River Flood Warning 11, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
1000 AM PST Fri Feb 10 2017

.SYNOPSIS...A surface low will move across the north part of
Western Washington today bringing windy and showery weather to the
area. Showers will decrease this evening as the associated upper
level trough moves east. An upper level ridge will build over the
region this weekend for calmer and mostly dry weather through at
least Monday. A weak warm front could bring some spotty light rain
to the area on Saturday, but will depart soon enough for mostly
sunny skies Sunday and Monday. Increasing moist southwest flow aloft
will probably bring some light rain to the area Tuesday or
Wednesday.

&&

.SHORT TERM...A 999 MB surface low was about 75 NM W of the north
coast at 7 AM this morning. Models are in good agreement that the
low will fill as it moves NE across the N interior of W WA late this
morning. It will move E of Vancouver by 4 PM this afternoon. S winds
are coming up as the low approaches especially over the NW interior
and along the coast. Winds still look like they will peak in the
advisory range. A westerly surge of wind is still expected through
the strait of Juan de Fuca today and that will probably be the last
area for winds to weaken, late this afternoon.

The upper level shortwave trough, associated with the surface low,
will move across W WA this afternoon. Scattered snow showers are
still falling over the Cascades this morning and are expected to
increase a little this afternoon as the upper level trough moves
through. A winter weather advisory remains in effect for the
Cascades through 6 PM. The passes are still a mess from the residual
heavy snow followed by heavy rain.

The flow aloft will shift to NW this evening as a second weak upper
level trough zips across the area. Showers will continue into this
evening then probably end for the lowlands by late evening while
continuing over the mountains through tonight.

A broad upper level ridge with modest amplitude will begin moving
over the region on Saturday and then continue over the PacNW through
Monday. This will bring a period of calmer and mostly dry weather
this weekend. A surface ridge will be over the area Saturday for
light winds. A weak warm front moving across the north side of the
ridge may bring some spotty light rain to mainly the coast on
Saturday. As the ridge axis moves overhead on Sunday the air mass
should dry enough for mostly sunny skies. Kam

.LONG TERM...The upper ridge will shift further inland Sunday
night and Monday, with weak low level offshore flow developing. It
will be another dry day, mostly sunny to partly sunny, and a
couple degrees warmer yet. Longer range models differ on when wet
weather will return to the forecast area. The new GFS run shows
some showers developing over the region Monday night, prior to the
arrival of more widespread frontal precipitation late Tuesday
through Wednesday. Other models keep the weather basically dry
until the front moves in Tuesday night and Wednesday. For now we
are discounting the outlying GFS solution. This will be a fairly
warm system with the snow level around 6000 ft. Thursday will
probably have some rain too. McDonnal

&&

.AVIATION...Upper trough just off the coast is expected to move
inland and east of the Cascades by 03z this evening. Strong S/SW
flow ahead of the trough to swing to W/NW behind the trough aloft.
At the surface, a weakening low just off the north coast will track
NE to near Vancouver BC this afternoon and continue inland this
evening. The air mass is moist and slightly unstable with MVFR to
lower-end VFR conditions and scattered showers that are forecast to
taper off through tonight.

Expect strong winds at the surface with peak gusts to 40-45 kt
possible - coast this morning to midday and inland, late morning
through midday tapering off through this evening. A ridge of higher
pressure is forecast to build over the region Saturday into early
next week for a drying trend and more stable air mass. Buehner

KSEA...Showers around the terminal today with varying cigs 2500-
4500ft. S/SW winds increasing late this morning peaking midday with
gusts up to 45 kt, tapering off later this afternoon and easing
this evening after 02z. Buehner

&&

.MARINE...The combination of high astronomical tide and low pressure
will result in 1-1.5ft tidal anomalies from 10 am through 2 pm this
afternoon along the central coastlines of WA. A minor tidal overflow
is in effect. Large wave action from swell up to 19 feet could
locally exasperate flooding near the central coast today.

A 995 mb surface low will move onto the north coast by noon...
followed by a period of strong onshore flow. Gales are likely across
most waters. Winds will ease tonight as this system weakens and
exits. High pressure will bring lighter winds on Saturday. The flow
will turn offshore on Sunday and continue through early next week.
dtm/33

&&

.HYDROLOGY...Showers today will end overnight and then we will
have a dry period through the rest of the weekend. The showers will
not be enough to exacerbate or renew flooding, though any heavier
showers may slow or delay recessions. Low snow levels will further
reduce any possible runoff.

The upper reaches of the rivers have crested with the crests moving
downstream to the lower reaches of most rivers today. The exception
will be the slow moving Chehalis where the flood crests will not
reach Thurston and Grays Harbor Counties until tonight and Saturday
afternoon respectively. The notoriously slow receding Skokomish
will likely drop below flood stage tonight.

Once this round of flooding comes to an end this weekend no
additional flooding is expected next week. Felton/JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 3 PM PST this afternoon for Bellevue and
     Vicinity-Bremerton and Vicinity-Everett and Vicinity-Hood
     Canal Area-San Juan County-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Whatcom
     County.

     Winter Weather Advisory until 6 PM PST this evening for Cascades
     of Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Flood Watch from this evening through Saturday evening for
     Central Coast-Lower Chehalis Valley Area.

     Wind Advisory until noon PST today for Central Coast-North Coast.

     Coastal Flood Advisory until 2 PM PST this afternoon for Central
     Coast.

     Wind Advisory until 8 PM PST this evening for Admiralty Inlet
     Area-Eastern Strait of Juan de Fuca.

PZ...Small Craft Advisory for rough bar until 4 PM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory from 3 PM this afternoon to 4 AM PST
     Saturday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 3 PM PST this afternoon for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Warning until 4 AM PST Saturday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Warning until 10 PM PST this evening for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

     Gale Warning until 6 PM PST this evening for Puget Sound and
     Hood Canal.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html

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