A heat advisory has been issued for Nassau County and northwest Suffolk, in effect until 8 a.m. Saturday, when an excessive heat watch will go into effect through Sunday evening, the National Weather Service said.

Heat index values, also known as “feels like” temperatures, of 100 to 103 can be expected through Thursday evening. They are expected to increase to between 100 and 105 for Friday and remain around 105 for Saturday and Sunday.

Strong thunderstorms were affecting parts of Suffolk County, and were expected to move into Nassau County, the weather service said in tweets about 3:35 p.m. Anyone outdoors is advised to find safe shelter in buildings or vehicles, the weather service said.

In some areas, thunderstorms that do develop “will be capable of producing torrential rainfall and localized flash flooding,” the weather service said earlier in a hazardous outlook.

The temperature at Long Island MacArthur Airport hit 89 degrees about 2 p.m. Thursday but the “feels like” temperature was 99 as “a very warm and humid air mass” settled over the area, the National Weather Service said.

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As of just before 2 p.m. the “feels like” temperature had also reached 99 in Shirley and was even higher farther west — 103 in Farmingdale, 105 at Kennedy Airport and 102 at LaGuardia, prompting the heat advisory.

A heat advisory is issued when heat combined with humidity is expected to make it feel like 100 to 104 degrees for at least two hours in a row, the weather service said. An excessive heat advisory is issued when the conditions could pose “a dangerous situation in which heat illnesses are possible.”

Hot and humid conditions increase risks for health issues, especially for the elderly, the service said, and for those with chronic problems such as heart and lung disease.

The forecast is for “warm and steamy weather into the weekend, with highs 80 to around 90,” said News 12 meteorologist Rich Hoffman. “More hit-and-miss thunderstorm threat.”

A severe thunderstorm that swept across the East End Wednesday “produced a small weak short-lived tornado in Mattituck,” the weather service said. “Wind damage was confined mainly to trees, where several trees were snapped in half,” the service said.

Thursday also brings a high risk for rip currents at ocean beaches that lasts through the evening.

The chance of showers and thunderstorms continues at 20 percent Thursday night, again, with heavy rain possible. Low temperatures are expected in the 70s.