Supercell thunderstorms, explained
By CNN Meteorologist Mary Gilbert
(CNN) — Supercell thunderstorms are dangerous severe storms that unleash destructive hail, roaring winds and powerful tornadoes.
They are responsible for nearly all of the United States’ most violent tornadoes and the largest hailstones.
Here’s what makes them so potent.
Thunderstorms 101
Thunderstorms need moisture, energy — which meteorologists call instability — and some sort of lift to form. Usually, the lift comes from cold and warmer air masses colliding along a line called a front.
Once thunderstorms bubble to life, they come in two main forms: single cell and multi-cell.
Single cell thunderstorms develop alone and stay solo — think the short-lived thunderstorms of spring and summer afternoons. Multi-cell thunderstorms occur when individual storms develop but then merge into lines or clusters.
Supercell thunderstorms are also single cell, but they last much longer — hours instead of minutes. The “super” in their name refers to that extended lifetime.
What makes supercells different
Supercells have one main feature that keeps them up and running longer than other storms: a powerful, rotating updraft.
An updraft is a column of air that rushes upward in a thunderstorm and feeds it the moist air and energy it needs to grow tall and strong.
All thunderstorms have an updraft, so it’s the rotation that makes supercells the dangerous standout they are.
An updraft rotates when there is enough shear — a change in wind speed and/or direction with height — in the atmosphere. Rotation keeps the updraft strong and steady for a longer time by pulling in more moisture and energy.
Once a supercell thunderstorm gets up and running, it’s almost guaranteed to produce some sort of severe weather in the form of hail or damaging wind gusts.
Not every supercell thunderstorm produces a tornado, in fact only about 20% to 30% of them do, according to the National Weather Service. But any tornadoes from a supercell are more likely to be on the higher end of the EF Scale.
Supercells also often develop a signature look on weather radar, especially when they may be about to produce a tornado. It’s called a hook echo and only supercells develop them. That spiral shape is an easy way to spot a supercell’s powerful rotating updraft.
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