Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 192144 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 244 PM PDT Tue Sep 19 2017 .SYNOPSIS...
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Low pressure aloft will remain over the region through Wednesday for cool and unsettled conditions. High pressure aloft will gradually move over the area from the west the latter part of the week for a drying and warming trend. An upper ridge will remain over the area into early next week.
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&& .SHORT TERM...
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An upper trough will be over the area tonight. There have been a few lightning strikes this afternoon over the Central and South Puget Sound area but these are expected to be gone by evening. Fairly widespread showers this afternoon should diminish early this evening as daytime heating ends. An upper short wave will push onshore tonight and Wednesday morning, brushing the southern part of the forecast area and bringing another shot of organized showers to the south. The mountains above 5000 feet will probably pick up a little more snow tonight and Wednesday. Paradise will probably get the most with maybe 2-4 inches. Further north, showers will be more isolated later tonight and Wednesday. Highs will be near 60. Drier northerly flow aloft will develop Thursday and Friday as the upper trough shifts inland. A few showers will linger but probably mostly over the mountains, especially by Friday. Highs will creep up a few degrees each day but remain a little below normal. Schneider .LONG TERM...An upper ridge will be just offshore Saturday through Tuesday. The ridge is looking more dirty compared to yesterday. The GFS is dry but the ECMWF brings some precipitation over the top of the ridge and into Western Washington at times. The forecast is dry for now but some PoPs may need to be introduced if models continue to trend the direction of the EC. Temperatures should warm back to near normal during this period. Schneider
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&& .AVIATION...
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An upper level trough is over the Pacific Northwest. The air is moist and unstable with onshore flow and scattered showers. The mountains will remain mostly obscured. KSEA...There will continue to be scattered showers that move across the area through this evening--and a Puget Sound convergence zone might develop this evening and focus more shower activity over Seattle. The southerly breeze will become variable if a PSCZ develops.
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&& .MARINE...
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One trough will shift inland through tonight and another will move ashore, mainly over Oregon, on Wednesday. High pressure will build into the area Friday and Saturday. Swell over the coastal waters around 13 feet will subside to around 8 feet Wednesday.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory for hazardous seas until 2 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-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory for rough bar until 2 AM PDT Wednesday for Grays Harbor Bar.
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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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