Subtropical storm? What's that?

A subtropical depression formed Wednesday morning east of the Bahamas, and since this system almost certainly will never affect the U.S. mainland, we can focus on something other than the storm's track.

For example, what does "subtropical" mean, and how is such a storm different from a "tropical" system?

Subtropical Depression 17 is expected to strengthen into a storm, and it'll get the name Otto. After that, all the computers agree that the system will move swiftly to the northeast, away from North America and out to sea.

It probably will convert to a tropical storm and might even become a hurricane.

And it's interesting to note that Otto would be the 15th named system this year. As I recall, that's pretty close to all the predictions, and it's indicative of a busy season. However, since no major storms actually made landfall in the United States, most people probably will think of 2010 as a quiet tropical season.

Anyways ... back to the issue at hand -- tropical vs. subtropical.

A tropical system gets its energy from the ocean's warm water. Its center, called the core, is warm. The strongest winds and heaviest thunderstorms tend to be near the center of a tropical system.

A subtropical system is different. Often, it's a low pressure area along a cold front that moves into the south Atlantic or Caribbean. Gradually, the storm system stops getting energy from the cold front and instead begins receiving fuel from the warm water below. Thunderstorms in the system give off latent heat, and that helps develop the warm core.

In a subtropical system, the strongest winds tend can be several hundred miles from the center. In other words, it's not organized quite as well. But subtropical storms can have sustained winds of up to 65 or 70 mph.

Often, subtropical systems develop a warm core and convert to tropical systems. The strongest winds and storms move closer to the center, and the system becomes better organized.

There's one more term to know -- extratropical storms. That's a system that was tropical in nature (either a tropical storm or hurricane) but moves into colder waters and loses its tropical characteristics.