<P> A bow echo is associated with squall lines or lines of convective thunderstorms . These echoes can range in size from 20 to 200 km , and have a life span of 3 to 6 hours . Bow echoes tend to develop when moderate to strong wind shear exists in the lower 2 to 3 km of the atmosphere . While similar to squall lines , bow echoes are smaller in scale and are moved by the wind inside them . They tend to push outward and after time die out . A bow echo also lowers the chance of a tornado being formed in the storm itself . The `` bow shaped '' echo is a result of focusing of the strong flow at the rear of the system . Especially strong bow echoes that cause devastating damage all along the width of the storm are often called derechos . </P>
which part of the mcs produces the bow echo on radar
the rear of the system