A storm system that was not a true tropical storm passed off the coast of the Carolinas on Monday, dropping rainfall of “historic magnitude” comparable to that of Tropical Storm Debby last month.

Although the storm reached tropical storm force of over 39 mph (62 kph), it was not technically the eighth named storm of this year’s Atlantic hurricane season, Helene. The storm more closely resembled a typical storm system over the United States, but its energy came not from rising warm, moist air from the ocean, but from interacting air masses.

But even without the name, the dangers are the same. The storm had significant impacts on Monday. Life-threatening flash flooding was reported in parts of southeastern North Carolina and wind gusts of over 60 mph were recorded along the coast, meteorologists at the National Hurricane Center said Monday afternoon.

Flash flooding began early Monday afternoon after more than 6 inches of rain fell in southeastern North Carolina since midnight. Meteorologists warn that flash flooding is likely to continue, with another 4 to 8 inches of rain possible by the afternoon.

Tropical Storm Debby dumped over a foot of rain on parts of the Carolinas in August, submerging vehicles and even endangering landfills. Meteorologists weren’t originally expecting that much rain on Monday, but by early afternoon, some places, like Sunny Point, North Carolina, had already experienced nearly 15 inches of rain, the most rainfall that fell in North Carolina during Debby.

In Carolina Beach, North Carolina, over 18 inches (46 cm) of rain fell since midnight, more than the 11 inches (28 cm) that fell in the beach community during Debby. Meteorologists in Wilmington, North Carolina, estimated that only once a year does it rain that much in just 12 hours.

In New Hanover County, where partial flash flood warnings were in effect Monday afternoon, all students were dismissed early and classes were canceled at Carolina Beach Elementary School, just a few blocks from the shore. All schools in neighboring Brunswick County canceled classes Tuesday.

WECT, a local Wilmington television station, showed footage of cars and vans stuck in the floodwaters, as well as road closures in the area. Several roads in Brunswick County collapsed in whole or in part on Monday, according to Facebook posts from the Brunswick County Sheriff’s Office.

Brunswick County government offices were closed Monday due to a declared state of emergency. Officials in the city of Southport, North Carolina, which is about 30 miles south of Wilmington, closed streets to all traffic and urged residents to shelter at home or their workplace, according to Facebook posts from the city.

The most important thing to know

• Most of the rain will fall near or north of the storm center. Rainfall totals of 4 to 8 inches are expected across the area, with some areas in North Carolina seeing nearly 10 inches.

• The storm system is expected to continue to slowly move inland overnight as rain spreads across North Carolina. On Tuesday, the remnants of the storm will move into Virginia, bringing the possibility of flooding across the state by Wednesday morning.

• As the storm approached land, its winds weakened and failed to reach tropical storm strength (39 mph or greater), prompting the Hurricane Center to lift tropical storm warnings for the coast.

A typical storm system moving across the United States draws its energy from the interaction of cold and warm air masses, which meteorologists say is occurring off the coast of the Carolinas this morning. A tropical cyclone, like a tropical storm or hurricane, draws its power from warm, moist air rising from the ocean surface.

Sarah Ruberg contributed to the reporting.