A “blockbuster” storm tearing up the East Coast that has already knocked out power to more than 500,000 customers and grounded hundreds of flights promises to bring flooding rain and dangerous winds through Monday.

The heaviest rain will likely fall across eastern New York and western Massachusetts, with as much as 5 inches (12 centimeters) hitting the ground, said Rob Carolan, owner of Hometown Forecast Services, which provides forecasts to Bloomberg Radio.

New York City has received about 1.5 inches of rain since midnight and could get another 1 to 2 inches through Monday, said Rich Otto, a forecaster at the US Weather Prediction Center. Flood warnings and watches reach from Maine to North Carolina for the storm that has already drenched Florida and Charleston, South Carolina.   

Rivers have reached major flood stage across New Jersey, with the Saddle River in Lodi rising more than five feet since midnight and the Passaic River set to crest seven feet higher by Tuesday, the National Weather Service said. In New Hampshire, the Saco River in Bartlett has risen more than 10 feet since Sunday night. Route 9, a heavily traveled secondary highway in eastern Massachusetts, was closed in Wellesley after it flooded.

“The two branches of the jet stream have come together and there is a nice, big blockbuster low going right up through New England,” Carolan said by phone.

From Virginia to Maine, 701,766  customers were without power, with more than 270,000 in the dark in Massachusetts, according to website PowerOutage.us.

On the air travel side, 488 flights were canceled through early Monday, with the majority at New York’s LaGuardia and John F. Kennedy airports and Boston’s Logan International Airport, according to data from FlightAware.

The storm is still strengthening, Otto said. Early Monday there was a low pressure system over Philadelphia and one off the Virginia coast, and the two will combine later. As it builds in power, coastal areas could have widespread wind gusts of 50 mph or more.

At Boston’s Logan airport, the weather service clocked a 68 mph gust and police departments in eastern Massachusetts are urging drivers to stay off roads blocked by trees and powerlines, according to postings on X, formerly known as Twitter. 

“I wouldn’t be surprised if some places reach 70 mph, this is a very strong system and there is a lot of wind with it,” Otto said.