The big picture: The mayors of Boston, New York City, Philadelphia and Washington, D.C., declared heat emergencies. New York City and Baltimore issued Code Red Extreme Heat Alerts.
In Arkansas, former New York Giants offensive lineman Mitch Petrus died on Thursday night after being struck by heat stroke, according to Kark-TV.
In Maryland, health officials attributed the heat to the deaths of a Prince George's County man and a Worcester County woman, per NBC News.
Baltimore logged a heat index of 122 degrees on Saturday evening. It's one of several locations experiencing a heat index in the triple digits, CBS News notes.
In Michigan, more than 200,000 properties were without electricity after storms, Michigan Live reports.
In New York, more than 9,000 customers lost power earlier Saturday in Far Rockaway, Queens and on Long Island, PSEG Long Island said. The outage disrupted some train services, said Mayor Bill de Blasio, who noted power was restored to most customers.
New York City authorities canceled a Times Square commemoration of the 1969 Moon landing and an outdoor festival at which soccer star Megan Rapinoe, musician John Legend and "The Daily Show" host Trevor Noah were due to appear, per AP.
In Chicago, several outdoor events were canceled, but the Pitchfork Music Festival went ahead with 3 air-conditioned "cooling buses" for festival-goers, the Chicago Sun-Times reports.