 Since they form from extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required is around 23 degrees Celsius for their formation, which is three degrees Celsius (5°F) lower than for tropical cyclones. This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.
