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James W. Duke
and
Joseph A. Rogash

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.

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Joseph A. Rogash
and
Jonathan Racy

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.

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Joseph A. Rogash
and
Richard D. Smith

Abstract

On 1 March 1997 violent tornadoes caused numerous fatalities and widespread damage across portions of central and eastern Arkansas and western Tennessee. In addition, the associated thunderstorms produced very heavy rainfall and flash flooding, with a few locations receiving up to 150 mm (6 in.) of rainfall in 3 h. The initial environment appeared favorable for strong tornadoes with unseasonably warm moist air at lower levels resulting in significant instability (convective available potential energy values between 1400 and 1800 J kg−1) where 0–2-km storm-relative helicities exceeded 300 m2 s−2 and the middle-tropospheric storm-relative flow was conducive for tornadic supercells. The most destructive tornadoes developed along a preexisting surface boundary where lower-tropospheric moisture convergence and frontogenesis were enhanced. Tornadoes and heaviest rainfall only ensue after upward motion associated with the direct circulation of an upper-tropospheric jet streak became collocated with lower-tropospheric upward forcing along the surface boundaries. From a flash flood perspective the event occurred in a hybrid mesohigh-synoptic heavy rain pattern as thunderstorms developed and moved along surface boundaries aligned nearly parallel to the mean wind. In addition, strong flow and associated moisture flux convergence in the lower troposphere favored the formation of cells to the southwest or upstream of the initial convection with thunderstorms, including a a tornadic supercell, traversing over the same area. The available moisture and ambient instability also supported both vigorous updrafts and high precipitation rates.

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