Winter Storm Warning 3, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
316 PM PST Tue Jan 15 2019

.SYNOPSIS...
Wet and windy weather is expected across the region through
Thursday with periods of heavy snowfall in the mountains. A brief
period of dry weather is expected before precipitation chances
return on the weekend.

&&

.DISCUSSION...
Water vapor imagery this afternoon distinctly shows the pair of
systems impacting Northern California. The lead shortwave is
currently moving through the Bay Area driving a broad axis of
light rainfall, generally on the order of 0.10" per hour. Mountain
impacts are already underway with accumulating snows over the
heavily traveled passes on I-80 and Highway 50. The heaviest burst
of snow is likely to occur early this evening into the overnight
hours with over a foot possible during this timeframe. In addition
to the increased coverage of precipitation, an uptick in the wind
speeds will be evident by later tonight. Winds are not expected to
slacken given the enhanced pressure gradient in advance of the
more vigorous system.

Lurking out around 150W longitude is the mentioned powerful
Pacific system expected to impact much of the state Wednesday
evening through Friday morning. Forecast anomalies with this
system with regards to typical atmospheric parameters (wind,
heights, etc.) are around 3 to 4 standard deviations from
climatology. The major hazards will include locally heavy
rainfall, several feet of mountain snows, high winds, as well as a
chance for some scattered thunderstorms.

The biggest hazard to travel will be the heavy mountain snows,
generally 5,000 feet and above. Snow levels will rise ahead of the
next storm which suggests the heaviest accumulations should sit
above 6,500 to 7,000 feet. The stronger winter storm is forecast
to spread several feet of snow to the Sierra and Southern
Cascades. Travel impacts will be high and travel is not advised
given intense snowfall rates coupled with high winds leading to
blizzard/whiteout conditions. Such hazards should persist through
Thursday night with scattered snow showers lingering into the
following morning.

While this will be a long duration rainfall event, the potential
threat for thunderstorms may ignite some short-term flash flood
concerns. This is particularly the case over any recent burns
scars so those will bear watching. Difficult to say this far out
where any such storms would develop, but instability appears
decent enough to support scattered convection Wednesday evening
into the overnight hours. Locally gusty winds would be possible
within any thunderstorms, but a more widespread high wind threat
remains given the strength of the approaching storm. 50 to 60 mph
southerly wind gusts cannot be ruled out across the Valley with
slightly higher numbers over mountain locales.

Once the system clears out on Friday morning, a transient upper
ridge moves across the western U.S. supporting a brief period of
improved weather. Global model agreement is rather good
suggesting high confidence in this pattern change ahead of the
weekend system.  BRO

&&
.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)

Precipitation spreads into the northern Coastal Range Saturday
afternoon as ridge shifts eastward and a Pacific trough moves in.
This system will bring widespread precipitation on Sunday but rain
and snow amounts will be relatively light. Snow levels look to be
around 7000 feet Sunday falling to 4500 to 5500 feet Sunday
night. Some mountain travel impacts will be possible but should be
limited to mainly high Sierra passes.

Drying northerly winds are expected to increase Monday and Tuesday as
the trough exits, with breezy winds on the west side of the Valley.
A dry pattern sets in through mid week as the area remains on the
east side of a Pacific ridge.

-CJM

&&

.AVIATION...

Southerly flow aloft with local MVFR/IFR conditions possible over
interior Norhtern California in precipitation or BR next 24 hours.
Local SE gusts to 20-25 kts possible over the southern Sacramento
and northern San Joaquin Valley.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 4 AM PST Friday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Blizzard Warning from 7 PM Wednesday to 7 AM PST Thursday for
West Slope Northern Sierra Nevada.

Flood Watch from Wednesday afternoon through Thursday morning
for Carquinez Strait and Delta-Central Sacramento Valley-Clear
Lake/Southern Lake County-Motherlode-Mountains Southwestern
Shasta County to Northern Lake County-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Shasta Lake Area / Northern Shasta County-
Southern Sacramento Valley.

High Wind Watch from Wednesday afternoon through late Wednesday
night for Carquinez Strait and Delta-Central Sacramento Valley-
Clear Lake/Southern Lake County-Motherlode-Mountains
Southwestern Shasta County to Northern Lake County-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Southern Sacramento Valley.

Winter Storm Warning until 4 AM PST Friday for Burney Basin /
Eastern Shasta County.

&&

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