Squall Line - Development and Movement

Development and Movement

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.