A squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front.
A summer squall line in Southern Ontario, producing lightning and distant heavy rains
A Squall Line contains heavy precipitation, hail, frequent lightning, strong, straight line winds, and possibly tornadoes and waterspouts.
Development and Movement
Areas of thunderstorm activity may reinforce pre-existing frontal zones and can 'outrun' cold fronts. This outrunning occurs within the upper level westerly airflow 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 and linear or curved, the MCS may be described as a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. 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.
Indications of Severe Weather
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.