Classic derechos occur with squall lines that contain bow- or spearhead-shaped features as seen by weather radar that are known as "bow echoes" or "spearhead echoes". Squall lines typically "bow out" due to the formation of a mesoscale high pressure system which forms within the stratiform rain area behind the initial convective line. This high pressure area is formed due to strong descending air currents behind the squall line, and could come in the form of a downburst. The size of the bow may vary, and the storms associated with the bow may die and redevelop.