A third round of Canadian wildfire smoke is forecast to dive into the upper Midwest beginning Tuesday, reach the Northeast around Wednesday, and spread into parts of the South by Wednesday and Thursday.
Weekend relief will be brief: thunderstorms helped clear some smoke Saturday, but another plume is set to move into the western Great Lakes on Sunday and then the Ohio Valley, Northeast and mid-Atlantic.
Code purple "very unhealthy" air hit cities including Cleveland, Columbus, Detroit, Pittsburgh and Washington on Friday, after the latest smoke event began Wednesday and spread from the northern Great Lakes into Boston, New York City and Toronto.
More than 800 active Canadian wildfires and flare-ups in northeast Minnesota are feeding the smoke, while northwest winds behind cold fronts are steering it south after a heat dome shifted into the Carolinas.
The smoke carries fine particulate matter that can penetrate deep into the lungs and bloodstream, prompting air-quality alerts across parts of the Midwest and Upstate New York.