The West has been sizzling in recent days, and some of that heat will be felt in the East Monday.
Forecasters are calling for afternoon highs approaching 100 in the nation's capital.
The Metropolitan Washington Council of Governments has issued a Code Orange alert, meaning air quality will be unhealthy for children, the elderly and people with heart or lung problems.
Free bus rides will be offered in DC's Northern Virginia suburbs. Officials hope that keeps some cars off the road.
An excessive heat warning is posted for the Philadelphia area, because of the combination of heat and humidity.
It was 96 in downtown Chicago yesterday, not unheard of in July, but well above the average high of 83.