What is a hook echo?

A hook echo is the classic radar shape of a rotating supercell thunderstorm: precipitation wrapped around the storm's mesocyclone in a curl, usually on the storm's back (often southwest) side.

Why hooks form

A supercell's updraft rotates. Rain and hail falling on the back side of the storm get pulled around that rotation, and the rear-flank downdraft wraps them into a curl. On reflectivity that curl looks like a hook or the number 6. The tornado, when there is one, is usually near the tip of the hook, close to the edge of the updraft.

How to check a hook on radar

• Switch to velocity. A real tornado threat has a tight velocity couplet at the hook.
• Check CC. A debris ball — a small round area of high reflectivity and low CC at the tip — suggests a tornado is already lofting debris.
• Watch the loop. Hooks that tighten and wrap up over several scans are more concerning than ones that stay broad.

Not every hook means a tornado

Many supercells show a hook without producing a tornado, and some tornadoes come from storms without a clear hook — especially in squall lines or when heavy rain wraps around the circulation (high-precipitation supercells). That is why forecasters combine reflectivity, velocity and CC, and why warnings come from the NWS.

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