In an atmosphere with rich low level moisture and vertical wind shear , narrow , convective rainbands form . These are known as squall lines , and are usually in the cyclone 's warm sector , ahead of strong cold fronts associated with extratropical cyclones . In an atmosphere with rich low level moisture and vertical wind shear , narrow , convective rainbands known as squall lines generally in the cyclone 's warm sector , ahead of strong cold fronts associated with extratropical cyclones .