The National Hurricane Center has issued a tropical storm warning for parts of South and North Carolina.
Subtropical storm Ana isn't a powerful system — it's only packing 45 mph winds — but it's a little outstanding, because it formed before the traditional June 1st start of the Atlantic Hurricane Season and because Ana bears sightly different characteristics from typical tropical cyclones.
Ana is forecast to make landfall Saturday evening, bringing winds and up to 6 inches of rain.
Now, why is Ana considered a subtropical cyclone, instead of a regular hurricane?
The Hurricane Center says that it is on its way to becoming a bonafide Tropical Storm, but:
"The storm still has less-intense convection than most tropical cyclones, however, and is still co-located with an upper-level low; consequently Ana remains best classified as subtropical for now."
In English, it means that the storm is being energized in part by a difference in temperature between the warm ocean and pool of cold air at high altitudes. Regular cyclones, or hurricanes, derive their energy from "latent heat," which Weather Underground explains is "when water vapor that has evaporated from warm ocean waters condenses into liquid water."