Saturday on Long Island will start out cloudy, before gradually turning mostly sunny, but the big story will be first widespread hot spell of the season.

The National Weather Service said Friday evening that on Saturday there will be areas of fog before 11 a.m. and cloudy skies through midmorning before the sun takes over.

The high temperature should be about 82 degrees, which would be 12 degrees higher than the normal high for the date in Islip.

Heat indexes on Saturday are expected to be well into the 90s away from the coast, which will increase the risk of heat-related illness for vulnerable populations, the weather service said.

Heat indexes measure how hot it actually feels when relative humidity is factored in.

Friday afternoon the state Department of Environmental Conservation issued an air quality health advisory for Saturday for Long Island, New York City and Westchester and Rockland counties.

Suffolk County Health Commissioner Dr. Gregson Pigott in a statement said, "All residents, especially young children, seniors, those who exercise or are involved in strenuous outdoor work, and those with preexisting respiratory or heart problems, [should] limit strenuous outdoor activity."

Ozone levels often are high from noon through early evening on hot, sunny days, Pigott added.

The weather service said Saturday night will be mostly clear with a low of around 62 degrees.

Sunday also should also be sunny, with highs near 78 degrees. There is a chance of showers overnight into Monday.

Other than the heat, another danger to watch out for is that, despite the warm weather, the water will still be cold.

"With the heat expected this weekend, it's important to keep in mind that ocean, nearshore, and inland water temperatures remain cold, in the 50s to around 60 degrees. Physical incapacitation and hypothermia are likely in as little as 10 minutes if you fall in the water, greatly increasing the risk of drowning," the weather service said.

After the weekend's heat, temperatures are expected to cool off Monday. A high temperature of about 73 is predicted, under mostly cloudy skies.

Tuesday should be partly sunny with highs in the upper 60s, and Wednesday mostly cloudy with highs again in the upper 60s.

Thursday there is a 40% chance of showers with a high temperature near 70 degrees.

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