Forecasters also warned that "isolated to scattered thunderstorms are possible each day through Sunday" with the greatest chances from Friday through Sunday.
"Any storm has the potential to produce damaging winds, large hail, and localized flash flooding," officials said.
New York City Mayor Zohran Mamdani urged in a statement on Thursday morning that residents remain vigilant as dangerous conditions are expected.
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"Please stay prepared and informed as these dangerously high temperatures continue," he said, adding that Friday's heat index is expected to reach 110°F (43.3°C).
"Plan for extreme temperatures. Drink more water than usual.
Reach out to vulnerable neighbors who may need help finding resources."
Mamdani also announced that the city is deploying cooling vans staffed with medical personnel, who are set to "provide wellness checks, medical care, water and other supplies such as electrolytes and sunscreen, as well as transport to cooling centers or healthcare facilities."
Across the mid-Atlantic, forecasters warn that "heat indices up to 110°F (43.3°C) are possible" on Thursday, and said that on Friday and Saturday, afternoon "showers and thunderstorms" are also possible "as heat persists" and "some storms could become severe."
Muriel Bowser, the mayor of Washington DC, urged residents to factor the extreme temperatures into their Fourth of July plans.
"Consider the heat as you make plans for the 4th of July and America250," she said, before encouraging residents to monitor weather alerts and safety guidance.
Weather officials for parts of New Jersey and the Philadelphia area said that Thursday and Friday are expected to be the hottest days of the week there, but warned that "dangerous heat will continue through Saturday."
Emergency responders in Boston are reminding residents that "extreme heat can affect anyone – not just older adults or people with underlying medical conditions."
"During a heat wave, Boston EMS typically sees a 10–15% increase in 9-1-1 calls, with patients of all ages experiencing heat-related illness," they said.
"So, stay somewhere cool, drink plenty of water and we're here if you need us."
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The heatwave in the US comes just days after parts of Europe endured unprecedented, record-breaking high temperatures, which brought dangerous conditions to parts of the continent.
