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A squall line is a 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. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts. Strong straight-line winds can occur where the squall line is in the shape of a bow echo. Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present. Some bow echoes which develop within the summer season are known as derechos, and they 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 be present, sometimes associated with a heat burst.
Polar front theory was developed by Jacob Bjerknes, derived from a dense network of observation sites in Scandinavia during World War I. This theory proposed that the main inflow into a cyclone was concentrated along two lines of convergence, one ahead of the low and another trailing behind the low. The trailing convergence zone was referred to as the squall line or cold front. Areas of clouds and rainfall appeared to be focused along this convergence zone. The concept of frontal zones led to the concept of air masses. The nature of the three dimensional structure of the cyclone was conceptualized after the development of the upper air network during the 1940s.
Organized areas of thunder storms activity reinforce pre-existing frontal zones, and they can outrun cold fronts. This outrunning occurs within the westerlies in a pattern where the upper level jet splits into two streams. The resultant mesoscale convective system (MCS) forms at the point of the upper level split in the wind pattern in the area of best low level inflow. The convection then moves east and toward the equator into the warm sector, parallel to low-level thickness lines. When the convection is strong linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. This feature is commonly depicted in the warm season across the United States on surface analyses, as they lie within sharp surface troughs. If squall lines form over arid regions, a duststorm known as a haboob may result from the high winds in their wake picking up dust from the desert floor. Squall lines are depicted on National Weather Service surface analyses as an alternating pattern of two red dots and a dash labelled "SQLN" or "SQUALL LINE", which are followed by mesoscale high pressure systems. Well behind mature squall lines, a wake low can develop on the back edge of the rain shield, which can lead to a heat burst due to the warming up of the descending air mass which is no longer being rain-cooled.
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 line. This high pressure area is formed due to strong descending motion behind the squall line, and could come in the form of a downburst. The pressure difference between the mesoscale high and the lower pressures along the squall line cause high winds, which are strongest where the line is most bowed out. Another indication of the presence of severe weather along a squall line is its morphing into a line echo wave pattern, or LEWP. A LEWP is a special configuration in a line of convective storms that indicates the presence of a low pressure area and the possibility of damaging winds, large hail, and tornadoes. At each kink along the LEWP is a mesoscale low pressure area, which could contain a tornado. In response to very strong outflow southwest of the mesoscale low, a portion of the line bulges outward forming a bow echo. Behind this bulge lies the mesoscale high pressure area.
A derecho (from Spanish: "derecho" meaning "straight") is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. Derechos blow in the direction of movement of their associated storms, similar to a gust front, except that the wind is sustained and generally increases in strength behind the "gust" front. A warm weather phenomenon, derechos occur mostly in summer, between May and August in the Northern hemisphere. They can occur at any time of the year and occur as frequently at night as in the daylight hours.
The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 miles per hour (93 km/h) during the storm as opposed to gusts, high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles (460 km; 290 mi) in length.) In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts. Although these storms most commonly occur in North America, derechos occur elsewhere in the world. Outside North America they may be called by different names. For example, in Bangladesh and adjacent portions of India, a type of storm known as a "Nor'wester" may be a progressive derecho.