An Excessive Heat Warning is in effect for much of the interior of Western Washington until 9 p.m. Friday.
A large area of high pressure building just off the coast will cut off onshore winds into the weekend. This will push highs into the 80s and 90s.
>>Related: Seattle cooling shelter locations
Thursday will be the hottest day with highs in the mid to upper 90s to near 100 degrees in spots.
You may have noticed the hazy conditions we have right now over much of the area. That's smoke! It's drifting down from southern British Columbia due to wildfires burning there.
That may have an effect on our high temperatures. Smoke particles in the air will filter some of the sun's energy, and we could see high temperatures several degrees cooler than forecasted a couple days ago.
High temperatures could impact Sounder train service during the heat wave. Sound Transit advises travelers to arrive to their station on time and pay attention to alerts, which would be sent if delays go past 10 minutes.
Temperatures will only cool gradually over the weekend and should be back into the 80s Sunday and Monday as onshore winds return, though only weakly.
"Record high temperatures will occur Wednesday and Thursday with highs on Thursday within 5 degrees of all time record highs," the National Weather Service said.
The National Weather Service says the hottest day in Seattle since the 1890s was 103 degrees on July 29, 2009.
>>Related: Full forecast
The Associated Press contributed to this report.