With heavy showers and thunderstorms "increasingly likely" Thursday afternoon and night, the National Weather Service has issued a flash flood watch for Thursday afternoon to Friday afternoon.
Average rainfall on Long Island for that time is expected to be 1 to 2 inches, but localized rainfall could amount to more than 2 inches an hour, the weather service said.
As for late afternoon Wednesday into the evening, there's a 40 percent to 50 percent chance Long Island will see scattered showers and thunderstorms -- some possibly severe -- as storms are developing to the north and west, said Mike Layer, weather service meteorologist in Upton.
Those and storms in other areas of the country, such as Virginia, the Midwest and Great Lakes, are affecting local air traffic. As of late afternoon Wednesday, some flight arrivals were experiencing close to half-hour weather-related delays at Kennedy Airport, where departures were being delayed up to 45 minutes, according to the Federal Aviation Administration's website.
Some LaGuardia arrivals were delayed an average of 43 minutes, with some departures from Long Island MacArthur Airport also affected, said the FAA, which advised travelers to check with their airlines.
The area could be in store for further "bouts of heavy showers and thunderstorms" during the weekend and into early next week, the weather service said.
The weather service forecast says more hot, humid weather is ahead, but a high-pressure system called a Bermuda High is going to help pump precipitation across Nassau and Suffolk counties, Westchester County, southern Connecticut and New York City.
Tim Morrin, a meteorologist with the Upton-based service, said between Wednesday afternoon and into Tuesday, Long Islanders can expect warm, humid days with high probabilities of thunderstorms.
Chances of rain between Wednesday afternoon and Sunday night range from 40 to 70 percent, the service said, with the likelihood that much of the rain will be in the form of short, heavy thunderstorms.
Morrin said it's not entirely clear exactly where and when the storms will strike, but chances are most will be daytime events.
"During the day, there's more of an increased chance because daytime heating and humidity fuel these types of intermittent rain patterns," Morrin said. "Some areas will get a quick shower; some won't."
The Bermuda High is a large, subtropical air mass that rests over the western North Atlantic near the islands for which it is named and often leads to several days of summer warmth for portions of the East Coast.
In many cases, Morrin said, the system creates a typical summer pattern: hot, humid conditions with sunny skies interrupted by a quick shower. "It's the chaotic summertime, a convective environment," he said.
For most of the next several days, daytime temperatures will be in the mid-80s, with nighttime highs in the mid-60s. Daily heat records are probably safe, Morrin said, because of some cloud cover.
Because the thunderstorms create a chance of flooding in low-lying areas or have poor drainage, the service issued a hazardous weather outlook through Tuesday.
Morrin said people on the golf course, at the beach or picnicking should be prepared for the rain and take cover if they see lightning.
"And even if you are impacted, the duration of the rain is such that it would not ruin your event or outdoor plans," he said.
With Gary Dymski