Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 141213 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 413 AM PST Thu Dec 14 2017 .SYNOPSIS...A strong upper ridge over the Pacific Northwest will quickly flatten and break down through tonight, allowing a cold front to move onshore with some rain on Friday morning. A warm front will then try to spread onshore on Saturday and Saturday night. A cold front will sag southward through Western Washington Sunday night and Monday. A developing low center could move near the area around Tuesday, followed by cool showery weather next Wednesday. && .SHORT TERM...This long streak of dry December weather is only about 24 hours from reaching its end. The strong and amplified upper ridge axis which has clung to the Pacific Northwest for the past week will flatten quickly today and tonight, and westerly flow aloft will become strong tonight. This will drive a cold front onshore late tonight and Friday morning, accompanied by light to moderate rain and mountain snow. Rain should first reach the North Coast after midnight tonight, then arrive in the Puget Sound region around 4-6 am. The front will be followed by weak, shallow shower activity. Onshore flow will focus shower activity on windward terrain and near a short-lived Puget Sound Convergence Zone. Early Saturday morning, deep warm advection will develop, bringing warm frontal lift and the development of a large stratiform precip shield that will persist for most of Saturday and Saturday evening. .LONG TERM...Models are in good agreement with the longwave pattern over the eastern Pacific and western North America through next Wednesday. An east-west oriented cold front will sharpen over Vancouver Island early Sunday and then sag southward with precip later Sunday and Sunday night. The front should turn up stationary over Southwest Washington on Monday. Meanwhile, an upper trough trailing southwest from the Gulf of Alaska will advance eastward on Monday. Large-scale lift in the right entrance region of an upper jet preceding the trough will interact with Monday`s leftover front to support cyclogenesis off the coast. The 00z ECMWF and the 06z GFS ensemble mean rapidly deepen the low offshore and bring it on an E-NE track across central Vancouver Island. The deterministic 06z GFS`s track through Western Washington appears to be a rightward outlier at this time. Still, it will support moderately windy conditions on Tuesday, first in southeast-wind prone areas in advance of a strong trailing cold front, then in the Puget Sound region following the front. This will be something to watch in the days ahead. In fact, the biggest change with this forecast package is to significantly increase precip chances on Tuesday. Once the deepening low passes east of our longitude, a fairly cold ocean-modified air mass will spread across the Pacific Northwest next Wednesday, along with typical post-frontal showers. For being a Day 7 forecast, there is strangely good agreement in models depicting Fraser Outflow initiating late next Wednesday or on Thursday, bringing even colder but drier continental air from the interior of British Columbia. Only two of the 06z GFS`s 20 ensemble members shows over 1 inch of snow at SEA and BLI next Wed. In other words, the odds of significant lowland snow appear very low right now through the middle of next week. Haner && .AVIATION...Surface high pressure is inland and the axis of the upper ridge is just off the coast. There is offshore flow with dry stable air--except for some high clouds and also patchy fog. Clouds will increase tonight and Friday morning, with areas of rain friday morning in a cold front. KSEA...Some cirrus at times and a chance of some fog this morning, winds will be light northeast and with the SEA-YKM gradient at -8mb we might see an east breeze develop today. A little rain and a southerly breeze should develop for Friday morning with a cold front. && .MARINE...Light offshore flow continues. Some areas of fog should develop by daybreak. East winds at the West Entrance to the Strait of Juan de Fuca and west of Grays Harbor will rise to around 20 knots at times today. A cold front will move through the waters after midnight tonight through Friday morning with onshore flow. Another frontal system will reach the area Sunday. && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Air Stagnation Advisory until 2 PM PST this afternoon for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity-East Puget Sound Lowlands-Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest Interior-Tacoma Area-Western Skagit County-Western Whatcom County. PZ...None. && $$ www.weather.gov/seattle

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