Some Meteorological Characteristics of Significant Tornado Events Occurring in Proximity to Flash Flooding

Abstract

A study was performed to determine meteorological aspects of environments in which thunderstorms produced both strong or violent tornadoes and flash floods within a limited temporal and spatial domain. It was found that the overwhelming majority of these episodes occurred in the spring and summer months and during the afternoon and evening hours. In most instances, at least some of the tornadoes were present when flash flooding was in progress. The ambient environment usually included an air mass that exhibited both relatively high convective instability and abundant lower-tropospheric moisture, including an average most-unstable CAPE of 3200 J kg−1 and mean surface dewpoint and precipitable water values of 70°F (21°C) and 41 mm (1.6 in.), respectively. Storm-relative helicity magnitudes indicated that the vertical wind shear ranged from marginally to moderately favorable for supercell formation in all cases. Surface patterns for each episode were generally similar to patterns earlier studies determined to be frequently attendant with flash flooding, in which preexisting surface boundaries acted to focus deep convection. Most events also occurred east of an approaching and well-defined upper-tropospheric trough and in the left-front or right-rear quadrant of an upper-level jet streak in which upward vertical motion is usually present.