The heat is on — and sizzling sirens Lady Gaga and Rihanna are turning it up a few notches just in time for the unofficial start of summer.

“S&M” singer Rihanna tipped her hat to the Memorial Day weekend with a “Today” show performance yesterday at Rockefeller Plaza, while “Born This Way” diva Gaga did a funky rope slide for “Good Morning America” fans in Central Park.

Forecasters say temperatures will be in the 80s for the long weekend, with a chance of a thunderstorm this afternoon, some stray clouds tomorrow, and another p.m. thunder-boomer on Memorial Day.

City beaches in Brooklyn, Staten Island and The Bronx officially open today — and there’ll be 500 lifeguards on duty to make sure everyone’s safe. A total of about 1,300 guards will be hired, said Parks Commissioner Adrian Benepe.

“We’ve gone instantly from a cool spring to a hot summer,” Benepe said. “We’re ready!”

Meanwhile, motorists heading for the Hamptons yesterday afternoon began the long, slow traffic dance to sun and fun. “[Traffic’s] already bad,” griped landscaper Gene O’Neill.

Also, in the city Monday, alternate-side parking regulations and garbage pickups are suspended.