Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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791 FXUS66 KSEW 181101 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 401 AM PDT Mon Jun 18 2018 .SYNOPSIS...An upper level ridge offshore will build into British Columbia today then shift inland over the Pacific Northwest Tuesday. The ridge will shift east Tuesday night and Wednesday. High temperatures over interior will be within a few degrees of records through Wednesday. A trough will bring cooler marine air into the area Thursday and Friday. High pressure will try and rebuild over the region next weekend. && .SHORT TERM...Satellite imagery shows just some cirrus embedded in the northeasterly flow aloft over the area moving over the northwest portion early this morning. Clear skies for the remainder of the area. It`s a mild early morning at 3 am/10z with temperatures in the 50s to mid 60s. Upper level ridge offshore building into British Columbia today while the upper level low to the southeast remains over Idaho. Little change in the pattern over western Washington for most of the day with northeasterly flow aloft and flat gradients at the surface. Surface gradients turn onshore slightly in the afternoon along the coast and through the Strait which will put a cap on the high temperatures in those locations. For the interior Sandpoint profiler shows the temperatures aloft are similar to slightly warmer that Sunday morning at least in the Seattle area. Even without any offshore flow at the surface will little change in the pattern will go with highs similar to Sunday with 80s and lower 90s for the interior. Along the coast and Strait with the onshore flow kicking in expect highs in the upper 60s to lower 80s, 5 to 10 degrees cooler than Sunday. Upper level ridge moving over the area tonight into Tuesday. All of the models have some degree of onshore gradients through Tuesday. Having the upper level ridge axis overhead does not bode well for much of a marine push Monday night and Tuesday for the interior. Will keep the cloud cover Tuesday morning west of the Puget Sound and only less than 5 degrees cooler on the high temperatures for the interior on Tuesday, 70s to mid 80s. Highs along the coast and Strait in the 60s to lower 70s. Lows tonight will be mild again, in the 50s to lower 60s. Upper level ridge shifting east Tuesday night into Wednesday with the flow aloft over western Washington becoming south southwesterly as an upper level trough offshore digs south. Onshore gradient go flat again allowing high temperatures in the interior on Wednesday to warm up a couple of degrees. The air mass over the mountains will become somewhat unstable in the afternoon and with the southerly flow aloft introducing some mid level moisture there is a chance of afternoon and evening showers or possibly thunderstorms in the Cascades and Olympics. Highs for the interior on Wednesday in the mid 70s to near 90. Highs for the coast and Strait remaining in the 60s to lower 70s. .LONG TERM...Extended models in good agreement with an upper level trough moving through the area Thursday morning. Increasing onshore gradients behind the front with the resulting marine push cooling high temperatures 10 to 20 degrees over the interior. Showers chances with the trough will be confined to the higher terrain. Models solutions become inconsistent on Friday with the GFS moving the trough through the area faster than the ECMWF. Onshore flow continuing at the lower levels keeping highs near normal, in the 60s to mid 70s. Some differences int he models over the weekend with the GFS a little deeper keeping an upper level trough offshore versus the ECMWF which is weaker but more progressive with the trough. Will stay with a dry forecast for now which is more in line with the GFS and keep high temperatures in the 60s to mid 70s. Felton && .AVIATION...Northeast flow aloft continues over Western Washington this morning as an upper ridge remains centered over the offshore waters and an upper level low makes slow eastward progress over southern Idaho. The air mass remains somewhat moist in the mid levels and will become unstable over the Cascades with daytime heating. This may lead to a few mountain showers and thunderstorms. Light low level onshore flow will increase this afternoon with some stratus reaching the coast this evening and pushing locally inland tonight. VFR conditions expected across the region with the exception of some IFR ceilings and visibilities in fog/stratus coastal areas tonight and locally into the interior Tuesday AM. KSEA...VFR through tonight. There is a possibility of some stratus briefly reaching Sea-Tac on Tuesday morning, but confidence is low in that occurring. Light S-SE wind this morning becoming W-NW 4 to 8 knots this afternoon. Wind becoming light S-SW tonight. 27 && .MARINE...Generally light surface flow over the waters today. Onshore gradients increase tonight with small craft advisory westerly winds likely in the central and eastern portions of the Strait. Higher pressure offshore and lower pressure inland will maintain weak onshore flow through Wednesday. Stronger onshore flow will develop Wednesday night into Thursday. Gale force westerlies are possible in the central and eastern Strait, especially Wednesday evening. Onshore flow will persist Friday into the weekend. DTM && .CLIMATE...Even though summer is right around the corner it is still hard to get a stretch of really warm weather in western Washington in June. Since records started in 1945 at Sea-Tac airport there have only been 3 streaks of 4 or more days in a row with highs 85 degrees plus in Seattle in June. Two 5 days streaks, June 17-21, 2004 and June 21-25, 1992 and one 4 day streak June 2-5, 1978. Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ www.weather.gov/seattle

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