River Flood Warning 37, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
354 AM PST Fri Feb 17 2017

.Synopsis...
Wet weather continues with several storm system passages expected
through the middle of next week. Sierra mountain travel issues at
times. Gusty winds Friday and Monday. Increased flood threat
Monday.

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. Discussion...
Scattered light showers are occurring this morning across the
area from about Sacramento northward in the warm advection ahead
of the next approaching Pacific cold frontal system. Significant
precipitation is not expected from these showers. As the offshore
upper low center shifts closer to the Bay Area this
morning...surface gradients will increase especially from about
Sacramento county southward. Winds will be on the increase as the
low shifts east but the strongest winds are not likely to kick in
until late morning and afternoon. Will leave wind advisory
starting this morning since winds will be on the increase by this
time. The bulk of the energy with this system is still expected to
shift into Socal as the low moves to the southeast so
precipitation amounts are not expected to be especially heavy and
likely not enough to re-ignite flood issues.  Snow levels will be
below pass levels with this system however so with several inches
of snow possible on Trans-Sierra roadways, current snow advisory
starting later this morning looks good. May see a few
thunderstorms from about Sacramento county southward this
afternoon and evening in the most unstable area closest to the low
center. As the upper low center drops south of the Bay Area this
evening...surface gradients relax which should bring a slow
decrease in winds overnight. Narrow upper trough over the state
will keep showers going tonight and Saturday but precipitation
should drop off fairly quickly Saturday afternoon as the upper
through axis shifts east of the state.

An upper level shortwave trough pushing into the Pacific Northwest
and Norcal on Saturday night will bring a slight increase in
precipitation over Shasta county through Sunday morning while
upper level troughing keeps a slight threat of showers going
elsewhere. Cold air associated with this trough will bring down
snow levels Sunday to between 4000 and 5000 feet or lower than
they have been for a while. Precipitation amounts will be light
however so mountain travel impacts will be minimal Sunday. Snow
levels begin to climb again by Sunday night in the warm sector
ahead of the next stronger Pacific storm system to move into
Norcal. Warm advection precipitation Sunday night will be
accompanied by increasing winds as surface gradients ahead of this
system increase. Snow levels will be low enough on Sunday night
and early Monday to impact Sierra travel but when the heavier
precipitation comes on Monday with the main frontal band...snow
levels will be on the rise and likely above pass levels by Monday
morning. The passage of this frontal band Monday and Monday night
will bring strong winds and heavy rain especially to the
orographically favored west slopes of the Sierra which could see
several inches of rainfall between Sunday and Monday nights.
Stability progs show a threat of thunderstorms over Shasta county
Monday afternoon. With the expected heavy rainfall and high snow
levels...flood issues are likely to crop up again going into early
next week.

&&

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

Precipitation activity will continue on Tuesday as the frontal
system continues to push southward but amounts should be
decreasing. Snow levels are expected to drop around 4500-5500
feet on Tuesday as cold air aloft ushers in behind the system.
Winds will be weakening on Tuesday.

The main upper trough is forecast to move inland later Tuesday
into Wednesday resulting in showers across the area, with lowering
snow levels and lighter precipitation amounts. Extended models
indicate a lull in precipitation activity on Thursday. Model
uncertainty is evident by the end of the forecast period
regarding an upper system dropping southward along the West Coast.
The GFS is the most bullish at this point, bringing shower
activity over the northern mountains and the northern Sierra but
the ECMWF keeps our area dry.

&&

.AVIATION...

Main band of precipitation is approaching the southern portion of
our CWA and will continue to spread northward throughout the day.
MVFR/IFR conditions across the Valley and IFR/LIFR conditions
across higher terrain. Increasing southeasterly winds with gusts
25-35 kts from around KSMF southward through 02z Saturday.
Precipitation activity should be decreasing mainly after 08z
Saturday with lingering shower activity.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 7 AM this morning to 10 AM PST
Saturday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Wind Advisory from 7 AM this morning to 10 PM PST this evening
for Carquinez Strait and Delta-Northern San Joaquin Valley-
Southern Sacramento Valley.

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