The Flanking Line, a Severe Thunderstorm Intensification Source

Leslie R. Lemon National Severe Storms Laboratory, Norman, Okla. 73069

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Abstract

An organized persistent severe thunderstorm on 25 June 1969 blends elements of the classic supercell (Browning and Donaldson, 1963) and the multi-cell storm (Marwitz, 1972), as revealed by detailed surface and radar data. Changes in supercell strength reflect contributions of cells from a flanking line that overtake and combine with the main storm. One cell contributed to an intensified mesodepression and increased surface convergence as the cell merged with the supercell weak echo (updraft) region. Cells move both to the right and left as well as with the mean winds. Surface data reveal distinctively arranged surface discontinuities, a large persistent mesodepression, an associated convergence area, and one principal downdraft.

Abstract

An organized persistent severe thunderstorm on 25 June 1969 blends elements of the classic supercell (Browning and Donaldson, 1963) and the multi-cell storm (Marwitz, 1972), as revealed by detailed surface and radar data. Changes in supercell strength reflect contributions of cells from a flanking line that overtake and combine with the main storm. One cell contributed to an intensified mesodepression and increased surface convergence as the cell merged with the supercell weak echo (updraft) region. Cells move both to the right and left as well as with the mean winds. Surface data reveal distinctively arranged surface discontinuities, a large persistent mesodepression, an associated convergence area, and one principal downdraft.

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