Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180539
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
939 PM PST Tue Jan 17 2017

.SYNOPSIS...A strong system will give areas of heavy rain and
windy conditions tonight through Wednesday. Cold air trapped in
the central Cascade passes will produce a combination of freezing
rain and sleet through Wednesday. A cool upper level will settle
over the region Wednesday night through the weekend giving lower
snow levels and more showery weather along with occasionally
breezy conditions.

&&

.SHORT TERM...
Wind speeds reached or were close to high wind warning criteria
over some areas on the coast and northwest interior; therefore, a
High Warning was issued earlier this evening. Winds will subside
over these areas somewhat overnight (but remain quite windy)
before briefly reaching high wind criterion again Wednesday
morning.

Meanwhile, rain (locally heavy) will continue overnight. Freezing
rain and sleet will continue in Snoqualmie Pass and Stevens Pass,
where temperatures were in the mid to upper teens (below 5,000
feet). It looks like about two inches of sleet may have fallen in
Snoqualmie Pass thus far.

From the previous discussion:
An atmospheric river extends from north of Hawaii into the
eastern Pacific around 35N 140W then northeastward into Western
Washington. This atmospheric river contains impressive
precipitable water amounts of 1.5 to 2 inches, is being forced
northeast into the area by impressively strong south to southwest
winds between 2000 and 5000 feet, and contains hourly rainfall
rates of 0.25 to 0.5 inches per hour offshore. At 2 PM, Abernathy
Mountain RAWS in the far south interior at 2000 feet had south
winds gusting to 67 mph. The short term NAM12 and GFS solutions
both show 850 mb winds (5000 feet) south-southwest at 60 to 75
knots tonight into Wednesday morning. All this points to a 24 hour
period of heavy rain that will continue until mid afternoon on
Wednesday. See the HYDROLOGY section below for more details on
this rainfalls affect on local rivers and urban runoff.

The strong winds aloft will continue to interact with the terrain
causing high-end advisory level winds across the north interior
(from north of Everett and Whidbey Island northward to the
Canadian Border) and along the coast. Sustained winds in these
areas already have been 25 to 35 mph in many locations and a few
gusts to 56 mph have occurred today. Expect these conditions to
continue through midday Wednesday. A wind advisory is in effect
for these areas.

Freezing levels are around 8000 feet this afternoon per the 18Z
special sounding taken at KUIL. Snow levels are above most
mountain observations sites. A cold dome of air banked up on the
east slopes of the Cascades is giving easterly flow in the passes
with temperatures well below freezing. Rain falling from the warm
air aloft into the cold air is producing a combination of freezing
rain and sleet in the passes down to around 1000 feet east of
North Bend this afternoon. Temperatures at the lower portions of
Snoqualmie Pass will go above freezing this evening, but the bulk
of the pass will see significant accumulations of either freezing
rain or sleet, and that will continue until mid or late afternoon
Wednesday. This is shown well by the 18Z NAM. The GFS appears to
be eroding the cold air to the east too quickly. A winter storm
warning for mainly freezing precipitation is in effect for
Snoqualmie and Stevens passes through Wednesday afternoon.

Pressures have been falling across the region and astronomical
tides remain high. Coastlines in the Admiralty Inlet area, on the
south facing shores of the San Juans, around Bellingham Bay, and
around Blaine, will have high tides Wednesday morning that reach
within a half-foot of the HAT value (HAT is the point at which
minor overflow begins). The combination of the high tide and
rather strong wind and waves from the south could give some minor
problems to areas susceptible to flooding around the high tide
Wednesday morning. A coastal flood advisory was issued for around
the high tide Wednesday morning in these areas.

Once the front and strong flow aloft moves off the the south of
the area Wednesday afternoon, a deep and cool upper trough will
settle in along about 130W. Difficult to time, and inconsistently
handled short-wave-systems moving around this trough will give
periods of rain or showers at times and temperatures around normal
for this time of year. Albrecht

.LONG TERM from the previous discussion...
Models generally agree on bringing a broad upper level trough
through the area over the weekend, though they differ on the
details concerning the strength and placement of all-important
short-wave features rotating around the upper trough. Starting
early next week, the models are trending drier as strong upper
riding develops over western portions of North America. At this
time the long term forecast shows temperatures near normal with
snow levels 2000-4000 feet through Sunday night followed by a
gradual drying trend early next week. Albrecht

&&

.AVIATION...A frontal system will move onshore tonight and
Wednesday with southwest flow aloft. The air mass is moist and
stable.

KSEA...Low clouds will persist through Wednesday. It looks like
IFR ceilings will affect the terminal this evening then generally
MVFR after that. Confidence not especially high about that
however. IFR ceilings possible again Wednesday night. Southerly
wind 5-10 knots will increase to 10-15 knots Wednesday. Schneider

&&

.MARINE...A series of frontal systems will move through the waters
this week. Gales will continue Coastal Waters, entrances to the
Strait, Northern Inland waters and Admiralty Inlet through tonight
and probably through at least the morning hours on Wednesday. There
will be a bit of a lull in between weather systems Thursday before
additional strong fronts affect the waters Friday through Sunday.
Schneider

&&

.HYDROLOGY from the previous discussion...
Heavy rain, associated with an atmospheric river that is tapped
to tropical moisture near Hawaii and is being transported
northeastward in strong low level flow, is expected tonight
through early Wednesday afternoon. A flood watch continues for the
Olympic Peninsula (Grays Harbor and Mason Counties northward), for
Whatcom County and for King and Pierce Counties. The rivers most
likely to flood in this pattern include the Skokomish (though the
Elwha and Satsop cannot be ruled out), the Nooksack, and the White
River (near the R Street Bridge).

In addition the the threat of some river flooding, it appears that
urban flooding is becoming increasingly likely in the central
Puget Sound area where confluence of the low level flow by the
terrain around the sound adds lift of the very moist air mass
flowing at rather high speeds northward. Several models show an
area of 2-3 inch rainfall from Shelton and Bremerton eastward
through the greater Seattle-Bellevue area tonight into Wednesday
morning. Highest rainfall rates appear that they will occur this
evening, then again Wednesday morning.

Once the atmospheric river moves to the south of the area
Wednesday afternoon and evening and snow levels fall, the threat
of river flooding will decrease. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watches/Warnings/Advisories are in effect for much of
 the CWA.

     High Wind Warning until 3 PM PST Wednesday for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom
     County.

     Coastal Flood Advisory from 7 AM to noon PST Wednesday for
     Admiralty Inlet Area-San Juan County-Western Skagit County-
     Western Whatcom County.

     High Wind Warning until 10 AM PST Wednesday for Central Coast-
     North Coast.

PZ...Small Craft Advisory for rough bar until midnight PST Wednesday
     night for Grays Harbor Bar.

     Gale Warning until 3 PM PST Wednesday 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.

     Gale Warning until 9 AM PST Wednesday for Admiralty Inlet-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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