The smoke is spreading so widely this week because the same heat dome that has led to brutally high temperatures in the Midwest and the Northeast has trapped the polluted air close to the ground.
There will be little improvement on Thursday. The stubborn weather pattern will continue to affect the same areas as on Wednesday, with concentrations similar to — and in some places worse than — the smoke from the Upper Midwest that crossed southern Ontario and into the Northeast yesterday.
By Friday, as another weather system pushes the hottest air away, the heat and the smoke are expected to ease in the Northeast, forecasters said.
But in the Upper Midwest, which is closer to the fires, the smoke and the heat are likely to linger through the weekend.