Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 181518 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 818 AM PDT Mon Sep 18 2017 .SYNOPSIS...
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An upper trough will be over the area through Wednesday for cool and showery weather. Drier northerly flow aloft will develop late in the week as an upper ridge offshore moves toward the area.
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&& .SHORT TERM...
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An upper trough will be over the Pacific Northwest for the next few days for cool weather an showers at times. There should be an uptick in the showers this afternoon and evening as an upper short wave moves onshore. There could also be isolated thunderstorms today, mainly towards the coast. Another short wave will push onshore Tuesday night. Models have trended further south with this feature and most of the shower activity will probably be south of the area. The upper trough axis will begin to shift inland Wednesday and the shower threat will be decreasing. Highs the next few days will be several degrees below normal. Regarding snow in the mountains...models show snow levels between 5000 and 6000 feet the next few days. This seems to be on track as web cams at locations like Hurricane Ridge in the Olympics and Paradise on Mount Rainier picked up a dusting of snow early this morning. The main threat for significant accumulating snow over the next couple of days would be at Paradise. The problem is the necessary ingredients - orographics, QPF, and low snow levels - are probably too marginal for Paradise to pick up 6+ inches of snow tonight or Tuesday night. While 6+ inches cannot be ruled out entirely, the more likely scenario is something like 1-3 inches. Unless there is some big change in the 12Z models this morning, the winter storm watch will probably be cancelled later today. Schneider .LONG TERM...Drier northerly flow aloft will develop over Western Washington Thursday. It looks like things will dry out entirely around Friday and remain dry into the weekend with northerly flow aloft continuing. Schneider
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&& .AVIATION...
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An upper level trough will continue to bring scattered showers to the area. The air is moist and unstable with onshore flow. There will be a chance of thunderstorms this afternoon and evening. The mountains and higher terrain will be obscured much of the time. KSEA...Scattered showers and sunbreaks with a southerly breeze.
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&& .MARINE...
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Low pressure will remain over the region through midweek. There is a trough near Vancouver Island that will move southeast through the area later today. There are stronger NW winds behind the trough that will clip the coast as the trough heads for Oregon tonight. Another trough will reach the area Tuesday night and Wednesday morning--the strongest pressure gradient with that one will be down on the Oregon coast. The winds over Western Washington will be onshore through Tuesday and then will probably become light northerly on Wednesday as the lower pressure passes to the south.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Storm Watch from 6 PM PDT this evening through Tuesday morning for Cascades of Pierce and Lewis Counties. PZ...Small Craft Advisory from 11 AM this morning to 11 AM PDT Tuesday 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-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Small Craft Advisory from 11 AM this morning to 11 PM PDT this evening for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.
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&& $$ www.weather.gov/seattle You can see an illustrated version of this discussion at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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