River Flood Warning 42, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
847 PM PST Tue Nov 14 2017

.SYNOPSIS...A cold front will move slowly across Western
Washington tonight, bringing areas of heavy rain, heavy mountain
snow, and windy conditions. An upper trough will follow the front
later Wednesday through Thursday night, bringing showery weather.
Friday and early Saturday could be relatively dry, but a
transition to a wet and warmer pattern is likely during the
weekend. A dry reprieve looks possible next Monday, then more
heavy rain around next Tuesday.

&&

.SHORT TERM...A slow-moving cold front will drag through Western
Washington tonight into Wednesday morning with various impacts
across the region: Windy conditions along the coast and north
interior, heavy snow in the mountains and river flooding in the
Olympics. There are also thunderstorms over the Coastal Waters
along the frontal boundary. So far, the forecast is pretty much on
track. The only update this evening was to add Clallam county to
the Flood Watch as the Bogachiel river is seeing a sharp rise.
Expect winds in the Puget Sound area of 15 to 30 MPH with winds
easing early Wednesday morning as the front pushes through.

An upper level trough will keep the air mass cool and showery on
Thursday. Snow levels will remain low in the mountains, around
2,500 feet. Temperatures will be a few degrees cooler than normal
with highs mainly in the 40s.

A weak ridge will start to build inland on Friday for drier
conditions. There is still a chance of showers in the Cascades but
overall less activity is expected. 33

.LONG TERM...Previous discussion...The GFS and ECMWF have finally
come into better agreement with the weekend weather. They both
now drop a slow- moving front southward through Western Washington
starting late Saturday, though it should take until Sunday
evening for the front to move south-southeast of the area. Should
be plenty of heavy rainfall focused along the slow-moving front,
similar to the current situation. Windy conditions will again
precede this front, especially in southeast-wind prone locales.

Looks like a dry respite next Monday and Monday night, as a
sharper transient ridge passes across the Pac NW.

Yet another heavy rain and southeast-wind producer is in our
sights for next Tuesday.  Haner

&&

.AVIATION...Strong moist southwest flow aloft will continue tonight
and Wednesday. A slow moving cold front just off the coast will move
inland tonight and weaken Wednesday. The air mass is moist and
unstable over the coast and somewhat unstable over the interior.
Strong southeasterly surface pressure gradients will continue to
produce windy conditions throughout most of Western Washington
tonight.

Due to the slow movement of the front, lower ceilings on the coast
have not yet spread over the greater Puget Sound region, where in
many places it is not even raining yet. Ceilings inland are still
VFR, and may not fall to MVFR until 12Z or later. Gusty southerly
winds 15 to 30 KT will continue over much of the area.

KSEA...The slow moving front has not made much progress eastward
so ceilings have stayed VFR. Ceilings may not lower to MVFR until
after 12Z. Gusty southerly surface winds 15-30 KT will continue. CHB

&&

.MARINE...A front just offshore will slowly move inland tonight.
Gales are occurring over most of the coastal waters and the inland
waters from Admiralty Inlet north. High end small craft advisory
south winds are occurring over Puget Sound. Winds will gradually
ease Wednesday as the front slowly dissipates as it crosses Western
Washington.

An offshore surface low will approach northern Vancouver Island
Wednesday, then move southeast over the Washington coast by Thursday
evening. A weak ridge will cross the region on Friday. Another
strong frontal system will slowly approach the coast on Saturday
then move across the area Saturday night and Sunday. Kam/CHB

&&

.HYDROLOGY...Another enhancement of heavy rain is expected
tonight. 24-hour rainfall of 5 or more inches is expected over the
windward Olympics, and 2-3 inches over the North Cascades, for the
24 hours ending mid-day Wed. This will keep the Skokomish river
above flood stage, and could possibly boost it into the Moderate
Flood category tonight. Will also be watching other rivers
flowing off the Olympics such as the Satsop and Bogachiel.

Snow levels will fall by Wednesday morning into the 3000-3500 foot
range, so that will lessen the contribution of further runoff at
that time.

Landslide indices are also creeping up and could become marginally
conducive for landslides during tonight's rain. Any particularly
intense burst of rainfall tonight could pose the first landslide
test of the rainy season.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 3 PM PST Wednesday for Cascades of
     Pierce and Lewis Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Weather Advisory until noon PST Wednesday for Cascades of
     Snohomish and King Counties.

     Wind Advisory until 4 AM PST Wednesday for Admiralty Inlet Area-
     San Juan County-Western Skagit County-Western Whatcom
     County.

     Flood Watch from 4 AM PST Wednesday through Wednesday afternoon
     for Central Coast-Lower Chehalis Valley Area.

     Wind Advisory until midnight PST tonight for Central Coast.

     Winter Storm Warning until 4 AM PST Wednesday for Olympics.

     Flood Watch through Wednesday morning for Eastern Strait of Juan
     de Fuca-North Coast-Olympics-Western Strait of Juan De Fuca.

PZ...Small Craft Advisory for rough bar until 6 AM PST Thursday for
     Grays Harbor Bar.

     Gale Warning until 6 AM PST Wednesday for Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from midnight tonight to 6 AM PST Thursday
     for Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm.

     Gale Warning until midnight PST tonight for Coastal Waters From
     James Island To Point Grenville 10 To 60 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm.

     Small Craft Advisory until 6 AM PST Thursday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm.

     Gale Warning until 10 AM PST Wednesday for Admiralty Inlet-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until noon PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Wednesday for Puget Sound
     and Hood Canal.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html