Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 181240 CCA AFDSEW Area Forecast Discussion...CORRECTED National Weather Service Seattle WA 440 AM PST Sat Nov 18 2017 Corrected a couple of minor typos. .SYNOPSIS...Weak high pressure aloft will prevail through tonight. Expect a Pacific storm to approach the area during the day Sunday. The cold front will sweep across the region during the late Sunday afternoon through Sunday evening time period. Expect locally strong winds and heavy precipitation ahead of and with the front. Weak high pressure aloft will return on Monday. && .SHORT TERM...
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There were scattered showers over the CWA at this time. Expect the showers to decrease in coverage this morning, becoming confined to the far northwest part of the CWA by this afternoon. Meanwhile, weak high pressure aloft will prevail over the region through tonight. The ridge aloft will weaken further early Sunday, allowing an upper level trof and associated cold front to approach from the northwest. The models were a little slower with this system than the previous runs; therefore, it appeared that the southeast part of the CWA will probably not see any precipitation until Sunday afternoon. The front will sweep across the area during the late Sunday afternoon through Sunday evening time frame. Expect locally strong southerly winds over parts of the coast and northwest interior ahead of and with the front. A Wind Advisory will probably be needed for some of these zones for Sunday. Concerning snowfall: The north Cascades will remain in the colder air throughout the day Sunday; therefore, the precipitation will fall as snow above the 3000 foot level. The Mount Baker ski area could pick up about a foot of snow during the day Sunday; therefore, a Winter Storm Watch was issued for Whatcom and Skagit Counties. The snow will taper off over this area behind the cold front Sunday evening. Locally heavy precipitation will continue on the Cascades of Pierce and Lewis Counties as the snow levels fall Sunday night; therefore, the Crystal Mountain ski area and the southwest slopes of Mount Rainier could receive up to about a foot of snow Sunday night before the precip tapers off. Therefore, a Winter Storm Watch was issued for this area. Expect advisory snowfall amounts (5-10 inches) in the Cascades of Snohomish and King Counties Sunday night. Stevens Pass may pick up about 10 inches of new snow. The GFS and ECMWF solutions were in better agreement as far as Monday was concerned. The Canadian model was the outlier. It looks as though a warm front will approach the region from the south during the day. Much of the CWA will be dry on Monday. .LONG TERM... Much of this period appeared mild, wet, and unsettled. Temps are anticipated to be above normal during this period. Expect locally windy periods, with the possibility of flooding on some area rivers. Confidence in the details was not high for this period.
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&& .AVIATION...A flat upper ridge centered over Western Washington early this morning will shift east into the Idaho Panhandle late tonight. A strong and wet cold front will sag southeast through British Columbia today then will shift southward through Western Washington on Sunday. Moderate westerly flow aloft. The air mass is moist and somewhat stable and will dry somewhat at lower levels today. Deep moisture will shift southward into the area late tonight into Sunday morning. Isolated convective showers over the coastal waters and extending into the interior from about KSHN southward continue to weaken and diminish in coverage early this morning. Conditions are variable with pockets of MVFR or IFR ceilings and other areas seeing ceilings 035-045. Expect the lower level moisture to decrease this morning leaving just mid and high level clouds above about FL100. Cigs will fall to 008-015 after 10Z tonight through Sunday morning as rain ahead of the incoming front shifts southeast into the area. Albrecht KSEA...South wind 8-11 knots. CIGS 030-035 this morning then above FL100 this afternoon and evening. CIGS falling to 010-015 after 10Z Sunday. Albrecht && .MARINE...A strong cold front extending WSW from the Queen Charlotte Sound into the Pacific will slowly drag southeast down the British Columbia coastline today then will push southeastward through the coastal and inland waters on Sunday. Winds will increase gradually from north to south through tonight with gales expected over all of the waters except for Puget Sound and Hood Canal and the Central Strait of Juan de Fuca late tonight into Sunday ahead of the front. Winds will rapidly ease behind the front later Sunday into Sunday night. High pressure will bring light winds to the waters on Monday. A strong warm front will push northward through the waters on Tuesday. A strong low pressure center developing well offshore will push northeast into Haida Gwaii on Wednesday. Albrecht && .HYDROLOGY...
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The models were not as wet with the Sunday system as the previous runs; however, the forecast rainfall amount for the Skokomish River basin was still enough to drive the uniquely flood-prone Skokomish River near or slightly above flood stage Sunday night. Beyond Sunday, the models did not exactly agree on the details. However, it still appears that much of this period will be wet. The snow levels will be atypically high (generally at or above 8K feet) for this time of the year Tuesday through early Thursday. This means that the precip will fall primarily as rain in the mountains. It looks like this firehose will be a bit different in that instead of staying on for X amount of time, it will be more of an on and off type of deal. Thus, expect occasional heavy rain in the mountains and on the coast. Although rainfall amounts may not be significant for any given time period, the cumulative rainfall for the Tuesday through Thursday period will probably be enough to drive some rivers above flood stage, not just the Skokomish River.
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&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch in effect for Mason County from Sunday morning through Sunday evening. Winter Storm Watch from Sunday afternoon through late Sunday night for Cascades of Pierce and Lewis Counties above 4500 feet. Winter Storm Watch from late tonight through Sunday afternoon for Cascades of Whatcom and Skagit Counties above 3500 feet. PZ...Small Craft Advisory until 10 PM PST this evening for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm- Northern Inland Waters Including The San Juan Islands. Gale Warning from 10 PM this evening to 3 PM PST Sunday for Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 6 PM PST this evening for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm. Gale Warning from 6 PM this evening to noon PST Sunday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm. Gale Warning from 10 PM this evening to noon PST Sunday for Coastal Waters From Cape Flattery To James Island Out 10 Nm- West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 3 AM PST Sunday for Coastal Waters From James Island To Point Grenville Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning from 3 AM to noon PST Sunday for Coastal Waters From James Island To Point Grenville Out 10 Nm. Small Craft Advisory from 9 AM this morning to 3 AM PST Sunday for Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning from 3 AM to 3 PM PST Sunday for Admiralty Inlet- Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 10 PM PST this evening for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 3 AM PST Sunday for Admiralty Inlet. Small Craft Advisory from 10 PM this evening to 9 PM PST Sunday for Puget Sound and Hood Canal. && $$ www.weather.gov/seattle An illustrated version of this forecast discussion can be seen at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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