HRRR model estimate of total precipitable water, an indicator of atmospheric moisture, at 7 p.m. Sunday. (WeatherBell.com)

The author, Matthew Cappucci, is a rising junior in atmospheric science at Harvard University.

When it rains, it pours! Just ask residents living just outside Harrisburg, Pa., where a quick-hitting but exceptionally intense downpour dumped an unbelievable amount of rain.

In a single hour, just before 7 p.m., the atmosphere unloaded 4.27 inches at Harrisburg International Airport in Middletown, Pa. Over an inch came down in a mere 10 minutes between 6:15 and 6:25 p.m.

Harrisburg's chance of getting even 3.5 inches in an hour was just 0.1 percent, much less over four inches, tweeted meteorologist Alex Lamers.

In all, 4.71 inches of rain fell Sunday, shattering records for the date and ranking as one of the top five daily rain totals ever recorded in the Harrisburg area.

Counting the 0.58 inches that fell early Monday morning, the 24-hour total climbed to 5.29 inches.

"It was definitely impressive," said Paul Head, a meteorologist at the National Weather Service office in State College, Pa.; his office serves much of central Pennsylvania. "We got a report up north in Montour County from the city of Danville where a resident saw nearly six inches."

However, the rainfall was extremely localized. "To give you an idea of just how hit-or-miss the rainfall was, just over the river at the York Haven Dam, they got less than a half an inch," said Head. "That's how it is with summertime storms, but this event was definitely up there."

The storm was able to generate so much rain because of unusually high amounts of atmospheric moisture. Computer models estimated total precipitable water, which integrates the total mass of water vapor from the top of the troposphere to the surface, at about 2.5 inches — an extraordinary level for the area around Harrisburg. Even in a more tropical location like New Orleans, this would be considered high.

When the thunderstorm erupted over the area, it unleashed blinding sheets of rain. The Weather Service reported visibility dropped to just 0.5 miles.

"All of a sudden it was just downpouring and within 10 minutes the streets were flooded," Crystal Holley, an eyewitness, told Fox 43, a local news affiliate. "You couldn't go nowhere, at all. It came and flooded everyone's basements, everyone's houses. It was a mess."

The excessive rainfall wasn't the only rough weather that the Keystone State had to contend with Sunday. Head's office issued over a dozen severe thunderstorm warnings, with tree damage from high winds reported in Morris Township.

In addition to Harrisburg, a number of locations in central and eastern Pennsylvania, northeast Maryland, northern Delaware and southern New Jersey were deluged by three to seven inches of rain. Wilmington, Del., picked up 4.83 inches.


24-hour estimated rainfall ending 7 a.m. Monday, July 24. (National Weather Service Middle Atlantic River Forecast Center)

More scattered storms are possible in this region Monday, and the Weather Service has placed it in a marginal risk zone for excessive rainfall.

A flash flood watch remains in effect for portions of northeastern Pennsylvania.

Cooler and drier weather is expected to move into this region Tuesday through Thursday.

Follow Matthew Cappucci on Twitter: @MatthewCappucci.

Jason Samenow contributed to this report.