Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSEW 171732
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
932 AM PST Tue Jan 17 2017

.SYNOPSIS...A weakening frontal system is moving through Western
Washington this morning. A stronger system will arrive tonight
through Wednesday bringing wet and windy conditions. Cold air
trapped in the central Cascade passes will produce a combination
of freezing rain, sleet and snow through Wednesday. A cool upper
level trough will settle over the area for the second half of the
week with showery and occasionally breezy weather.

&&

.SHORT TERM...A weakening frontal system associated with a surface
low moving toward Haida Gwaii is pushing through Western
Washington this morning. Rain in the lowlands and a combination of
freezing rain, sleet and snow in the passes will temporarily pause
this afternoon as the front dissipates to the east.

A second, stronger, front with an impressive slug of moisture can
be seen on satellite imagery from off the northern Oregon coast
southwestward to around 33N 150W then westward into the Pacific.
This moisture will move into the area this evening through
Wednesday bringing an impressive 18-24 hour of moderate to heavy
precipitation to the area. Temperatures aloft will remain quite
warm and winds at 5000 feet will be southwesterly 60 to 75 knots.
The winds aloft have trended stronger. At the low levels, flow
will remain easterly across the passes and there remains an
impressive 4000 to 5000 foot deep cold pool that will remain
banked up against the east slopes of the Cascades.

Based on the above:
1) Will maintain the wind advisory for the coast and north
interior (areas north of Everett to the Canadian border) through
Wednesday. Winds in these areas will be 25 to 35 mph with gusts 50
to 55 mph, mainly tonight and Wednesday.
2) Will maintain the flood watch for the Olympic Peninsula from
Grays Harbor and Mason counties northward and for Whatcom County
in the north interior. Will also expand the flood watch to include
King and Pierce counties since hydrologic guidance suggests that
portions of the White River will reach flood stage.
3) Will issue a winter storm warning for a heavy accumulation of
a combination of freezing rain, sleet and snow in Snoqualmie and
Stevens passes. This warning will go through Wednesday afternoon
as the winds in the passes will stay easterly through the period.

Once the front moves northeast of the area late Wednesday
afternoon, a cool upper trough will settle over the region
with lowering snow levels and cool showery conditions. Albrecht

.LONG TERM...From the previous long term discussion: The models
generally agree on bringing a broad upper level trough into the
offshore waters by late Friday. They then begin to diverge in
solutions over the weekend in regards to where and how quickly to
bring the trough onshore. The GFS is considerably faster than the
Euro. Despite the differences...a strong jet stream is poised to
remain to our south into the weekend. Coastal areas and the
Southwest Interior may see an uptick in precipitation over the
weekend, but Oregon and California are likely to see the bulk of
the precipitation in the extended period. 27

&&

.AVIATION...A weakening frontal system will move onshore this
morning with southwest flow aloft. A stronger front will reach the
area later today. There is high pressure centered over southern
Idaho and deep low pressure moving to Haida Gwaii. The air mass is
stable with increasing moisture all levels and the mountains are
obscured.

KSEA...Cigs will lower as more rain moves in, likely dropping to
MVFR later today as heavier rain arrives. Southerly breeze.

&&

.MARINE...There is high pressure centered over southern Idaho and
deep low pressure moving to Haida Gwaii. There will be gale force
winds at times for the Coastal Waters, entrances to the Strait,
Northern Inland waters and Admiralty Inlet today through Wednesday.
There will be a bit of a lull in between weather systems Thursday,
then additional fronts will affect the waters Friday and Saturday.

&&

.HYDROLOGY...Heavy rain associated with a warm front will move
through the area late this afternoon through early Wednesday
afternoon. The moisture from this system is tapped into tropical
moisture around the Hawaiian Islands. Winds aloft at 5000 feet are
expected to rise to 60-75 knots from the southwest as snow levels
remain over 7000 feet tonight into Wednesday. Peak 24 hour rainfall
amounts at this time look to be 4 to 6 inches in the Olympics...3
to 5 inches in the North Cascades and 2 to 4 inches in the Central
and Southern Cascades.

Rivers coming off of the Olympics have the best chance to flood in
this scenario with the flood prone Skokomish the most likely of the
bunch. Other rivers that could reach flood stage will be the rivers
coming off the North Cascades in Whatcom County.

Some reaches of the White River that borders King and Pierce
counties could flood as well.

The flood watch will be expanded to include King and Pierce
counties tonight into Wednesday evening. The flood watches for the
Olympic peninsula and for Whatcom county will be maintained.
Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from 6 PM PST this evening through Wednesday 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-Tacoma Area-Western Strait of
     Juan De Fuca-Western Whatcom County.

     Wind Advisory until 3 PM PST Wednesday for Admiralty Inlet Area-
     Central Coast-North Coast-San Juan County-Western Skagit
     County-Western Whatcom County.

PZ...Small Craft Advisory for rough bar until 2 AM PST Wednesday for
     Grays Harbor Bar.

     Gale Warning until noon 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 2 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 10 AM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST Wednesday 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


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.