Original sentence: A squall line is an elongated line of severe thunderstorms that can form along and/or ahead of a cold front.In the early 20th century, the term was used as a synonym for cold front.On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst.Severe weather, in form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line which bows out the most.Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory.The squall line contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts.Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present