Emergency plans activated for 8.5m as brutal heat and severe storms threaten the East Coast TODAY

Emergency plans have been activated for millions as brutal heat and severe storms threaten to unleash dangerous flash flooding.

New York City Mayor Zohran Mamdani ordered the citywide response Thursday as all five boroughs entered a heat advisory lasting through 8pm Friday.

Similar heat alerts stretch across parts of Connecticut, New Jersey, Pennsylvania, Delaware, Maryland, Massachusetts and New Hampshire, as well as Washington DC.

The National Weather Service (NWS) warned that the heat index could reach around 100°F in New York City, 103°F across parts of Pennsylvania, New Jersey and Delaware and more than 105°F in southern Connecticut.

Overnight temperatures are expected to remain in the low 70s, offering little relief before the oppressive heat returns Friday.

The extreme conditions will coincide with thunderstorms capable of unleashing torrential rain, powerful winds and frequent lightning across the Northeast and Mid-Atlantic.

Forecasters warned that the storms could trigger localized flash flooding, topple trees and bring down power lines through Friday.

New Yorkers living in basement apartments were issued the most urgent warning and told to identify somewhere safer to shelter before floodwaters begin rising.

New York City officials urged residents to remain indoors during severe weather and avoid flooded roads.

The most urgent warning was issued to people living in basement apartments, who were told to identify somewhere safer to shelter before floodwaters begin rising.

Anyone trapped or facing immediate danger should call 911, while downed trees, clogged catch basins and other nonemergency hazards can be reported to 311.

‘City government is mobilizing every resource we have – from clearing catch basins and preparing emergency crews to sending COOL vans to check on older adults,’ Mamdani said.

‘Protecting New Yorkers means showing up before the storm, not just after it.’

Hundreds of cooling centers will open across the five boroughs Thursday and Friday, including accessible and pet-friendly facilities. Locations can be found online or by calling 311.

The dangerous conditions stretch well beyond the city, with heat advisories covering large parts of the Northeast and Mid-Atlantic.

In New York, eastern Ulster and western Dutchess counties could experience heat index values of up to 98°F through Friday evening.

Nearly all of Connecticut is also under an advisory, with only northern Litchfield County excluded.

Heat index values could climb above 105°F in the hardest-hit areas of southern Connecticut, while Hartford could feel as hot as 102°F and New Haven could reach 103°F.

Actual temperatures are forecast to rise into the upper 80s and low 90s, but oppressive humidity will make conditions feel significantly hotter.

Scattered thunderstorms are expected across Connecticut on Thursday afternoon and evening, particularly in western and northwestern areas.

The storms could produce heavy rain, frequent lightning and powerful wind gusts, creating a risk of localized flooding in communities recently hit by downpours.

Friday will bring another round of punishing heat and a greater chance of thunderstorms, with the likelihood of storms reaching between 50 and 60 percent in parts of the state.

Heat advisories have also been issued across sections of New Jersey, Pennsylvania, Delaware, Massachusetts and New Hampshire.

Much of New Jersey is also under a heat advisory, with heat index values potentially reaching 103°F. 

The warning covers communities including Trenton, Camden, Cherry Hill, New Brunswick, Glassboro, Freehold, Somerville and Mount Holly and remains in effect through 8pm Saturday.

Northeastern counties including Passaic, Bergen, Essex and Union also face thunderstorms Friday that could produce powerful winds, torrential rain and localized flash flooding.

Farther south, heat index values could reach 103°F across a heavily populated region encompassing Philadelphia and parts of Pennsylvania, New Jersey and Delaware.

The advisory there will remain in effect until 8pm Saturday, potentially exposing residents to several consecutive days of dangerous heat.

Communities covered by the warning include Philadelphia, Trenton, Camden, Cherry Hill, Wilmington, New Brunswick, Glassboro, Doylestown and West Chester.

Advisories were also in effect across parts of the Baltimore and Washington DC, region as the hot, humid air spread along the East Coast.

The NWS said a frontal boundary moving through the eastern US would fuel scattered showers and thunderstorms, with some capable of producing torrential downpours.

Extreme heat and humidity can prevent the body from cooling itself efficiently, increasing the risk of heat exhaustion and potentially fatal heat stroke.

Symptoms of heat exhaustion include dizziness, weakness, nausea, headache and heavy sweating. Heat stroke can cause confusion, loss of consciousness and dangerously high body temperatures and requires immediate medical attention.

Older adults, young children, outdoor workers and people with chronic health problems face the greatest danger.

Residents across the affected states have been urged to drink plenty of water, remain in air-conditioned spaces, postpone strenuous outdoor activity and never leave children or pets inside parked vehicles.