Development
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system (MCS) forms at the point of the best upper level divergence 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 with the mean tropospheric flow. 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.
Derechos are squall lines that are bow- or spearhead-shaped on radar and, thus, also are called bow echoes or spearhead radar echoes. 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 size of the bow may vary, and the storms associated with the bow may die and redevelop.
During the cold season within the Northern Hemisphere, derechos generally develop within a pattern of southwesterly winds at mid levels of the troposphere in an environment of low to moderate atmospheric instability (caused by heat and moisture near ground level or cooler air moving in aloft and measured by convective available potential energy) and high values of vertical wind shear (20 m/s (45 mph) within the lowest 5 kilometres (16,000 ft) of the atmosphere). Warm season derechos in the Northern Hemisphere form in west to northwesterly flow at mid levels of the troposphere with moderate to high levels of instability. Derechos form within environments of low-level warm air advection and significant low-level moisture.
Read more about this topic: Derecho
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