Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 182317
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
317 PM PST Sat Feb 18 2017

.Synopsis...
Wet weather with renewed flooding concerns into the middle of
next week with wettest storm of the bunch expected for Monday into
Tuesday. Gusty winds likely again Monday. Mountain travel will
also be impacted at times due to heavy snow.

&&

.Discussion...
Snow levels have risen today between 6500 to 8000 feet. CalTrans
webcams show that roads are clear on I-80 and Hwy 50. Biggest
concern now is the upcoming storm arriving Sunday night into
Tuesday. Forecast still on track for another atmospheric river
with impressive moisture to approach the West Coast on Sunday.
Some showers could pop up during the day on Sunday, but the
heaviest precipitation is expected to arrive Sunday evening and
Monday with continued rain and snow on Tuesday. Widespread
moderate to heavy rain and snow will impact our region in this
timeframe.

Impressive amounts across the Sierra due to orographic
nature with totals ranging between 4-10 inches. Amounts in the
Valley likely 1.5-3 inches. With heavy rain and saturated soils
throughout the area, flood and mudslide potential is heightened
Sunday evening through early Tuesday. Flood Watch remains in
effect.

Snow levels will start Wednesday around 4500-5500 feet Sunday but
should rise to around 6000-7000 feet during the day Monday as
system pushes in. Still a bit of uncertainty with this. Colder air
will then wrap in Monday night into Tuesday dropping snow levels
back down to around 5000 feet. Heavy snow likely in this timeframe
with several feet of accumulation at pass levels, hindering
travel and causing delays.

Another round of gusty winds will accompany system on Monday,
this time perhaps more widespread across the Valley. MFR-SAC
pressure gradient increases to around 12 mb with 925 mb winds in
the 60+ kt range during the day Monday. Have boosted up winds a
bit from going forecast with gusts in the 45-55 mph range which
still may be underdoing things but will need to watch trends going
forward. Additional wind/tree damage and power outages highly
likely.

&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)

General agreement in extended models for Wednesday with upper
trough over the West coast although models differ on depth.
Precipitation amounts do not look all that impressive with
scattered showers across the region but localized convection could
bring locally heavier amounts. Cannot rule out valley
thunderstorms Wednesday afternoon with moderate instability
indicated. Cool airmass will bring low snow levels Wednesday at
about 3000 to 4000 feet. Wednesday night and Thursday would see
drying northerly flow most areas under current model scenario with
upper trough moving into the Great Basin. Have left slight chance
of showers over the Sierra Thursday as it remains under cyclonic
flow.

Models really diverge going into Thursday night and Friday. GFS
brings another Pacific trough into the Pacific Northwest and
NorCal Thursday night and Friday for another round of rain, wind
and mountain snow while ECMWF and GEM models keep NorCal under
flat ridging and drier outcome. Have tried to split the difference
for now leaving a threat of showers over the mountains and
keeping most of the valley dry. Models then diverge even further
going in to next weekend ranging from the GFS solution of fairly
significant low moving in to California to the ECMWF solution of
dry northerly flow. Obviously, confidence in forecast for the end
of next week will be quite low until models become more
consistent.

&&

.AVIATION...
Periods of MVFR across interior NorCal for the next 24 hours.
Gusts around 25 kts possible after 09z for the KRDD and KRBL
vicinity.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Sunday evening through late Monday night for
Burney Basin / Eastern Shasta County-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-West Slope Northern
Sierra Nevada-Western Plumas County/Lassen Park.

Winter Storm Watch from Sunday morning through late Monday night
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

$$



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