A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form between the equator and the 50th parallel.[33]
As early as the 1950s, meteorologists were unclear whether they should
be characterized as tropical cyclones or extratropical cyclones, and
used terms such as quasi-tropical and semi-tropical to describe the
cyclone hybrids.[34] By 1972, the National Hurricane Center officially recognized this cyclone category.[35] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[33]
They have broad wind patterns with maximum sustained winds located
farther from the center than typical tropical cyclones, and exist in
areas of weak to moderate temperature gradient.[33]
Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold by three degrees Celsius, or five degrees Fahrenheit, lying around 23 degrees Celsius.[36] 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.[37]
Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold by three degrees Celsius, or five degrees Fahrenheit, lying around 23 degrees Celsius.[36] 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.[37]
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