Heat advisory in effect for Nassau, part of Suffolk, NYC

The National Weather Service has issued a heat advisory for Nassau and northwestern Suffolk through Friday as heat index values will reach the mid-90s before showers hit Long Island. 

Temperatures on Thursday reached the mid-90s. 

Friday is expected to be mostly sunny but with a 20% chance of showers and thunderstorms after 11 a.m., then increasing to 50% at night. Temperatures will drop slightly to highs of around 86 degrees, but high heat index values in the mid-90s will return with high humidity. 

The advisory, which is also in effect for New York City and northeast New Jersey, may be extended into Saturday, but it’s too soon to tell, meteorologist Bryan Ramsey said.

"The reason we're still seeing a high heat index is because the humidity is going to increase every day," Ramsey said. "Even though temperatures may cool a couple degrees each day, the humidity will be ticking upward ever so slowly, which will keep us feeling hot."

The heat stretch will not be as bad as the previous one in July, Ramsey said, as that streak had a heat index value in the upper 90s.

"If we get a heat index value of 95 for two days straight, that is what triggers that heat advisory," he said.

There is a 50% chance of showers and thunderstorms on Saturday and Sunday.

"Our main concerns with those storms will probably be just isolated, heavy downpours, but otherwise nothing looks too extreme for now," he said. "Just your typical summertime showers and thunderstorms."

Monday will most likely be dry, with temperatures in the mid- to high 80s.