Long Island weather: Warm afternoon Thursday; heat advisory issued for region on Friday

Following a seasonably pleasant Thursday, extreme heat, stifling humidity and the threat of scattered but severe thunderstorms could make for a volatile, oppressive weather day for much of Long Island on Friday, according to the National Weather Service.

Thursday promises good beach and outdoor weather, with temperatures in the mid-to-upper 80s and moderate humidity for much of the day. The weather service warns of a moderate rip current risk for all South Shore ocean beaches.

Starting Thursday, Long Islanders will "start to notice an increase in heat and humidity," said meteorologist Dave Radell, with the National Weather Service’s Upton office.

But Friday brings the threat of isolated and scattered severe thunderstorms, which the weather service said could pack wind gusts of up to 58 mph and torrential rains that could lead to localized flooding.

In parts of New Jersey it'll be even more unpleasant, with the weather service predicting heat index values of up to 107.

Heat index values are based on a combination of heat and humidity and indicate how that feels in terms of "apparent temperature." That is, while the weather service is calling for temperatures in the mid-to-upper 90s on Friday for much of Long Island, those temperatures, combined with predicted high humidity, will make it feel like it's well in excess of 100 degrees.

According to heat index charts provided by the weather service, a heat index between 90 and 105 is considered cause for "extreme caution," bringing with it the threat of sunstroke, muscle cramps and heat exhaustion with prolonged exposure.

A heat index above 105 is cause for danger, the weather service said.

"Seniors and those with chronic health problems or mental health conditions are at increased risk," the weather service said in a statement Thursday, adding: "Homes without air conditioning can be much hotter than outdoor temperatures."

As a result, the weather service is urging those without air conditioning to "seek out cool spaces" — or to go to a cooling center or some other place that has air conditioning.

Saturday promises to feature more moderate weather, with temperatures again back in the mid-to-upper 80s and mostly sunny skies, though there's a chance of showers and isolated thunderstorms again on Sunday.

"It will feel a little bit better, but still pretty hot and pretty humid on Saturday," Radell said. "On Sunday, we’re looking at low to mid-80s for temperatures and a little bit more cloud cover around."