An overview of a cool season tornadic supercell over central Mississippi

During the evening hours of 9 December 1999, a low topped (echo top less than 9 km) supercell moved along the Big Black River in central Mississippi, producing a short track F3 tornado near the small town of Bentonia. This supercell developed in advance of a pre-frontal squall line, and was the only known tornadic supercell in the region for this event. With the storm developing around 0000 UTC and about 50 km northwest of the Jackson, MS, rawinsonde site, the 0000 UTC 10 December sounding from Jackson provided a near-proximity sounding for this storm. This paper will begin by looking at this sounding, which was characterized by weak instability and relatively low values of storm-relative helicity, but strong deep-layer shear (Bulk Richardson Number Shear near 100 (m/s)2). The role of the pre-storm environment in the evolution of this storm will be discussed. Then, radar data will be presented using the National Severe Storm Laboratory's WATADS software package, which is very similar to the Warning Decision Support System (WDSS) software which was used operationally at the Jackson National Weather Service Office for warning operations on this day. The radar discussion will include the evolution of several distinct reflectivity features which were observed, as well as the role a boundary may have played in tornadogenesis. Additionally, the use of WDSS time series of velocity parameters in operationally determining the mode of tornadogenesis (descending versus non-descending from Trapp, et.al. 1999), and the utility of the knowledge of the tornadogenesis mode in real-time warning operations, will be presented.