<P> A squall line is an elongated line of severe thunderstorms that can form along or ahead of a cold front . In the early 20th century, the term was used as a synonym for cold front . The squall line contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts . Severe weather in the 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 that bows out the most . Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present . Some bow echoes in the summer are called derechos, and move quite fast through large sections of territory . 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 . This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese: 石 湖 風--shi2 hu2 feng1, Simplified Chinese: 石 湖 风) in southern China . </P> <P> Supercell storms are large, usually severe, quasi-steady - state storms that form in an environment where wind speed or wind direction varies with height ("wind shear"), and they have separate downdrafts and updrafts (i.e., where its associated precipitation is not falling through the updraft) with a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the supercell storm cloud (or anvil) can break through the troposphere and reach into the lower levels of the stratosphere, and supercell storms can be 24 kilometres (15 mi) wide . Research has shown that at least 90 percent of supercells cause severe weather . These storms can produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (10 centimetres or 4 inches diameter), straight - line winds in excess of 130 km / h (81 mph), and flash floods . In fact, research has also shown that most tornadoes occur from this type of thunderstorm . Supercells are generally the strongest type of thunderstorm . </P> <P> A thunderstorm is classed as severe if winds reach at least 93 kilometres per hour (58 mph), hail is 25 millimetres (1 in) in diameter or larger, or if funnel clouds or tornadoes are reported . Although a funnel cloud or tornado indicates a severe thunderstorm, a tornado warning is issued in place of a severe thunderstorm warning . A severe thunderstorm warning is issued if a thunderstorm becomes severe, or will soon turn severe . In Canada, a rainfall rate greater than 50 millimetres (2 in) in one hour, or 75 millimetres (3 in) in three hours, is also used to indicate severe thunderstorms . Severe thunderstorms can occur from any type of storm cell . However, multicell, supercell, and squall lines represent the most common forms of thunderstorms that produce severe weather . </P> <P> A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more . A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake - effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and they generally form near weather fronts . Most mesoscale convective systems develop overnight and continue their lifespan through the next day . The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours . </P>

The type of thunderstorm that has the most damaging winds and precipitation is a