Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 180506 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 920 PM PDT Sun Jun 17 2018 .SYNOPSIS...
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An upper level ridge offshore will build into British Columbia Monday and over the Pacific NW Tuesday and Wednesday. High temperatures over interior Western WA will be well above average through Wednesday. A trough will bring cooler marine air into the area Thursday and Friday. High pressure builds over the the region next weekend.
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&& .SHORT TERM...
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Convection fired up over the Cascades from King county southward into Oregon this afternoon. Radar indicates most the thunderstorm activity has dissipated with loss of daytime heating and instability. The large upper low over the Great Basin brought some wrap around moisture into the south and east parts of WA today. Models show the low gradually shifting away from the region the next couple days but the air mass will stay warm and weakly unstable. Thunderstorms may still pop up during the afternoon and evening hours along the Cascades and possibly the Olympics through Wednesday. The flow aloft will weaken which will probably prevent convection from pushing into the lowlands through mid week. High temperatures today were rather impressive, topping out in the mid to upper 80s around Puget Sound and even parts of the north coast, Strait and north interior. Temperatures will remain well above average through Wednesday with upper heights gradually building over WA. A partial marine push Monday night into Tuesday may hinder the warming trend, especially along the coast. Meso models show shallow marine air and low stratus along the coast seeping into the western part of the SW interior, south/west parts of Puget Sound, and maybe the Strait Tuesday morning. Any clouds will burn off quickly by late Tuesday morning and will probably only hinder high temperatures by a few degrees. The warming trend resumes Wednesday with some warmer interior spots reaching near 90. This will likely be the warmest day of the week as a strong push appears likely for Thursday. Mercer .LONG TERM...Previous discussion...An strong onshore push will develop Wednesday night and Thursday, bringing cooler marine air inland. This will knock temperatures down into the mid 60s to 70s. There is a chance of showers Thursday afternoon, mainly across the northern half of the CWA with a passing trough. We may also see showers in the north sound with a Puget Sound Convergence Zone. An upper level ridge will cross the area Friday and Saturday for more dry weather. Temperatures will be close to normal with onshore flow. A dry cold front may cross the region next Sunday for increasing onshore flow. 33
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&& .AVIATION...
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High pressure over the NE Pacific will build into British Columbia through Monday. Light NE flow aloft. The air mass will be weakly unstable and somewhat moist in the mid/upper levels over the Cascades. Cumulus build ups and possibly a few thunderstorms are possible Monday afternoon/evening near the Cascade crest. Drier and more stable conditions are expected over the lowlands. Patchy low clouds or fog possible later tonight/early Monday near the central/south coast. KSEA...VFR with high clouds at times through Monday. NE winds 5-8 kt tonight becoming N-NW 5-10 kt by Monday afternoon. DTM
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&& .MARINE...
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Northerly flow will prevail through Monday. Winds will generally remain below 15 kt throughout the waters. Onshore gradients increase Monday night, with UIL-BLI peaking near +3 to +3.5 mb. This could induce a brief westerly gale in the central Strait late Monday evening. Westerly small craft winds are likely in the central and east Strait Monday night into Tuesday. Anticipate generally weak onshore flow Tuesday and Wednesday. Westerly or onshore flow will strengthen Thursday for a higher threat of gale force winds over parts of the Strait of Juan de Fuca. DTM
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale watch central Strait Monday evening.
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&& $$ www.weather.gov/seattle

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