Abstract
A case study was conducted of the development and early evolution of a severe squall line that occurred 9 April 1991. The squall line formed near the border between Arkansas and Tennessee, then raced toward the northeast during the next 14 hours. Damaging winds were widespread with the squall line; thus, the 9 April 1991 event fits the definition of a derecho. Radar observations of the evolving squall line show signatures often correlated with damaging surface winds, including a bow echo, strong reflectivity gradients, and weak echo channels. Synoptic conditions under which the 9 April 1991 event occurred were significantly different in many respects from those associated with warm-season derechos. Differences include absence of important low-altitude signatures and more vigorous weather systems. There were similarities to the warm-season pattern, however, including moderately strong winds aloft, a very unstable thermodynamic profile, and cool dry air at midlevels. The similarities suggest refinements of the derecho model.
