The advisory, warning of temperatures in the upper 80s to mid-90s, was right on time for Seafair weekend.
At Sea-Tac International Airport, the official high was 93 degrees on Saturday. It was even hotter in Shelton and Quillayute, where the mercury hit 95 degrees. But the prize for hottest spot in Western Washington on Saturday goes to Vancouver, where the high temperature was a staggering 103 degrees.
Other high temperatures on Saturday were 92 at Bellevue, 91 at Joint Base Lewis McChord, 93 at Olympia, 90 in Hoquiam, 84 in Everett, 86 in Sequim and 80 at Anacortes.
Sunday could be even hotter, with highs topping out in the lower to mid-90s at many locations, according to the National Weather Service. Overnight temperatures also are expected to be a bit warmer than normal, dropping mainly to the upper 50s to mid-60s.
The heat advisory is in effect for the Seattle-Bremerton-Tacoma area, the east Puget Sound lowlands, the Hood Canal area and the southwest interior, including Chehalis, Centralia and Olympia.
KOMO-TV's Steve Pool reports that this is Western Washington's hottest weekend since 2010.
Area residents - who are not used to temperature extremes - are warned to take precautions against the heat, such as rescheduling strenuous activities to the early morning or evening, wearing lightweight clothing, applying sunscreen and drinking plenty of water.
The hot spell is also expected to lure many people to area lakes and rivers over the weekend, and all are urged to play it safe and wear a life jacket.
The heat advisory is set to expire at 8 p.m. Sunday.