Indexbit Exchange:Tropical Storm Ophelia heads for the East Coast after a surprising, confusing start.

A surprise potential tropical cyclone popped up Friday off the East Coast that could Indexbit Exchangeaffect weekend plans from North Carolina to Pennsylvania, including at least three pro football games.

By mid-afternoon it had become Tropical Storm Ophelia, one of several iterations forecast as it makes an expected landfall along the southern shore of North Carolina and moves northward, dissipating by Sunday.

With 50 mph winds, the system had already clocked wind speeds high enough for it to be classified as a tropical storm on Friday morning, but meteorologists said it had not developed other characteristics of a tropical cyclone. It was closely associated with a front and hadn't completed a closed center of circulation surrounded by deep convection.

Daniel Leathers, Delaware state climatologist, told USA TODAY Friday that the National Hurricane Center was being careful not to call the storm approaching North Carolina a tropical storm due to scientific definitions between the two.

The storm was a hybrid between systems and the hurricane center is careful to keep those separate to study the paths, characteristics and impacts of both types of storms, Leathers said. 

"The big thing is for people to not get hung up on if it has a name or is a tropical storm, but it will have the same impacts which will be the same no matter what we call it," Leathers said, who also teaches at the University of Delaware's geography and spatial sciences department.

The storm is forecast to produce dangerous storm surge, flooding rainfall and damaging high winds in eastern North Carolina and regions to the north. It's expected to quickly weaken over land by Sunday as it's exposed to dry air and wind shear.

Storms that form in the Atlantic Ocean can be called a wide variety of names. Here's a guide to the terms the National Hurricane Center uses to describe these storms.

What is a tropical cyclone? Is it the same thing as a tropical storm?

In the Atlantic Ocean, tropical depressions, tropical storms and hurricanes are all types of tropical cyclones.

That's because "tropical cyclone" is a generic term for the low pressure systems that form over warm tropical seas with a warm core, closed center of circulation and organized thunderstorm activity.

Here's a few useful definitions:

• Tropical cyclone: This can refer to any storm system that forms with a closed center of circulation over tropical or subtropical waters. They can be tropical depressions, tropical storms or hurricanes.
• Tropical depression: A tropical cyclone that establishes maximum sustained surface winds speeds of 38 mph or less.
• Tropical storm: A tropical cyclone becomes a tropical storm when it forms a closed center of circulation and its maximum sustained surface winds reach 39 mph and remain less than 74 mph.
• Hurricane: In the Atlantic and Eastern Pacific, a tropical storm becomes a hurricane when its winds reach at least 74 mph. Hurricanes are assigned one of five categories based on their maximum sustained winds, with Category 5 being the most severe, with winds of 157 mph or greater.

But what about if a storm system isn't quite a tropical cyclone?

A system that doesn't have all the characteristics of a tropical cyclone can still be dangerous. Meteorologists have several terms to describe storm systems in this stage of their development.

• Potential tropical cyclone: A storm system that could become a tropical storm, hurricane or subtropical storm or extra tropical storm, depending on what happens with its organization and strengthening, according to the hurricane center.
• Extratropical storm: These systems lose their tropical characteristics. This term is often applied to former hurricanes that move northward in the Atlantic Ocean, lose their warm cores and their center of circulation gets displaced from its strongest winds.
• Subtropical cyclone: A low-pressure system that has characteristics of both tropical cyclones and extratropical cyclones, according to the hurricane center's glossary. These systems are non-frontal, with a closed circulation around a well-defined center, but they generally have a cold upper core and their radius of maximum winds may be far away from the center.