Long Island weather update: Extreme heat arriving Wednesday, with triple-digit highs possible

An extreme heat watch with possible thunderstorms begins Wednesday and will continue through Friday for Long Island and the rest of the metropolitan region, according to the National Weather Service, which warned that daytime temperatures could hit the triple digits and last through the Independence Day weekend. 

By Wednesday, a "giant ridge of high pressure" will be moving in from the Southwest, according to Dominic Ramunni, a meteorologist with weather service's Upton office.

Forecasters called the phenomenon a "heat dome."

The highs Monday and Tuesday will top out in the mid-80s with a mix of clouds and sun. The low both nights will dip to the low 70s, forecasters said.

The heat is expected to build over several days as this "large sprawling dome of high pressure" settles across the Northeast, Ramunni said. Temperatures Wednesday will be above 90, and then "we could be even making a run at triple digits, at least across partial parts of the Island as we go into Thursday and Friday," Ramunni said.

"The combination of heat and humidity could lead to heat index values from 95 to 110 degrees," the service warned.

"Afternoon/evening showers and thunderstorms are possible each afternoon later this week," the weather service said.

Meteorologist Jim Connolly said last week the heat index may feel as high as 105 degrees on Long Island. While the heat watch is in effect until Friday evening "these conditions could extend into Saturday as well," the service said in a Monday briefing. 

On the East End, the heat will not be quite so oppressive; temperatures will likely top out in the high 80s or low 90s.

Connolly also said areas on the South Shore will feel cooler compared to the North Shore. 

With the intense hot air mass in place, Connolly said there will be a chance for showers and thunderstorms on Wednesday and Friday, but the forecast could change. The service will have a better understanding on the timing of showers and estimated rainfall as it get's closer to Wednesday.

"With this air mass in place, the door will be open for showers and thunderstorm chances," Connolly said. 

The evenings across the Island should bring some relief, as temperatures are expected to drop to the high 70s.

"This is dangerous heat," Ramunni said, noting that more people die in extreme heat than in any other type of weather disaster including flooding, hurricanes and tornadoes.

People should "take it seriously," he said, "stay hydrated, take breaks in the shade if you're going to be spending any time outdoors."

Wearing loose, light-colored clothing can help you stay cool. "Of course, never leave kids or pets unattended in vehicles," Ramunni said.

Older people, infants and young children and those with chronic health conditions, such as lung or heart problems, are especially at risk in high heat. Experts advise people to check on vulnerable neighbors and family members and help them get to cooling centers if necessary.

Europe also has been battered by extreme heat in the past week, as temperature records have been shattered from Denmark to Germany to Slovakia. Thousands of heat-related deaths have been reported.

Periods of extreme heat are happening more frequently, lasting longer and becoming more intense because of human-caused global warming, according to climate scientists.

By Saturday, Independence Day, the intense heat will lift somewhat with a high of around 93, but that will still be "one of the hottest July Fourths on record," Ramunni said.

The record is 97, set in 2010.

Last year, 21 people died in New York City as a direct result of the heat, and nearly 500 New Yorkers die each year of causes exacerbated by heat, according to the city Health Department. New York State doesn’t keep a tally of heat-caused deaths, but 155 people visited emergency rooms in Nassau County for heat-related illness in 2024, according to the county Health Department.