New York (CNN) -- A heat wave continued to blanket the U.S. Northeast with scorching temperatures on the first day of summer, as residents across the region sought to keep cool amid national heat advisories.
Severe heat and humidity stretched from Vermont to North Carolina on Thursday with temperatures reaching the mid- to upper 90s, the National Weather Service said.
The Northeast will begin cooling slightly Friday, the service said, but the heat is expected to continue with forecasts in the low 90s and high 80s across the mid-Atlantic states.
Cities across the Northeast designated cooling centers, turned on outdoor sprinklers and splash stations and issued advisories in an effort to help keep residents safe, emergency management officials in several cities said.
"[More than 400] cooling centers will continue to remain open Friday," said New York Emergency management spokeswoman Judith Kane. The city advised residents to turn on their air conditioners or to go to air-conditioned locations such as museums, malls and movie theaters to stay out of the sun.
In Boston, 15 indoor public pools, outdoor wading pools and splash stations were available, said John Guilfoil, spokesman for Boston Mayor Thomas Menino, in addition to air-conditioned community centers.
"We actually weren't going to come out, because it was so hot today, but they turned on the sprinklers and we're going to have some fun," first-grade teacher James Bernardini said in Manhattan's Central Park.
More than 700 sprinklers were available in parks across New York City, said Parks Department spokeswoman Vickie Karp.
"It's 90 degrees outside. Imagine how it is in a classroom without a fan, without an AC," Bernardini said.
According to the National Weather Service, Baltimore and Washington reached high temperatures of 98 degrees on Thursday. Farther north, Boston, New York and Philadelphia reached, respectively, 95, 93 and 97 degrees, the service said.
The service advises residents to drink plenty of water, to stay out of the sun, to take frequent breaks in shaded or air-conditioned environments and to check on relatives and neighbors during the heat wave.
Young children and the elderly are especially at risk for heat-related injuries, the service said.
Last year, it reported 206 heat-related deaths. In the past decade, heat has been the second-leading cause for weather-related fatalities, the service said.
Residents are advised to call 311 or to go to their city's website online to find cooling centers and to get further information on how to keep cool as the heat continues.