Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 200600
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
955 PM PST Sun Feb 19 2017

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

&&

.Discussion...

Precip has tapered to a few lingering light showers over our CWA
this evening...the lull before the next big storm. Offshore warm
front will be lifting NEwd by/after 12z Mon and the rain should pick
up again. Winds will also increase as the initial surface low
passes-by a couple hundred miles off the N coast. Strongest winds
now forecast around the KSMF early Mon morning at 925 mbs. The second
surface low moves closer to the N coast about 24 hrs later by 12z
Tue with about a 12 mbs N-S gradient forecast and the potential for
even higher wind gusts Mon nite. Thus, High Wind Warning in effect.
Look for precip to focused along the I-80 corridor and over the W
Slope Sierra later tonight and Mon. By late morning and into Mon
afternoon, another break or lull is forecast over the coastal range
and Shasta drainage. The second warm front will also continue to
drench the Sierra Mon nite and into Tue while precip gradually
decreases over the Wrn portion of the CWA. Chance of T-storms late
Mon nite and Tue as the second front and strong embedded waves move
through the broad Swly flow aloft. Strong Sly winds will provide an
upslope convergence boundary vicinity of RDD as well with
repeat/training echoes possible. Be prepared for additional
flooding.  JHM


Take this afternoon and evening to make final preparations ahead
of the incoming storm with flooding, strong winds and heavy snow.
If you were given less than 15 minutes to evacuate your home would
you have everything you need and gas in your vehicle? If roadways
are closed from flooding, downed trees, or mudslides, do you have
enough supplies to last you several days? If there are widespread
power outages that last more than a day would you be prepared? If
you are stuck on a road, do you have supplies in your car?

Northern California has become very vulnerable after being hit
continuously with storms since early January and we want all
residents in our region to plan ahead and be prepared.

Another very wet pattern starts later tonight with precipitation
lingering into Wednesday. However, the strongest storm will be
Monday into Tuesday when we expect the greatest precipitation and
strongest winds. An atmospheric river will bring another plume of
intense moisture to the west coast. Models vary slightly on where
this plume will hit directly, but we have good confidence that it
will be somewhere in our CWA. Another concern is that this plume
could stall for a period of time which would significantly
increase flooding issues for wherever that stall occurs. Keep in
mind that significant flooding can also occur outside of this
intense moisture plume since soils are so saturated and there will
be added stress to levees, rivers and streams. That`s why
residents should stay tuned to social media, news and updates
during the storm so they can act quickly if needed.

Specifics of the Monday-Tuesday storm: Precipitation amounts
could range 1 to 3 inches in the valley with up to 10 inches along
the Western Sierra Slopes. Remember that runoff from the
mountains will eventually flow downstream and add impacts to the
valley. Snow amounts will be heaviest above 6000 feet with a range
of 2-5 feet of new snow but we could see up to a foot of snow
down to 5500 feet. Snow levels will be near or slightly above
Sierra pass levels on Monday, then lower below pass levels on
Tuesday. Strong wind gusts in the valley and foothills are
forecast to range 45-65 mph with stronger winds over mountains
Monday afternoon into Monday night.

Summary of Warning & Advisory products: An Areal Flood Warning
remains in place for interior Norcal through much of the week. A
Winter Storm Warning has been issued for the Sierra Cascade range
through 10 pm Tuesday. A Wind Advisory for the Northern Sacramento
Valley is in place now until 10 am Monday and then the Wind
Advisory includes the entire Sacramento Valley, the Northern San
Joaquin Valley and Delta region from Monday 10 am to 4 pm. From 4
pm Monday to 4 am Tuesday we have a High Wind Warning in place
across all our valley and foothill regions while also including
the eastern slopes of the Coastal range and the northern mountains
in Shasta County.

Precipitation drops off on Tuesday behind the main front as do
winds but snow levels drop as colder air filters into the region.
Some snow levels could lower down into the 4000-5000 ft range, but
there won`t be as much precipitation occurring so snow
accumulations at the lower levels will not be high amounts.

Main upper trough shifts through on Wednesday keeping the showers
going and could lower snow levels even farther but precipitation
amounts will be fairly light. Cold air aloft that brings down the
snow levels will also destabilize the atmosphere so it is likely
that the valley will see isolated afternoon and evening
thunderstorms Tuesday and Wednesday.

JBB

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Mainly dry conditions are expected on Thursday as weak ridging
moves in, with just some lingering mountain and foothill showers.
Precipitation amounts should be light. Model uncertainty is
evident beyond Thursday as an upper trough drops southward along
the West Coast. The GFS and the Canadian models paint a wet
scenario Friday into Saturday but the ECMWF limits precipitation
activity to the northern mountains. Given model disagreement,
have kept precipitation chances on the lower end. Another upper
system is depicted by the extended models Sunday night into
Monday. However, there are some differences in terms of
precipitation amounts and coverage. At this point, looks like
precipitation chances will be favored across higher elevations.


&&

.AVIATION...

Scattered showers this afternoon before the next Pacific storm
moves in. Widespread precipitation expected generally after 06z
Monday with MVFR/IFR conditions across the Valley and IFR/LIFR
conditions across higher terrain. Gusty southerly winds will
continue overnight, especially over the northern Sacramento
Valley. Stronger winds expected 03z Monday - 18z Tuesday, with
gusts up to 55 kts.

&&

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

High Wind Warning from 4 PM Monday to 4 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.

Wind Advisory from 10 AM to 4 PM PST Monday for Carquinez Strait
and Delta-Central Sacramento Valley-Northern Sacramento Valley-
Northern San Joaquin Valley-Southern Sacramento Valley.

&&

$$



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