Hook echo

The hook echo is the visual signature of hydrometeors being wrapped around a mesocyclone by the rear-flank downdraft. It typically appears on the inflow side (south or south-east in UK setups) of a mature supercell.

A hook alone isn't a tornado signature — you need to correlate with velocity data (velocity-couplet, tvs) and BWER aloft. But a sudden tightening of the hook ('hook tightening') is a common pre-tornadic evolution and worth flagging in nowcasting.