Residual rain from Hurricane Ida hit the Northeast. Despite early forecasts suggesting 3 to 6 inches for the day, the region was walloped by the post-tropical cyclone. At least 40 people died in the region amid tornadoes, record-breaking rainfall, flash flooding and power outages.

The large weather system spanned from Baltimore to Boston Wednesday, dropping at least 2 inches of rain on 58 million people. Isolated areas saw up to 10 inches of rain.

A man waded through the flooded Major Deegan Expressway in the Bronx in New York on Thursday.

Photo: Spencer Platt/Getty Images

The intensity of the storm caught officials and commuters off guard. The Philadelphia and New York City corridor experienced rain rates of 2 to 3 inches per hour. Newark, N.J., recorded 3.24 inches of its 8.44 inch total between 8 p.m. and 9 p.m. In Central Park, 3.15 inches fell in an hour, the highest amount on record.

When declaring flash-flood guidance, the National Weather Service said that less rainfall is required to produce flash flooding in urban areas.

Write to Max Rust at max.rust@dowjones.com