Long Island forecast: Heat, humidity before midweek cool-down 

A heat advisory issued for large swaths of Long Island on Monday and Tuesday means high humidity and hot temperatures to start the workweek before a reprieve of cooler air and possible showers.

The National Weather Service issued the heat advisory for northern Nassau and northwest Suffolk as the muggy conditions will make it feel like the mid- to upper 90s.

The heat is forecast to peak both Monday and Tuesday between noon and 6 p.m. The weather service warned of an increased risk of heat-related illness for vulnerable populations, including young children, the elderly and people who work outdoors.

So, what's causing Long Island in mid-July to feel like Miami in late August?

“A combination of high temperatures and the additional moisture that is coming up from the south … is causing these very warm and humid, uncomfortable conditions,” said David Radell, a meteorologist with the weather service's Upton office.

The forecast for Monday also includes fog for some areas of Long Island during the morning commute, which then will gradually burn off and be replaced by cloudy skies. Light variable winds are forecast, but the heat index will be near 97. Nighttime temperatures will dip into the 70s with fog into Tuesday morning.

The New York State Department of Environmental Conservation issued an air quality alert for Sunday, signifying unhealthy air quality “for sensitive groups. For Monday the agency forecasts “moderate” air quality conditions on Long Island.

The DEC did issue an Air Quality Health Advisory on Sunday night for New York City as well as Rockland, and Westchester counties on Monday. The advisory notes that levels of pollution, either ozone or fine particulate matter, are expected to exceed an Air Quality Index value of 100, indicating greater health concerns. New Yorkers are advised to conserve fuel and reduce exhaust emissions.

Tuesday will start off with some patchy fog before 11 a.m. before partly sunny skies enter the mix. The heat index is expected to be in the upper 90s and a 30% chance of showers and thunderstorms is forecast at night with temperatures around 71.

“We have a little bit of a reprieve on Wednesday, Thursday into Friday with slightly cooler temperatures and shower chances each of those afternoons,”

The high Wednesday will top out in the mid-80s.

Showers and temperatures in the mid-80s are expected for the rest of the week until Sunday, which has an early forecast of sunny skies and a high near 90.