Long Island weather: A chance of heavy rain after dark, then cooler

Clouds that battled the sun Wednesday across Long Island could thicken before dark and bring heavy rain into the night, the National Weather Service said.

Daytime temperatures and soupy humidity Wednesday nearly matched Tuesday's high of 90 degrees.

There is a 20% chance of heavy rain through the rest of the afternoon, increasing to 30% at night with a low of 68.

In addition, the weather service said, "There is a low potential for flash flooding this afternoon and early this evening, mainly across southern portions of the forecast area."

And then look for a cool down: Temperatures are not expected to climb out of the 70s from Thursday until at least next Tuesday, according to the National Weather Service.

As the weather service's Weather Prediction Center explained, "A cold front will make slow progress southward across the Great Lakes and Northeast U.S. through the end of the week, bringing some welcomed relief to the recent hot weather for these areas by Thursday and especially on Friday."

Thursday should be mostly sunny with a daytime high of 78 degrees.

Friday may begin with showers before noon — though the odds are just 20% — and the daytime high should top out at 70 degrees, the weather service said.

Saturday and Sunday should offer the same delightful weather: sunny with daytime highs of 72 degrees, courtesy of a high pressure system.

High pressure systems allow air to cool and dry as it falls to earth, so they typically bring clear skies.

Low pressure systems, like the one that may affect the start of the work week on Long Island, deliver rain, as their rising air condenses into clouds and raindrops.

While there is a 40% chance of showers on Monday, the high for the day should be a comfortable 75 degrees.

Tuesday looks like it will be mostly sunny and a bit warmer, with temperatures hitting 78 degrees during the day, the weather service said.

Still, it cautioned that "showers with some afternoon and evening thunderstorms will be possible on Monday and Tuesday."