Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
439 FXUS66 KSEW 151529 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 829 AM PDT Tue Sep 15 2026 .SYNOPSIS... Upper level trough moving through the area today. Upper level ridge building into British Columbia tonight and Wednesday. Thermally induced surface trough moving up the coastline Wednesday night will shift inland Thursday. Upper level ridge over the area Thursday remaining in place through Sunday with low level onshore flow. Weak system approaching Monday. && .SHORT TERM /TODAY THROUGH THURSDAY/... The latest forecast remains on track with no updates this morning. Upper level trough over Southern British Columbia this morning moving through Western Washington later this morning into the early afternoon hours. Skies remaining cloudy/mostly cloudy as the trough moves through with a slight chance of showers. Dry northerly flow aloft behind the trough with skies clearing from the north this afternoon over the northern half of the area. Highs below normal, in the 60s. Upper level ridge building into British Columbia tonight and Wednesday as the upper level trough moves further south. North northeasterly flow aloft will dry out the air mass clearing skies out in the evening. Light flow in the lower levels overnight combined with the recent rains will allow fog to form mostly in the fog prone areas like the Southwest Interior. There will be a little chill in the air Wednesday morning with lows in the mid 40s to lower 50s. Dry northeasterly flow aloft continuing Wednesday with a thermally induced surface trough making its way up the Oregon coast. Low level flow turning offshore. What little fog that does form will dissipate in the morning with the drying air mass in the lower levels. Under sunny skies with a little offshore flow temperatures warming back up to near normal. Highs in the mid 60s to mid 70s. In this scenario it is possible the warmest temperatures will be along the coast. Thermally induced surface trough moving up the coast Wednesday night shifting inland Thursday. The transition to onshore flow along the coast will cool highs back to near 70. Over the interior Thursday will be the warmest day in the forecast period with highs in the 70s. Warmest locations like the Cascade foothills and Southwest Interior could get to 80. Lows Thursday morning in the upper 40s to mid 50s. September is off to a cool start in Western Washington. Seattle average temperature for the first two weeks of the month is 61.8 degrees, 2.7 degrees below normal. This is the coolest start to September in Seattle since 2010 ( 61.0 degrees ). Felton && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Models in good agreement with a flat upper level ridge remaining over Western Washington through the weekend. Low level onshore flow will keep temperatures near normal. Highs in the mid and upper 60s along the coast and upper 60s to mid 70s inland. Lows in the upper 40s to mid 50s. Upper level trough moving out of the Gulf of Alaska first part of next week. Operational models are much slower with this feature versus the NBM which has a system near the coast Monday. For now will leave the low pops in the forecast Monday. Will not be surprised if future runs push this back into Tuesday. There is general agreement that a pattern change is in the offing next week. It is just a matter of when the weekend ridge breaks down. Felton && .AVIATION... Westerly flow aloft will turn northerly tonight as an upper trough drops southward across the Pacific Northwest. A mix of MVFR and VFR ceilings will continue through roughly 20Z. Ceilings will improve this afternoon with clearing tonight and VFR expected areawide. There`s roughly a 30-40% chance of IFR/LIFR redevelopment overnight/Wednesday morning in pockets of the interior (as winds become light or calm overnight out of the north). Some of the lower ceilings may drop to the ground for pockets of fog/mist. KSEA...Mix of MVFR and VFR ceilings likely to continue through 20Z. Ceilings will lift and scatter out later this afternoon and evening. Surface winds northerly 4 to 7 knots will rise to 9 to 13 knots late this afternoon. Winds become light or calm out of the north tonight/Wednesday morning. There is a 20-30% chance of IFR/LIFR conditions developing Wednesday morning, but at this point the ceiling probabilities are more elevated over the chance of lower visibilities. 27/HPR && .MARINE... The flow will turn northerly today then offshore flow at times Wednesday and Thursday as thermally induced low pressure briefly expands northward along the coast. Onshore flow returns Friday and into the weekend as series of systems pass well north of the waters. 27 && .FIRE WEATHER... Minimal fire weather concerns through next week. A weak weather system will move through the area today bringing a chance for light precipitation. Drier and warmer pattern Wednesday into Thursday. Surface winds will briefly transition offshore late Wednesday morning and remain offshore into Thursday morning. This will decrease minimum RH values toward the mid 20s to low 30s, especially in the North Cascades and Southwest Interior. Minimum relative humidity values rebound Friday as onshore flow returns. The onshore flow pattern will continue through the weekend. Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$