Heat alerts are in effect in at least 19 states in the Midwest and Northeast, with some areas expecting to see their hottest days in 30 years.
Record highs were shattered Monday in the Midwest and Northeast: Toledo, Ohio, hit 99 degrees; Chicago reached 97 degrees; Cleveland recorded 96 degrees; and Syracuse, New York, soared to 94 degrees.
On Tuesday, temperatures are forecast to climb to a scorching 97 degrees in Pittsburgh and Syracuse; 93 degrees in Chicago and Detroit; 92 in Washington, D.C.; and 90 in Boston.
New York City is expecting a five-day heat wave with temperatures above 90 degrees. The city hasn't experienced five consecutive days above 90 degrees since June 1988.
"This is one for the ages," New York Gov. Kathy Hochul warned Tuesday. "The real feel temperature will exceed 100 degrees starting as early as today, which is abnormal for this time of year."
MORE: Heat stroke vs. heat exhaustion: Extreme heat safety tipsIn New York, the regions along the Interstate 90 corridor from Buffalo to Albany will experience the most severe conditions, said Commissioner Jackie Bray of New York’s Homeland Security and Emergency Services.
National Guard units have been stationed in Syracuse and Albany to be ready to help as needed, officials said. Cooling centers are being readied and water supplies are being secured across the state, officials said.
"We urge residents to take this seriously,” Bray said.
MORE: As sweltering heat approaches, experts share hot car safety tipsIn Boston, a heat emergency is in effect from Tuesday to Thursday.
"Everyone should make sure to stay hydrated, limit outdoor activity when possible, wear plenty of sunscreen, and check on your neighbors and loved ones," Boston Mayor Michelle Wu said in a statement. "The City is here to support residents through a variety of resources, and I urge anyone with questions to call 311 for assistance in staying safe in the heat."
The dangerous heat is forecast to continue from the Ohio Valley and eastern Great Lakes to the Interstate 95 corridor through the end of the week and into the weekend.
By Thursday, temperatures could hit 99 degrees in Pittsburgh and Manchester, New Hampshire.
By Saturday, the temperature could reach 100 degrees in Washington, D.C.
There 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.