Original: 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.
Simplified: 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.