DETROIT (AP) — 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, meteorologists say. Temperatures in parts of the East won’t drop below the mid- to upper-70s or even 80 degrees at night, he said.

The heat wave will likely be “short and searing,” said Greg Carbin, forecast branch chief for the weather service’s Weather Prediction Center.

A high pressure system stretching from coast-to-coast is keeping the heat turned on. The heat and humidity are made to feel worse by the large amount of moisture in the air coming from the Gulf of Mexico, much of it left over from Hurricane Barry.

The heat index, which is what the temperature feels like, should hit 110 in Washington, D.C., on Saturday and 109 in Chicago and Detroit on Friday, said Jeff Masters, meteorology director of Weather Underground. Wednesday marked Washington’s seventh straight day with temperatures of at least 90 degrees, and that streak was expected to last for another five days.

An experimental weather service forecast projects that nearly 100 local records will be broken Thursday and Friday in Texas, Oklahoma, parts of the Midwest and a large swath of the East Coast. On Saturday, 101 records could fall in an area stretching from Texas to Iowa and east to Maine and Florida, according to projections.