Flood Watch 3, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
332 PM PST Sun Feb 5 2017

.Synopsis...

Series of Pacific storms will bring periods of moderate to heavy
rain and mountain snow with strong wind to Interior Northern
California beginning tonight through Friday.

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.Discussion...

Next storm is rapidly approaching the coast this afternoon with
main moisture plume currently located between 130W and the CA
coast. Forecast for the next 24 hours still largely intact with
only minor changes. Latest hi-res guidance shows precipitation
moving into the coastal range in a few hours, into the Valley by
4-6pm, and into the Sierra shortly thereafter. Winds will also
begin to pick up in this timeframe, strongest late tonight into
Monday morning. Wind advisory remains in effect with gusts of 40
to 50 mph across the central Valley and 50+ at highest elevations.
Possible downed trees or branches and localized power outages
during this period. Moderate to heavy precipitation will continue
overnight into Monday morning with substantial orographic flow
across the Sierra. Snow levels will remain around 5500-6000 feet
with heavy snow and gusty winds causing treacherous travel and
whiteout conditions at times across the passes. Winter storm
warning remains in effect.

Main front moves through the Valley by Monday morning with
precipitation becoming more showery during the day. Moderate to
heavy precipitation will continue across the Sierra throughout the
day. Winds will taper off after noon but will still remain
breezy. Less instability noted in the models today than yesterday
as there likely will be too much cloud cover around and not much
of a break before the next storm moves in. Have kept thunderstorms
out of the forecast again.

NorCal then will be hit by another moist Pacific storm, this one
wetter and warmer than tonight's. Another round of gusty winds is
likely as the storm moves in. Heavy precipitation begins again
Monday night into Tuesday. Latest guidance has bumped up
precipitation amounts slightly for this period, especially across
the mountains. Healthy rainfall totals and little break between
storms will lead to rising water levels on area rivers and
streams. Some road and localized urban flooding as well. Something
to monitor for upcoming forecasts...have held off any sort of
areal flood products for now to assess how much falls with the
first storm. Tricky part of the forecast remains just how fast
snow levels will rise. Current projects have snow levels starting
around 6000 feet Monday night with heavy snow impacting the passes
through about Tuesday morning. Then steadily rising above pass
levels (8000-9000 feet) Tuesday afternoon into evening. Exact
timing will be key to impacts and travel hazards. Precipitation
turns more showery again Wednesday.

CEO

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.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Another storm system is on track to affect northern California
this Thursday. Upper level trough deepens off the coast and
strong low to mid level jet stream is anticipated to round the
base and into the forecast area Thursday into Thursday night. The
influx of stronger winds aloft and pressure gradient over the
forecast area will create strong southerly winds, especially for
higher terrain locations.

Plenty of moisture being advected ahead of the trough will allow
for widespread moderate to heavy precipitation to develop. The
system will be on the warm side with snow levels starting at 8000
feet and dropping to between 4000 and 5000 feet by Friday night.
With previous snowfall and rain leading up to this incoming
system, flooding is possible with the additional rainfall
expected. Heavy rain should shift to heavy snow Friday as snow
levels fall to below pass level.

The storm system will begin to exit northern California for the
coming weekend when an upper level ridge of high pressure builds
in from the west. Expecting conditions to dry out with
temperatures warming to near normal.

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.AVIATION...

MVFR ceilings for the northern Sacramento Valley with VFR
conditions elsewhere through this evening before widespread
MVFR conditions become prevalent as rain affects the terminals.
Strong southerly winds expected overnight for the valley with
gusts upwards of 50 mph for the Sierra. MVFR conditions should
continue through tomorrow afternoon.

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.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 7 PM this evening to 4 PM PST Tuesday
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Wind Advisory until noon PST Monday for Carquinez Strait and
Delta-Central Sacramento Valley-Clear Lake/Southern Lake County-
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.

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