow for areas N of Point
Conception late this afternoon. By this evening, strong southeast
winds will ramp up quickly as a 984mb surface low develops just
off the coast of San Francisco this evening into early Saturday
morning.

At 500 mb level, the center of this storm system will be located
west of San Francisco by Saturday morning, with a powerful
southwest jet of 100 knots along the base of trough aimed at the
Central Coast and SBA County. Near the surface, the NAM model
advertises a strong 983 mb surface low off the coast of Monterey
by Saturday afternoon. As a result, an impressive southeast-south
pressure gradient will develop in association with this storm
system. At this time, the GFS model continues to project an LAX-
San Francisco gradient just over +15 mb mb by Saturday morning.
This is indicative of a very strong southerly wind component.
This storm system will be tapping to a deep subtropical moisture
plume or a moderate Atmospheric River towards SLO/SBA Counties
just before sunrise on Saturday. This plume will then sag south
across southern SBA County, Ventura County, then L.A. County later
in the day. Model cross sections were showing strong dynamical
ascent along with very strong 50 to 70 knot southerly winds at 850
mb level which is expected to bring significant orographic
rainfall enhancement to south facing foothills and coastal slopes.
This will be a very efficient rain maker, especially across south
favored foothills and mountains. Behind the main cold front, the
NAM, GFS and now show a fairly significant secondary wave of
shower activity on Saturday night, with fairly strong lift in the
cross section and more unstable conditions. A slight chance of
thunderstorms were added across the northern portions of SLO
County and adjacent coastal waters Saturday evening. There is
still the potential for some heavy shower activity continuing into
Saturday night, especially near the foothills and mountains.

A Flash Flood Watch was issued earlier last evening valid for all
the recent burn areas in Santa Barbara, Ventura, and Los Angeles
counties Saturday into Saturday night. The most widespread heavy
rainfall is expected during the day Saturday, when peak rates
between 0.50 and 1.00 inches per hour will be likely. However, due
to the strong southerly orographic component, there will be the
potential for localized rates up to 1.25 inches across the south
facing foothills and coastal slopes (especially the Santa Ynez
mountains, Ventura county mountains, and San Gabriels). On
Saturday evening, there is still the potential for brief heavy
downpours as the rainfall may turn more convective in nature. A
slight chance of thunderstorms have been added to the northern
portion of San Luis Obispo County and adjacent coastal waters.

Shallow mud and debris flows will be likely with this event on
Saturday, with the potential for more significant and damaging
debris flows if hourly rates approach 1.0 inch or greater.
Rockslides and mudslides will bring an increased threat of road
closures in the burn areas. Projected storm totals are expected to
generally range between 1.5 and 3.0 inches with amounts between
4.00 and 7.00 inches possible across south facing foothills and
coastal slopes. A Hydrologic outlook (LAXESFLOX) was also issued
last evening to highlight flooding potential in non-burn areas as
well, which include possibility of widespread roadway flooding,
as well as flooding of small creeks and streams. Due to the
potential for significant rains in the mountains of western
Ventura County with this event, there is also the potential for
flooding of the Ventura River. Other rivers will likely see
significant water flows which could increase the threat of swift
water rescues.

In addition to the heavy rain potential with this storm, the very
strong southerly winds could be an equally concerning impact with
this storm system. In fact, the southerly wind component of 55 to
75 knots at 850 mb level along with the strong south-southeast
pressure gradient near the surface could bring one of the
strongest south-southeast wind events we have seen in recent
years associated with a storm. High Wind Watches are in place for
the the mountains, Central Coast, interior SLO County as well as
Cuyama Valley. Also a high wind watch was added to the Antelope
Valley. A Palmdale Wave scenario is possible during the day on
Saturday which could bring damaging winds, especially around
Quartz Hill, Littlerock, Pearblossom and Valyermo along the
southern foothills of the Antelope Valley.

Mountain areas (including Interstate 5 near the Grapevine) are
expected to see powerful wind gusts of 65 to 80 mph, which could
make for very dangerous driving conditions across mountain
roadways on Saturday. In addition, much of SLO/SBA counties could
see damaging wind gusts of 60 to 65 mph, with foothills areas
seeing gusts up to 75 mph! The strong and damaging southerly winds
will also likely impact the Santa Monica mountains and Santa
Clarita Valley where gusts up to 65 mph will be possible. Since
the grounds are wet with recent rains, we could see significant
wind damage with this event, with numerous downed trees and
powerlines.

In addition, the southerly flow ahead of the main front will keep
the snow levels above 7000 feet initially Friday night into
Saturday morning. When the cold front passes by late Saturday
afternoon it will usher in a much colder air mass and snow levels
should fall to 5000 or even 4500 feet. Most of the snow will be
ahead of the front across the high elevations. 1 to 2 feet of
snow is possible above 7000 feet. In the wake of the front
Saturday night, 3 or 46 inches of additional snow could fall as
low as 5000 feet through Sunday night. The snow and strong winds
has prompted the issuance of a Winter Storm Watch from tonight
through early Sunday evening for the mountains of Ventura and Los
Angeles Counties.

There will continue to be a broad upper trough across the area
with a few upper level disturbances moving over the area Saturday
night with a few more that will keep a showery forecast through at
least Tuesday. Snow levels are expected to really drop by Monday
morning and could be problematic for driving across the Interstate
5 Corridor. Winds should not be as strong after Saturday evening.
Rainfall totals From Saturday night through at least Sunday will
be light but some foothill locations could see moderate showers at
times.

High temps across the region are expected to remain several
degrees below normal today through Sunday, altho some sunshine on
this morning should help make it the warmest of the 3 days.

.LONG TERM (MON-THU)...01/338 AM.

Another large upper level trof off the coast of CAL/OR will move
south into Central California from Monday into Tuesday. The best
chance of showers during this period should be Monday when likely
POPs cover the forecast area. Mountain snow showers above about
4000 to 4500 feet Mon and 3000 to 3500 feet Tue can be expected,
with some snow accumulation possible. The upper trof will move
east by Wednesday, with weak upper ridging off the coast expected
to move into the area by Thu. Dry weather can be expected Wed and
Thu across the region, along with moderating daytime temperatures.

&&

.AVIATION...01/1226Z.

At 12Z, there was no marine inversion at KLAX.

Low to moderate confidence in the current forecast. There is a
chance of MVFR conditions at coastal and valley terminals and
LIFR to IFR conditions at desert terminals through 17Z. VFR
conditions are likely through at least 01Z, then IFR to MVFR
conditions in rain will spread northwest to southeast through
through Saturday morning.

KLAX...There is a 30 percent chance of MVFR conditions through
17Z. VFR conditions are likely through at least 05Z, then there is
an increasing chances of IFR to MVFR conditions in rain through
17Z Saturday. Southeast to south winds will likely increase to as
high as 25 kt possible after 10Z.

KBUR...There is a 30 percent chance of MVFR conditions through
17Z. VFR conditions are likely through at least 06Z, then there
is an increasing chance of IFR to MVFR conditions in rain through
Saturday morning.

&&

.MARINE...01/414 AM.

Moderate to high confidence in forecast. SE to S winds will
increase quickly today and this evening to reach Small Craft
Advisory (SCA) levels. Winds will likely reach storm force tonight
through Saturday for the northern nearshore waters, outer waters
and the Santa Barbara Channel. Winds will shift to SW to W and
diminish some Saturday afternoon and evening. Winds could remain
at Gale Force levels in most areas into Saturday evening, then at
SCA levels into Sunday. SCA level NW winds are possible again
Monday and Tuesday, especially in the outer waters.

Hazardous seas are expected to build late tonight through Saturday
night as large west to northwest swells mix with building steep
southerly wind waves. There is a 40 percent chance that a
northwest swell could be larger than forecast between Saturday
night and Sunday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Storm Warning in effect from 8 PM this evening to 9 AM PST
      Saturday for zone 34. (See LAXMWWLOX).
     High Surf Advisory in effect from 9 PM this evening to 9 AM
      PST Monday for zones 34-35. (See LAXCFWLOX).
     High Wind Watch in effect from this evening through Saturday
      afternoon for zones 34-35-51-52. (See LAXNPWLOX).
     High Wind Watch in effect from late tonight through Saturday
      afternoon for zones 36>39. (See LAXNPWLOX).
     Flash Flood Watch in effect from late tonight through late
      Saturday night for zones 36-39-52. (See LAXFFALOX).
     Beach Hazards Statement in effect from late tonight through
      Monday morning for zones 39>41-87. (See LAXCFWLOX).
     Flash Flood Watch in effect from Saturday morning through
      Sunday morning for zones 40-41-45-46-54-88-547. (See LAXFFALOX).
     High Wind Watch in effect from Saturday morning through
      Saturday afternoon for zones 46-59-88. (See LAXNPWLOX).
     Winter Storm Watch in effect from this evening through Sunday
      afternoon for zones 53-54. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect from 2 PM this afternoon to 8
      PM PST this evening for zones 645-673. (See LAXMWWLOX).
     Small Craft Advisory in effect from 2 PM this afternoon to 11
      PM PST this evening for zones 650-676. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 PM this evening to 3 AM
      PST Saturday for zone 655. (See LAXMWWLOX).
     Gale Watch in effect from late tonight through Saturday
      afternoon for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 9 AM this morning to 6 PM
      PST this evening for zone 670. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SUN-THU).

Large surf will bring dangerous breaking waves and rip
currents to coastal areas Sunday.

&&

PUBLIC...Kaplan/Gomberg
AVIATION...Hall
MARINE...Hall
SYNOPSIS...Kaplan

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