A 4-Yr Climatology of Cold-Season Bow Echoes over the Continental United States

Abstract

A search of radar mosaics and level-II Weather Surveillance Radar-1988 Doppler (WSR-88D) data revealed 51 cold-season (October?April) bow echoes that occurred in the contiguous United States from 1997?98 to 2000?01. Proximity soundings indicated mean 0?2.5-, 0?5-, and 5?10-km shear values of 14, 23, and 19 m s?1, respectively. Mean CAPE was 1366 J kg?1. Most bow echoes developed from squall lines, groups of cells, or squall lines overtaking cells that originated in the path of the squall line. Overall, cell mergers occurred just prior to the development of 34 (67%) of the 51 bow echoes, and embedded supercells were present in the mature stage of 22 (43%) bow echoes. Nine severe, long-lived bow echoes (LBEs) were identified, and seven of these had damage paths that met derecho criteria. LBEs developed in strongly forced, dynamic synoptic patterns with low to moderate instability. As in previous observational studies, proximity soundings suggested that LBEs are possible within much wider ranges of sampled CAPE and shear than idealized numerical modeling studies have indicated. Cold-season bow echoes formed overwhelmingly (47 of 51) in southwesterly 500-mb flow. Twenty (39%) bow echoes formed in a Gulf coast synoptic pattern that produced strong shear and moderate instability over the southeastern United States. Nineteen (37%) and seven (14%) bow echoes, respectively, formed in the plains and east synoptic patterns, which resemble classic severe weather outbreak patterns. Four (8%) bow echoes developed in a northwest flow synoptic pattern that produced strong shear and moderate instability over the southern plains.