Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 181735
Area Forecast Discussion
National Weather Service Seattle WA
935 AM PST Wed Jan 18 2017
.SYNOPSIS...A frontal system is bringing rain and windy conditions
to the area this morning. Rain will taper off to showers and
winds will diminish this afternoon. Cold air remains trapped in
the central Cascade passes; this will result in a combination of
freezing rain and sleet today. A cool upper level trough will
settle over the region tonight through the weekend giving lower
snow levels and more showery weather along with occasionally
.SHORT TERM...A frontal system lifting northeast through the area
this morning continues to produce heavy rainfall and is giving
windy conditions. 24 hour precipitation totals have been
impressive in the lowlands with 2 inches to locally 4 inches in
the central Puget Sound area. 4 to 6 inches have been reported in
the Olympics, and Mt Baker has seen 4.5 inches of rain. For flood
and river flooding related impacts, see the hydrology section
below. Rain will gradually taper off this afternoon and become
showery in nature as freezing levels fall tonight.
Wind appears to have peaked but southeasterly pressure gradients
continue to be high. Will keep an eye on observations over the
next hour or two and will take down the high wind warnings for the
coast and north interior if the decreasing trend continues and do
a forecast update.
The cold pool east of the Cascades never eroded out and cold air
continues to be dammed up along the east side of the mountains.
This cold air is squirting westward through the passes giving
surface temperatures in the teens. The warm air aloft is more
elevated then it has been, so precipitation is now falling mostly
as sleet with some snow mixed in. Will continue the winter storm
warning for Snoqualmie and Stevens passes through this afternoon.
Precipitation rates will decrease later this afternoon and tonight
and snow levels will gradually fall reducing the threat for
additional freezing rain and sleet.
A cool upper level trough will settle over the region Thursday
into Friday for periods of showers, lower snow levels, and near
seasonal temperatures. Albrecht
.LONG TERM...From the previous long term discussion: The models
are general agreement with the idea of a broad upper level trough
moving into the offshore waters by the weekend with a series of
disturbances rotating around it and bringing periods of
precipitation. The 00Z runs of both the GFS and the Euro agree
that Sunday could be the wetter day this weekend. By the early
next weeks, the models attempt to build a reasonably high
amplitude shortwave ridge into the Pacific Northwest. So we might
finally get a couple of dry days. 27
.AVIATION...Strong southwest flow aloft today will ease tonight
as an upper trough approaches. At the surface, strong southeast
gradients will also ease. The air mass is moist and stable,
except it will become slightly unstable off the coast. Most
ceilings will remain MVFR 2-3K ft, with occasional excursions into
VFR 4-5K ft, or below 1K ft in showers.
KSEA...Discussion above applies. Southeast wind 10G20 KT today
easing to 5-10 KT tonight. CHB
.MARINE...A series of frontal systems will move through the waters
this week. Gale force winds over the coast, strait entrances,
northern inland waters, and Admiralty Inlet will ease later today.
There will be a bit of a lull between weather systems Thursday
before more strong fronts affect the waters Friday through Sunday.
The combination of high astronomical tides and low atmospheric
pressure will cause minor tidal overflow this morning on low lying
land adjacent to the inland waters. West swell around 20 feet could
affect the Coastal Waters on Saturday. Schneider/CHB
.HYDROLOGY...Heavy rain will come to an end by early this
afternoon as a frontal wave now over north-central portions of
western Washington moves northward. Rainfall amounts have been
sufficient to push the Skokomish River above flood stage. Other
rivers may still flood, but the threat appears to be decreasing.
Will look at new hydrologic model output in the next couple of
hours and will drop the flood watch if the threat continues to
Minor urban flooding continues to be reported in the greater
Seattle metro area this morning as a result of the heavy rainfall
amounts from late Tuesday afternoon through this morning. The
Flood advisory for the central lowlands of the Puget Sound area
will be allowed to continue through 1015 AM this morning, then
will be allowed to expire as rainfall will be diminishing.
Renewed flooding after this evening is not expected as additional
precipitation amounts will be lighter and snow levels will be
falling to 2000-4000 feet. A ridge of high pressure at the
surface and aloft will build into the area early next week giving
another extended dry period. Albrecht
WA...Flood Watch through this evening for Admiralty Inlet Area-
Bellevue and Vicinity-Cascades of Pierce and Lewis Counties-
Cascades of Snohomish and King Counties-Cascades of Whatcom
and Skagit Counties-Central Coast-East Puget Sound Lowlands-
Eastern Strait of Juan de Fuca-Hood Canal Area-Lower
Chehalis Valley Area-North Coast-Olympics-Seattle and
Vicinity-Southwest Interior-Tacoma Area-Western Strait of
Juan De Fuca-Western Whatcom County.
Coastal Flood Advisory until noon PST today for Admiralty Inlet
Area-Bremerton and Vicinity-Everett and Vicinity-Hood Canal
Area-San Juan County-Seattle and Vicinity-Tacoma Area-
Western Skagit County-Western Whatcom County.
High Wind Warning until 3 PM PST this afternoon for Admiralty
Inlet Area-San Juan County-Western Skagit County-Western
High Wind Warning until 10 AM PST this morning for Central Coast-
PZ...Small Craft Advisory for rough bar until midnight PST tonight
for Grays Harbor Bar.
Gale Warning until 3 PM PST this afternoon for Admiralty Inlet-
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-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 this afternoon for Central
U.S. Waters Strait Of Juan De Fuca-Puget Sound and Hood
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