The big picture: The mayors of Boston, New York City, Philadelphia and Washington, D.C., have declared heat emergencies.
New York City and Baltimore issued Code Red Extreme Heat Alerts.
The extreme heat has been blamed for at least 3 deaths in the U.S., NBC News reports.
In Maryland, health officials attributed this week's extreme heat to the deaths of a Prince George's County man and a Worcester County woman, per NBC News.
Earlier this month, a Baltimore City man and an Anne Arundel County woman died.
In Washington, D.C., Medical crews were on standby at the National Mall, ready to treat heat-related illnesses at this weekend's outdoor Apollo 11 festival, according to CBS News, which notes D.C. was among several East Coast cities set for high heat and humidity that would make it feel like it's over 100 degrees.
Go deeper: The "heat dome" has arrived This is a developing news story.