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Area forecast discussion 
National Weather Service Seattle Washington 
900 PM PDT Tue Oct 17 2017 


Synopsis...a strong front will move into the area Wednesday with a 
period of heavy rain and windy conditions. Showers are expected 
Thursday and Friday. A strong warm front will arrive for the weekend. 


&& 


Short term...no updates this evening, forecast is in good shape. A 
strong front will move over western Washington on Wednesday. Heavy 
rain will fall in the mountains as a moist plume of air in strong 
southwest winds aloft collides with the Olympics and mainly the north 
Cascades Wednesday afternoon and evening. A Wind Advisory is in 
effect 
rather blustery weather. A Flood Watch is also in effect. See the 
hydrology section below for more information. The snow level will be 
high as the front moves over the area and will fall to around 4000 
feet behind the front as the rain turns to showers later Thursday 
through Friday. 


Long term...a strong warm front will move into the area on 
Saturday and may result in another period of heavy rainfall and 
strong winds. The GFS takes the brunt of the energy with this next 
system into northern Oregon, while the European model (ecmwf) is a bit farther 
north and gives more a prolonged period of heavy rain. This 
system will need to be monitored closely for the threat of 
flooding. 


Later Sunday or Monday Flat ridging aloft will build to the south 
of the area taking systems more into western portions of British 
Columbia. The forecast will call for drying and high temperatures 
warming to near 60 by next Tuesday. Albrecht 


&& 


Aviation...a vigorous frontal system will move through the area 
Wednesday. Ceilings will gradually lower tonight. Low clouds will 
become widespread Wednesday morning. 


Ksea...high clouds tonight. MVFR low clouds and rain will develop 
Wednesday morning as a front arrives. Southerly wind 5-10 knots 
will rise to 10-20 knots later Wednesday. Schneider 


&& 


Marine...a vigorous frontal system will bring gales to most 
waters Wednesday. A period of onshore flow will follow the front 
Thursday and Friday. Another strong front will move through the 
area Saturday. West swell 20-24 feet are expected to affect the 
coastal waters later Thursday through Friday morning. Schneider 


&& 


Hydrology...heavy rainfall is expected Wednesday through 
Wednesday night. The main focus of the precipitation will be on 
southwest facing slopes of the Olympics and north Cascades. 
Precipitation amounts of 5 to 8 inches can be expected in the 
Olympics, while the north Cascades see 4 to 6 inches and The 
Lowlands see up to 3 inches. The heavy rainfall rates are 
accompanied by high snow levels well over 7000 feet. Models do not 
show flooding on area rivers, but bring the river stages close 
enough (within a foot) on the skokomish, Nooksack, stillaguamish, 
and Skagit that flood watches have been issued for Wednesday 
afternoon through midday Thursday for Snohomish, Skagit, Whatcom 
and Mason counties in case rainfall is any heavier than currently 
forecast. Lowland locations may see urban and small stream 
flooding due to the combination of high rain rates and drainage 
issues due to leaves clogging drains. 


Rivers will recede later Thursday through Friday night as snow 
levels drop to around 3500 feet and precipitation becomes more 
showery. 


Rivers could quickly rise later Saturday into Sunday with the 
weekend system. The European model (ecmwf) is warmer and heavier with precipitation 
amounts and shows a more classic atmospheric river pattern for 
about 24 hours. With rivers starting from a higher point, minor 
flooding is possible on some rivers across the area. 


Conditions will dry out starting later Sunday or Monday of next 
week allowing all rivers to recede. Albrecht. 


&& 


Sew watches/warnings/advisories... 
Washington...Wind Advisory from 11 am to 11 PM PDT Wednesday for Bellevue and 
vicinity-Bremerton and vicinity-Hood Canal area-Seattle and 
vicinity-Tacoma area. 


Flood Watch from Wednesday afternoon through Thursday morning 
for Bellevue and vicinity-Cascades of Snohomish and King 
counties-Cascades of Whatcom and Skagit counties-east Puget 
Sound lowlands-Everett and vicinity-Hood Canal area-lower 
Chehalis valley area-Olympics-western Skagit County-western 
Whatcom County. 


High Wind Warning from 11 am to 11 PM PDT Wednesday for 
Admiralty Inlet area-central coast-Everett and vicinity- 
north coast-San Juan County-western Skagit County-western 
Whatcom County. 


Pz...Small Craft Advisory for rough bar until 11 PM PDT this evening 
for Grays Harbor bar. 


Small Craft Advisory until 4 am PDT 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 from 4 am to 9 PM PDT 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. 


Small Craft Advisory until 11 PM PDT this evening for East 
Entrance U.S. Waters Strait of Juan de Fuca-northern inland 
waters including the San Juan Islands. 


Gale Warning from 4 am to 6 PM PDT 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 11 PM PDT Wednesday for West Entrance 
U.S. Waters Strait of Juan de Fuca. 


Small Craft Advisory until 5 PM PDT Wednesday for central U.S. 
Waters Strait of Juan de Fuca. 


Gale Warning from 11 am to 9 PM PDT Wednesday for Puget Sound 
and Hood Canal. 


&& 


$$ 



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