Detroit, July 18 -- The heat wave that has been roasting much of the U.S. in recent days is just getting warmed up, with temperatures expected to soar to dangerous levels through the weekend.

Communities are preparing by offering buildings as cooling centers and asking residents to check in on relatives and neighbors. Officials also are concerned about smog, which is exacerbated by the heat and makes it more difficult for certain people to breathe, including the very young, the elderly and people with asthma or lung diseases.

More than 100 local heat records are expected to fall Saturday, according to the National Weather Service. Most won't be record-daily highs but record-high nighttime lows, and that lack of cooling can be dangerous,...