Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 210458
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
858 PM PST Mon Feb 20 2017

.Synopsis...

Significant flooding concerns this week as heavy rain will add
stress to waterways. Wettest and windiest period expected into
Tuesday morning. Strong winds will cause numerous downed trees
and power outages. Mountain communities and travel will be
impacted with heavy snow and mudslides. Valley thunderstorms
possible Tuesday and Wednesday.

&&

.Discussion...
Winds have been coming up this evening behind the front. Very
strong winds do not look likely overnight so earlier downgraded
the High Wind Warning to an Advisory. Peak wind gusts should
generally be in the 40`s but isolated stronger gusts are still
possible. The winds should taper after 6 am Tuesday morning.

Snow continues to come down at a heavy clip in the mountains.
Snow level was near 6000 feet but may briefly rise slightly this
evening before lowering towards midnight and beyond. Heavy amounts
of snow is expected during the heaviest time period tonight and
in the morning.

Flooding continues to be an issue. One levee break so far along
the San Joaquin River southwest of Manteca on the east side of
the levee. A number of small river and creek`s are beginning to
reach bank full and may become an issue for some locations
overnight. A number of mudslides have been reported as well and
may continue to be an issue overnight. Rain within the valley will
continue to transition to showers overnight but the upslope areas
in the mountains will take longer and transition late tonight and
Tuesday morning. Slight chances of thunderstorms overnight for
the Sacramento Valley and adjacent foothills. Thunderstorms may
become an bigger issue over the Sacramento Valley for Tuesday
afternoon.

There is currently a line with very heavy rain moving
over the north end of the valley that may bring some
localized flooding.

.Previous Discussion (Today through Thursday)...

Current water vapor imagery and satellite derived winds show a
strong jet stream advecting moisture into northern California
ahead of a trough of low pressure. Radar trends show a widespread
area of light to heavy precipitation occurring across the forecast
area. Gusty southwest winds were occurring for the north and
central Sacramento Valley up to 40 mph at times.

Forecast concerns for this period include flooding for the entire
area, including flash flooding for locations south of the I-80
corridor. Heavy snow and high winds with gusts greater than 70
mph for higher terrain is also a concern, including damaging wind
gusts between 55 and 65 mph for the valley and foothills.
Thunderstorms are possible with increased instability tonight and
Tuesday. Latest short term model forecast guidance has suggested
an area of excessive rainfall for locations south of the
Interstate 80 corridor in an area of enhanced orographic lift this
afternoon through tonight, so a flash flooding threat with
landslides is increased through tonight.

Heavy snowfall for the Sierra will continue through tonight as
snow levels drop overnight and Tuesday. One to two feet of snow is
expected for Donner and Echo Passes with 3 to 4 feet of new snow
for passes south of Highway 50. Up to 5 feet can be expected for
the highest elevations above pass level through Tuesday.

Heavier precipitation will transition towards lighter
precipitation amounts Wednesday with some additional light
accumulations of rain for the Valley and snow for the Sierra. A
brief dry spell is expected for northern California on Thursday.

&&

.EXTENDED DISCUSSION (Friday THROUGH Monday)

Models showing upper low dropping south off the West Coast Friday
into Saturday with system swinging inland Saturday night into
Sunday followed by a secondary low Monday. Result will be
unsettled weather through the extended forecast period with best
chances for more widespread showery precip occurring as the lows
move through. Overall storm precip totals attm look to be more
typical of an average winter storm for Interior NorCal.


&&

.AVIATION...

Pac fntl sys movs thru tngt with shwrs Tue. In Cntrl Vly, wdpsrd
MVFR/IFR conds ovngt with sct MVFR conds Tue in shwrs and isold
aftn tstms. Omtns, wdsprd IFR/LIFR thru Tue mrng bcmg areas
MVFR/IFR Tue aftn. Stg Sly sfc wnds tngt in Cntrl Vly of 25-35kts
with gusts up to 50 kts til arnd 12z Tue. Stg SWly wnds omtns
tngt and Tue with gusts up to 60 kts or gtr ovr hyr trrn. Sn lvls
abv 060 tngt lwrg to arnd 040-050 Tue aftn.


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 10 PM PST Tuesday for West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Flash Flood Watch until midnight PST tonight for Motherlode-
Northern San Joaquin Valley-West Slope Northern Sierra Nevada.

Wind Advisory until 8 AM PST Tuesday 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.

&&

$$



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