MIKE MOSS SAYS: Keith, A supercell thunderstorm is one in which a rotating updraft develps in such a way as to create a self-sustaining circualtion that can lead to a long-lived system. Supercell development is related to the amount of moisture available, the instability of the airmass in which it develops and to the direction and speed of ambient winds in the larger environment (especially how they vary with increasing altitude). Not all supercells produce tornadoes, but they are much more likely to produce significant tornadoes than any other type of thunderstorm (airmass/pulse or multi-cell).
We do see supercell thunderstorms develop in North Carolina, and they are responsible for the most significant and damaging tornadoes that occur in our state. They are most likely during the spring and fall months, when surface temperatures are still warm and humidity is moderate to high, in combination with favorable vertical wind profiles and lift provided by midlatitude disturbances - during the summer, we often have plenty of moisture and instability, but lack the proper wind profiles and lack organized sources of lift for widespread storm initiation.
To speak more directly to your question, the incredients that favor supercell formation tend to occur in the right combinations less frequently and somewhat less intensely, on average, in our state than they do in parts of the Plains and Midwest, including the states you mentioned. For that reason, "tornado alley" derives it's name for the greater numbers of strong to violent tornadoes which are in turn spawned by greater numbers of more highly organized and intense supercell thunderstorms.
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