Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
954 FXUS66 KSEW 171635 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 935 AM PDT Thu Sep 17 2026 .SYNOPSIS... High pressure aloft and a thermal trough pushing inland will bring warm and dry conditions to the interior of western Washington today. Onshore flow will resume on Friday, bringing more seasonable temperatures and morning clouds to the area through the weekend. Light showers will be possible for portions of the area early next week as a weak system moves into the region. && .SHORT TERM /TODAY THROUGH SATURDAY/... The latest forecast remains on track this morning with no updates. Upper level ridging will continue to nudge into the area today as an upper low over northern CA/OR starts to lift northeastward to exit the region. A thermal trough situated along the coast will push inland today, allowing for offshore flow to transition back to onshore this afternoon. A few areas of patchy, shallow fog remain possible across the area early this morning with generally light winds across the area and some residual moisture from recent rains. Expect the extent of fog to be less than that of yesterday, however, as offshore flow continues to promote drying in the low levels. Any shallow fog that does develop will dissipate rather quickly this morning. The thermal trough pushing inland will continue to promote warming across the interior, with highs expected to be in the mid to upper 70s for most spots. A few spots, mainly along the Cascade valleys and across the southern interior, could even make it into the low 80s this afternoon. Onshore flow will resume by this afternoon and persist into the weekend, bringing slightly cooler, more seasonable temperatures to the area on Friday and Saturday. The return of onshore flow will also bring the return of morning stratus to the interior both days. Stratus will scatter and retreat back to the coast by the afternoon on Friday and looks to linger a bit longer on Saturday before giving way to some afternoon sun. Afternoon highs will be in the low to mid 70s across the interior and mid 60s along the coast both days. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... Seasonable temperatures and dry conditions will continue through Sunday, with onshore flow continuing to promote morning stratus across the interior and mostly sunny afternoons. Ensembles then hint at the pattern changing early next week as upper level troughing starts to develop over the northeastern Pacific. This southwest flow aloft will open the door for systems to move into western Washington. At this time, the next weak system looks to approach the area later Monday into Tuesday and will bring the next chance of light rain to the area. Shower chances and slightly cooler temperatures, with highs in the mid 60s to low 70s, then look to persist into midweek. 14 && .AVIATION... Light flow aloft will turn northerly later today as high pressure remains centered offshore with an upper low well south of the region. Light offshore surface flow will turn onshore tonight as thermally induced low pressure shifts inland from the coast. Patchy valley fog in outlying areas this morning will burn off within the next couple of hours, VFR conditions will prevail today. IFR/LIFR stratus will return to the immediate coast tonight. KSEA...VFR. Surface winds N/NW 8 to 12 knots today. Winds will back to light southerly after 06Z tonight. 27/41 && .MARINE... Thermally induced low pressure along the coast will shift inland later today. Northerly or weak offshore flow will turn onshore tonight. Light onshore flow will then persist Friday through the upcoming weekend as a series of systems pass well north of the waters. The trend will likely persist into early next week. 27 && .FIRE WEATHER... A thermal trough pushing inland today will bring one last day of warmer and drier conditions to the interior. Offshore flow will gradually transition back to onshore this afternoon, though relative humidity values still look to dip into the 20s and 30s across portions of the Cascades. The lowest humidities will be concentrated over the North Cascades this afternoon. Onshore flow will then bring cooler temperatures and increased moisture to the area Friday through the weekend, helping to minimize fire weather concerns into early next week. 14 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$