Rain was expected to begin before 11 p.m. tonight and last through tommorrow afternoon with totals reaching up to two inches, the National Weather Service said.

A flash flood watch for Long Island is in effect from now through Friday afternoon, the service said, telling of the potential for periods of heavy rain.

“From late Thursday and into the morning hours Friday we should see moderate to heavy rainfall,” said Jay Engle, a meteorologist with the service’s Upton office.

The impending bad weather caused arrival delays at LaGuardia and Kennedy airports, according to the Federal Aviation Administration.

How much rainfall and exactly where it falls across Nassau and Suffolk counties isn’t entirely clear, Engle said. But “an inch to an inch and a half can’t be ruled out.”

Central Suffolk County has been deemed to be in severe drought, according to the Thursday update from the U.S. Drought Monitor, leaving the rest of the Island in moderate drought, the monitor’s least intense drought category.

advertisement | advertise on newsday

The upgraded severe level means that “crop or pasture losses likely, water shortages common, water restrictions imposed,” the monitor says.

The rain is expected to start after 9 p.m. Thursday and last through noon Friday, the service said. A threat of flash flooding exists from late Thursday into Friday afternoon, the service said in a hazardous weather outlook.

Most of Thursday is hot and humid, with temperatures flirting with 90 degrees.

As of just before 11 a.m., the temperature hit 90 at Long Island MacArthur Airport, according to weather service data. By just before noon, the temperature was 87, but humidity made it feel like 93.

News 12 Long Island meteorologist Matt Hammer said the rain on Friday could be especially heavy during the morning commute. “It’ll be pouring at times,” he said.

Daily highs Friday are in the low 80s, and the rain is expected to taper off starting at about noon.

“We need the rain,” Hammer said, noting that Long Island is more than 6 inches below its normal rainfall total for this time of year.

Engle agreed: “We do need the rain.”

With Gary Dymski