After a warm and sunny weekend, Monday will be the warmest day of the week. Fog and low clouds are expected in the morning, but skies will clear in the afternoon, with temperatures reaching about 21°C. On Tuesday, the day of the autumnal equinox, the season will officially change at 5:05 p.m. A weak convergence zone will bring drizzle and clouds, though the sun may appear after noon; temperatures will reach only 18–19°C.
Cloudy conditions will continue Wednesday and Thursday, while a more organized weather system will approach the region overnight Thursday into Friday. Between 6 and 13 millimeters of rain could fall by Friday, and temperatures will drop to around 18°C. The rain will move out by the weekend, but clouds will remain, with highs reaching only 16–18°C. As fall deepens, average high temperatures will continue to decline: by Oct. 1, they will be around 19°C, so Seattle residents should get used to a cool, damp and gray season.
Based on: Seattle fall equinox weather: ‘Cool start’ as clouds roll in