Heat alerts are in effect for 16 states from Maine to Kentucky as the dangerous heat wave continues.
Boston and Manchester, New Hampshire, both reached a record high of 98 degrees on Wednesday.
In Caribou, Maine, the heat index -- what the temperature feels like -- hit an all-time high of 103 degrees.
The heat won't relent on Thursday.
Temperatures are forecast to hit a scorching 99 degrees in Concord, New Hampshire; 97 degrees in Hartford, Connecticut; 95 in Boston and Pittsburgh; 93 in Philadelphia and Indianapolis; and 91 in New York City.
In the Northeast, the heat might force Amtrak trains to slow down, causing up to one-hour delays, according to Amtrak.
In Washington, D.C., the heat is forcing Metro trains to slow to below 35 mph.
MORE: The summer solstice is here, but that doesn't mean the season's hottest temperatures are, tooThis weekend, the heat dome is heading south and west, with the heat index forecast to climb as high as 110 degrees in some places.
By Sunday, the temperature is forecast to skyrocket to 101 degrees in Oklahoma City and Washington, D.C., and 99 degrees in Nashville, Tennessee.
MORE: As sweltering heat approaches, experts share hot car safety tipsThere are hundreds of deaths each year in the U.S. due to excessive heat, according to CDC WONDER, an online database, and scientists caution that the actual number of heat-related deaths is likely higher.
Click here for tips on how to stay safe in the heat.