Acton Weather

000
FXUS66 KLOX 230639
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1039 PM PST Fri Jan 22 2021

.SYNOPSIS...22/930 PM.

A cooler and unsettled weather pattern is likely for much of the
next week. The first of a series of three storm systems will
continue to move through the region through late Saturday, then a
stronger storm system is expected for Sunday through Monday with
better chances of rain and mountain snow. Significant rain and
mountain snow could develop for the latter half of next week as a
strong storm system could affect the region.

&&

.SHORT TERM (FRI-MON)...22/929 PM.

The latest infrared imagery shows a cold upper-level trough of
low pressure centered over the central Sierra Nevada Mountain
range. A frontal boundary associated with the system is exiting
the area this evening, while another piece of the storm system
near Point Arena dives south down the state. This piece has much
colder air associated with it. The latest NAM-WRF solutions
continue to suggest -30 degree Celsius cold pocket that will dive
south to near Point Conception by late Saturday morning. Isolated
thunderstorms could develop across the coastal waters, and maybe
some coastal areas late in the day on Saturday, as this area of
instability rotates around the area and moves over the region. A
showery pattern will likely persist with showers becoming more
scattered to numerous on Saturday afternoon as daytime heating
interacts with the cold air mass aloft to steepen lapse rates.
Snow levels near 4000 feet will likely descend to near 3000 feet
on Saturday afternoon or evening, possibly bringing a mix of rain
and snow to the foothill areas and Santa Monica mountains. Much
will depend how much moisture remains from the moisture starved
trough currently moving over the region.

For now, PoPs and cloud coverage look reasonable given the upper-
level trough in place and no changes are planned at this time.
Widely scattered to scattered showers will likely continue for
tonight and into early Saturday morning until the next piece of
the storm system arrives.

A wind advisory for the Los Angeles County Mountains and Antelope
Valley was cancelled before the expiration time. Winds have
diminished across the area and only local gusts to 40 mph remain
possible this evening.

***From Previous Discussion***

Additional showers will affect the forecast area tonight through
early Sat evening, with snow levels lowering further to 3500-4000
feet by later tonight and continuing into Sat. There will also be
some increased instability by Sat afternoon, and a slight chance
of thunderstorms is expected over the L.A. County coast and
adjacent coastal waters.

Additional rainfall amounts during the period are expected to be
about 0.15 to 0.33 inch across the region, with local amounts of
to 0.70 inch over the eastern San Gabriel mtns. Accumulating snow
of up to 2-6 inches in are forecast for the mtns above about 4000
feet, with most of the accumulating snow over the eastern San
Gabriel mtns. There could be a some icy conditions across the
Grapevine with up to an inch accumulation there by Saturday
afternoon. Snow accumulations are not expected to be high enough
to justify a Winter Weather Advisory, but motorists should be
careful driving through the Grapevine and I-5 Corridor.

Dry weather will prevail later Sat night thru Sun, except for a
slight chance to chance of rain moving into NW SLO County for Sun
afternoon as the next weather system approaches.

A much colder upper level trof will dig down into swrn CA from
the N Sun night and Mon, with H5 heights plunging to 534-535 dm
over the region on Mon. A cold air mass will move into the region,
as 850 mb temps fall to -1 to -3 deg C by Mon afternoon, and 950
mb temps dip to as low as 4 to 6 deg C Mon morning). This system
will spread rain and mountain snow across the region Sun night
into Mon, with a chance of pcpn lingering into Mon evening. Snow
levels are expected to be around 3500-4000 ft Sun night then drop
to around 2000 to 2500 ft Mon into Mon evening. There will also be
gusty NW winds at times especially in the foothills and mtns.

Rainfall with this system is expected to be about 0.25 to 0.50
inch overall across the forecast area, with local amounts up to
about 0.80 inch in the mtns. Snow accumulations are expected to be
around 3 to 6 inches between 3500-4500 ft, and 6 to 10 inches
above about 4500 ft. With the expected snow and gusty winds, a
Winter Storm Watch has been issued for the SBA/VTU/L.A. County
mtns (excluding the Santa Monica Mtns) from late Sun evening to
late Mon evening. Potential impacts include wintry driving
conditions and poor visibilities. Travel will likely be impacted
along I-5 in NW L.A. county, including over the Grapevine, as well
as the higher portions of Highway 14.

Temps across the region thru Mon are forecast to be several
degrees below normal, altho the coolest day will be on Mon with
highs 8-16 deg below normal.

.LONG TERM (TUE-FRI)...22/235 PM.

12Z EC/GFS are in generally good agreement for the extended
period. A cold upper level trof lingers over swrn CA Tue with a
few more showers and snow showers possible. In fact, it looks
like snow levels could be down to as low as 1500 feet late Mon
night into early Tue morning, with a slight chance of snow even in
the hills in and around the VTU/L.A. County vlys.

This system ejects eastward later Tue. However, a much stronger
upper level trof and surface frontal system along and off the nrn
CA coast will move into the area quickly late Tue night and Wed.
Warm-frontal pcpn will start over SLO/SBA Counties later Tue
night then spread into VTU/L.A. Counties on Wed. The main cold
front with this system is expected to push into SLO/SBA Counties
Wed afternoon or early evening, then into VTU/L.A. Counties later
Wed night into Thu. The front will move little thru the day Thu
with the potential of many hours of moderate to heavy rain over
the area. The heaviest rain amounts thru Thu will depend on
exactly where the front stalls, and the models are not in complete
agreement on this. The strong upper level trof will linger just
off the coast thru Thu night with the front probably remaining
over L.A. County. Finally, the upper level trof is forecast to
move over the area on Fri, with the surface front moving E and
pcpn diminishing.

The models have been consistent with this system, including the
mean ensembles. Our confidence in increasing that the forecast
area could have a high impact rain and mtn snow event Wed thru
Fri. There will be above normal PWATs as well as plenty of
orographically enhanced pcpn. Very early rainfall estimates for
Wed thru Fri are up to 3-4 inches for the coast and vlys, and 5-10
inches in the mountains, with the highest amounts along S facing
slopes. These numbers will likely change some as additional model
runs come in, but it does appear a heavy rain event will affect
the region and bring potential impacts such as flash flooding,
mud and debris flows out of recent burn areas, and heavy snow in
the mtns mainly above 5000 feet.

&&

.AVIATION...23/0635Z.

At 05z at KLAX... there was a weak inversion at the surface. The
top of the inversion was at 250 feet with a temperature of 13
degrees Celsius.

Overall... Moderate confidence in the current TAFs. Scattered
showers and occasional MVFR CIG conditions will continue through
much of the forecast period with VLIFR/LIFR VSBY conditions at
KPRB 13z-19z and IFR/MVFR VSBY conditions at KSBP and KSMX 14z-
18z. Otherwise and elsewhere, VFR conditions will prevail.

KLAX... Moderate confidence in the current TAF. Scattered
showers and mostly MVFR conditions will continue through 02z.
Otherwise VFR conditions will prevail. There will not be any east
winds greater than 8 kts during the forecast period.

KBUR... Moderate confidence in the current TAF. Scattered
showers and mostly MVFR conditions will continue through 02z.
Otherwise VFR conditions will prevail.

&&

.MARINE...22/932 PM.

The coastal waters will have cool and unsettled weather through
at least next Friday with an increasing amount of rain with each
storm. There is a slight chance of thunderstorms on Saturday.

Across the outer waters... Winds will continue at Small Craft
Advisory (SCA) level through this evening. After a lull overnight,
winds will increase again on Saturday and will remain at or above
SCA level through at least Wednesday. Gale force winds are likely
on Monday. Seas will increase to SCA level on Saturday and remain
at or above SCA levels through at least Wednesday.

Across the nearshore waters of the Central Coast... SCA level
winds will develop on Sunday and continue through at least
Wednesday with a thirty percent chance of Gale Force winds on
Monday. Seas will increase to SCA level on Saturday and remain at
or above SCA levels through at least Wednesday.

Across the inner waters south of Point Conception... There is a
forty percent chance of SCA level winds tonight, mostly across
the western portion of the Santa Barbara Channel. SCA level winds
will develop again late Sunday and continue through at least
Wednesday with a twenty percent chance of Gale Force winds on
Monday. Seas will increase to SCA level on Monday.

&&

.BEACHES...22/929 PM.

Surf 6 to 9 feet with local sets to 10 feet will develop along
the Central coast on Saturday. The surf will diminish somewhat on
Sunday, then higher surf will develop quickly late Sunday along
the Central Coast and on Monday south of Point Conception. At the
peak Monday afternoon and evening... surf of 13 to 18 feet with
local sets to 20 feet is likely along the Central Coast and 10 to
14 feet is likely south of Point Conception. The surf will
diminish slowly on Tuesday and early Wednesday then increase again
by late Wednesday.

There will be strong rip currents and at least minor coastal
flooding on Monday and Tuesday, particularly at high tide.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Watch in effect from Sunday evening through
      Monday evening for zones 52>54. (See LAXWSWLOX).
PZ...Small Craft Advisory in effect until 3 AM PST Monday for
      zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (MON-FRI).

Light to moderate precipitation and low elevation snow is
possible Monday through Tuesday. A stronger system will impact
the area Wednesday and Thursday with significant rain and mountain
snow.

&&
$$

PUBLIC...Hall/Sirard
AVIATION...Kj
MARINE...Kj
BEACHES...Kj
SYNOPSIS...Hall

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