18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001
Left-moving thunderstorms in a high plains, weakly-sheared environment
John F. Weaver, CIRA/Colorado State Univ., Fort Collins, CO; and J. F. Dostalek and L. Phillips
Poster PDF (354.1 kB)

         Convective environments having moderate-to-high values of Convective Available Potential Energy (CAPE), and strong winds that veer and increase in strength from the surface to 3 km AGL (or higher), frequently produce long-lived supercell storms that move to the right of the mean cloud-layer wind. In many cases, the right-mover forms after the original storm undergoes a process called ‘storm-splitting.’ In this process, storms split when pressure gradients on the flanks of the original updraft enhance lift, and produce two new updrafts; one of which moves off to the right, the other to the left. In the case of a cyclonically-curved hodograph, a region of high pressure develops above the low pressure area on the left flank. In the majority of cases, the left-moving updraft dissipates within 30 min, while the right-mover often continues on for 1-3 h, or longer.

          Most forecasters are aware that severe thunderstorms (producing large hail, damaging winds and/or tornadoes) can occur in environments with substantially less shear than that required to produce classical splitting storms and supercells. This paper presents a brief overview of a case in which left-moving thunderstorms formed in a weak-flow/weak-shear environment. The mean wind from 0-6 km AGL was from 265o at 19 kt, and the midday estimated SREH was less than 100 (m/s)2. In this case, the right- and left-moving storms neither formed nor behaved in the classical manner. Rather, the left-movers developed along the northward advancing portion of the original storm’s low-level outflow boundary. These left-moving storms were long-lived and produced severe weather at least as intense as their right moving partners.

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