Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 251551 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 AM PDT Tue Sep 25 2018 .SYNOPSIS...A stationary offshore upper level ridge will produce sunny, warm days and cool, clear nights. At low levels, light northerly flow will turn offshore on Friday making it the warmest day of the week. The ridge will weaken and move northwest over the weekend with a weather system moving into the area Sunday. && .SHORT TERM... Mostly clear skies this morning with fog over many of the river valleys and parts of Lake Washington. The shallow fog will burn off quickly this morning to sunny skies. The offshore ridge will continue to build along 135W today boosting heights downstream over the area with some warming aloft. At low levels, northerly surface gradients will drive light northerly winds. Sunny skies and the slight warming aloft will help bump high temperatures a bit over yesterday despite the northerly low levels flow - reaching into the mid 60s to mid 70s. The pattern will remain the same through Thursday - low level northerly flow, with clear, cool nights and sunny warm days. Will likely see at least patchy shallow fog develop again Wednesday and Thursday mornings in the preferred locations, burning off to sunny skies and warm temperatures. .LONG TERM...from previous discussion...Multiple runs in a row including the 00z run showing the low level flow turning offshore Thursday night into Friday with the thermally induced surface trough along the Oregon coast expanding northward over the Washington coast. In the upper levels the ridge offshore begins to drift north and west with 500 mb heights falling over the area. Even with the falling 500 mb heights, the low level offshore flow will make Friday the warmest day of the week with the warmer locations with exposure to the offshore flow, west slopes of the Cascades and Olympics, getting close to 80 degrees. For the remainder of the area upper 60s to upper 70s. It was much warmer on September 28th last year with Seattle setting a record high with 86 degrees. Confidence low in the forecast for the weekend with the extended models remaining inconsistent. Model solutions start to diverge on Saturday with the ECMWF kicking an upper level low off the California coast further north versus the GFS solution. The ECMWF also has a deeper trough developing along the British Columbia coast. With the further north solution the ECMWF has some light precipitation along the coast by 00z Sunday while the GFS is dry. The weaker GFS trough moves further south into the northern portion of the area on Sunday while the ECMWF kicks what is left of the upper level low over the area. Monday the GFS has a flat ridge trying to rebuild over Western Washington while the ECMWF keeps a weak trough in the area. Will stay with the general idea of chance pops for Saturday night and Sunday. Models are pretty dry on Monday but with the solutions bouncing around will keep the low chance pops in the forecast. Either solution temperatures will cool down over the weekend into Monday with highs back into the 60s Sunday and Monday. Felton && .AVIATION...Northwest flow aloft today with northerly winds at the surface. High pressure will remain in control this week with mostly clear skies. Morning fog in the river valleys, at the coast, and in the southwest interior will scatter out over the next few hours but may return again toward daybreak Wednesday. KSEA...VFR conditions through the period with north to northeast winds 6 to 12 kts. CEO && .MARINE...High pressure offshore and thermal low pressure to the south will continue through the end of the week. Northerly flow will continue for the next few days. Onshore flow may return by the weekend as our next weather system approaches the area. CEO && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.