Residents of Western Washington can expect a brief warm spell this week: temperatures could reach 70°F (about 21°C) on Sunday and Monday. Meteorologists attribute this to an area of high pressure bringing warm air into the region. Variable cloudiness and light winds are expected, but temperature records for these dates, set in 1977 and 2007, will not be broken.
However, the situation will change Monday evening: cool marine air from the Pacific will begin to arrive, leading to a noticeable cooldown. On Tuesday and Wednesday, thermometers will fall to 56–58°F (13–14°C), and nighttime frost is possible midweek. Despite this, forecasters describe the overall outlook for the coming days as calm and not indicative of major weather anomalies.
Based on: Seattle weather: Temperatures could reach 70 before cooling midweek