The slow-moving line of showers is expected to cross the region starting Sunday afternoon, with as much as 4 inches of rain possible in some locations, the National Weather Service said. Flood watches are in effect for areas of New York, New Jersey, Connecticut, New Hampshire and Vermont.

Philadelphia, Baltimore and Washington are under a severe thunderstorm watch.

Rain may fall at rates of 2 inches per hour through early Monday in New York, Bryan Ramsey, a National Weather Service meteorologist in Upton, New York, said Sunday. Because the storm is moving so slowly, accumulations may pile up in harder-hit areas.

“We are going to be looking at some very heavy rainfall, this will include the I-95 corridor,” said Andrew Orrison, a forecaster with the US Weather Prediction Center. “In general, the region up here has been wet, so we are looking at significant impacts.”

While about 80 million people are in the storm’s path, the highest rain totals are expected across portions of the northeast and New England, forecasts predict, with some seeing a month’s worth of moisture in just a few hours. Because the front is moving slowly, the bands of rain and flooding threat could linger until Monday morning.

As of Sunday afternoon, more than 300 flights have been canceled at New York’s LaGuardia and John F Kennedy airports, and more than 150 at Newark, according to FlightAware, an airline tracking company.

New Yorkers are urged to keep a close eye on the forecasts and prepare for possible flooding.

“Throughout the weekend, parts of the state will continue to be at risk for flooding from storms bringing heavy rain, especially in those areas already hard-hit by rains and flooding over the past couple of days,” New York Governor Kathy Hochul said in a statement.