Scattered fast-moving thunderstorms moved across Long Island Sunday afternoon, interrupting what had been an ideal beach day.

At 3:14 p.m. the National Weather Service put out an advisory that the storms were moving across the area with possible winds in excess of 40 mph and "frequent cloud to ground lightning."

Heavy rain fell in some areas and the service put out a flood advisory that lasted until 4:15 p.m.

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The bad weather developed suddenly.

Earlier, Kevin Kacan, a meteorologist for the National Weather Service in Upton had said, "Sunday's temperatures will hover around 90 degrees in the inland northern areas of Suffolk and everywhere else there'll be highs in the 80s . . . There'll be plenty of sunshine."

Kacan said temperatures for much of the week will be higher than normal and the days will be muggy. He said it might feel like a heat wave but it won't be.

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"We're looking at a warm and humid air mass but a heat wave is when temperatures reach about 90 degrees for three consecutive days or more," Kacan said, "but it'll definitely be a warm week for this time of year. He said that temperatures at times will average about five degrees above normal.

Lows Sunday night will be around 70, Kacan said. "We're pretty much staying dry until Wednesday for the most part."

"On Monday it'll be a little bit cooler but we'll still be in the mid- to upper 80s," Kacan said. "There'll be plenty of sunshine with lows at night in the low 70s and maybe the high 60s toward the Forks."

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"On Tuesday we'll kind of settle in towards the mid-80s -- Islip this time of year averages about 81 so we'll still be three or four degrees above average," Kacan said. He said Islip could actually reach 86 for a high.

Lows at night will be in the upper 60s for most of the island," Kacan said. "It may be 70 in a few spots towards Nassau County."

"On Wednesday, it'll be about the same as Tuesday for highs, with increasing cloud cover as we have the chance for some thunderstorms during the day, beginning around noon," Kacan said. "There'll be an on and off chance that tapers off more toward the late evening hours then the chance comes back in the overnight hours."

Temperatures will be around 70 at night.

There will be another chance of thunderstorms throughout the day Thursday when it will be mostly cloudy heading into the evening hours. Look for highs in the mid-80s with nighttime lows in the low 70s.

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"The lows at night will be a little bit above average," Kacan said. "In Islip it's usually about 65."

"We're not going to start with partial clearing skies until Friday when it'll be partly cloudy," Kacan said. He said highs will be in the mid-80s "then we're going to see temperatures well below 70 overnight, 67 is closer to normal."