What's a subtropical storm?

Subtropical Storm Ana formed in the Atlantic overnight... So what's a subtropical storm? Some meteorology basics first:

Meteorologists divide storms, or cyclones, into two general types: "extratropical" and "tropical."

Extratropical cyclones are the familiar storms that occur in all seasons and can form over land or water. They are represented as an "L" on weather maps. Temperature contrasts are their major source of energy and they have fronts, or boundaries between warm and cold air.

Tropical cyclones include tropical storms, hurricanes and typhoons. They form mainly in the summer and fall and form over warm oceans, especially tropical oceans. They get most of their energy from the extraction of heat from the warm ocean water. Also, they do not have fronts. Instead, tropical cyclones form in warm, humid air masses.

A few storms, however, combine elements of both of these types of storms. The National Hurricane Center calls these "hybrid" storms subtropical storms, which are like tropical storms in that they don't have fronts. But they also get their energy from contrasting temperatures, as do extratropical storms.

Subtropical cyclones usually form out of old extratropical storms.

Additionally, tropical systems have the potential to quickly grow into hurricanes, while extratropical or subtropical storms do not.

Historically, subtropical storms are not well documented, according to WeatherBug. In fact, the hurricane center only began to name subtropical storms in 2002.

Subtropical storms are also more likely than tropical storms to form outside of a region's designated hurricane season. For instance, the "official" start to the Atlantic hurricane season is June 1.