Winter Storm and Flooding in California, December 11 to 14, 2021

Area Forecast Discussion
National Weather Service Sacramento CA
322 PM PST Sat Dec 11 2021

.SYNOPSIS...
A significant storm will bring rain, wind and very heavy mountain
snow tonight through Tuesday. Mountain travel will be hazardous
Sunday through Tuesday with chain controls and possible road
closures. Another system will bring additional rain and mountain
snow Wednesday into Thursday with mountain travel impacts likely.

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...Widespread hazardous travel Sunday through Tuesday...
Travel will become dangerous, especially for the mountain passes,
and is discouraged. Roads through the passes are likely to
experience chain controls and possible closures. Whiteout
conditions, downed trees, and power outages are all possible.

.DISCUSSION...
Long wave troughing currently over the Gulf of Alaska will dig and
build over the West. This is going to bring an extended period of
active weather with moderate to heavy rain, heavy mountain snow
with whiteout conditions and strong southerly winds. This system will
be very dynamic and will have good upper level support to bring
large-scale ascent and strong upslope flow. We are seeing some
light showers already over the NW part of the state into NW Shasta
County in an area of weak warm air advection. Rain and snow
chances are going to become more widespread tonight into tomorrow
as the first short wave pushes into the PacNW pushing a cold front
into the region. This cold front will stall over the Bay Area
east into the Sacramento to Lassen Park area tomorrow afternoon
as the long wave trough digs over the Pacific. Rain will continue
to be more steady along and ahead of the stalled front with
activity more showery behind it.

Sunday night and Monday will be the most intense part of the
storm as rain and snow will increase in coverage once again as the
deep trough digs into the region. This will be the period of
heaviest precip over the area and will be heaviest along and
ahead of the stalled front. Low level winds will also be
increasing Sunday night into Monday with 850 mb winds 40-60
knots. This will bring strong upslope flow resulting in heavy
precip in the mountains. It is also going to bring strong
southerly winds with wind gusts 35 to 45 mph in the Valley,
locally higher and 40 to 60 mph over the higher elevations with
locally higher gusts over the peaks. A Wind Advisory has been
issued for lower elevations.

Snow level are somewhat challenging with this system. Cold air is
in place and we will see low snow levels tonight in the 2000 to
3000 foot range in Shasta County and in the 3000 to 4500 foot
range over the Sierra. Snow levels will rise throughout the day on
Sunday reaching 4000 to 5000 feet by the evening. Snow levels
will be slower to rise though in northern Shasta County and may
only reach 3500 feet.

The cold front and trough will push to our south Tuesday afternoon
with showers lingering mainly in the mountains. Once this happens
cold air will really build into NorCal and we will see snow levels
bottom out Tuesday night into Wednesday morning potentially
falling below 1000 feet. Snow accumulation at this time will be
very light with little to none.

Rain totals with this system will range from 2.0 to 3.5" in the
Valley, less in shadowed areas and the foothills with an
impressive 4.0 to 8.0 inches. Snow totals are going to be even
more impressive with this system with multiple feet falling. Much
of the Sierra should see 2 to 5 feet with 6+ feet above 6000 feet.

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.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...
There won't be much of a break in precipitation as the first
storm departs and the next system arrives by early Wednesday, just
a brief lull in intensity. Snow levels drop further, though, as
more cold air advects in, down to around 1500 feet and potentially
lower. Snow showers behind the trough and with another Gulf of
Alaska trough could bring additional hazardous travel conditions
into Thursday. Another couple feet of snow are possible in the
mountains, with a few inches into the upper foothills. Some light
accumulations could even extend into part of the northern
Sacramento Valley. Gusty southerly winds are also possible
Wednesday night into early Thursday.

Thursday will see additional precipitation across the area as the
trough moves inland, with around another foot or so in the
mountains. Friday has trended drier, with better confidence for
ridging. A weak system brushing to the north could bring some
light precipitation Saturday to the northern mountains and
northern Sacramento Valley. EK

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.AVIATION...
A Pacific weather system will spread mountain snow and rain late
today through Tuesday. Mainly MVFR/ conditions expected across the
Valley with IFR/LIFR over the mountains. Southerly wind gusts
increasing to 20-35 kts in the Valley after 04Z-06Z Sunday; up to
40 kts over higher terrain.

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.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 10 PM this evening to 10 PM PST
Tuesday for Burney Basin / Eastern Shasta County-West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Winter Storm Warning until 4 AM PST Tuesday for Mountains
Southwestern Shasta County to Western Colusa County-Shasta Lake
Area / Northern Shasta County.

Wind Advisory from 4 AM to 10 PM PST Monday for Carquinez Strait
and Delta-Central Sacramento Valley-Motherlode-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Southern Sacramento Valley.

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