In the past fifteen years, at WFO Greenville – Spartanburg (GSP) we have investigated several severe weather events produced by quasi-linear convective systems (QLCS) that occurred in a high shear-low CAPE (HSLC) environment. The severe weather events were mainly tornadic in nature and most received attention because they happened without a warning in effect. In recent years, Justin Lane and I have spent considerable amounts of time studying these events with the goal of improving our understanding, awareness, and performance during future events.
Over the next few days, we would like to take the opportunity to begin the discussion about severe weather in HSLC environments by sharing some of our thoughts and experience. While the severe weather threat and occurrence in HSLC environments is not limited to tornadoes, naturally our attention is drawn to them, especially when the tornadoes occur without a warning in effect. I will discuss some recent high profile events (item number 1 on the list of CI contributions) with this entry. Justin plans to discuss operationally useful parameters and radar reflectivity patterns in the coming days.
With the exception of some events that were associated with tropical cyclone remnants, most of the tornado events associated with HSLC environments were produced by a QLCS in the “cool season” (defined loosely as the months from October through March). A brief discussion of our tornado climatology might provide some perspective to our problem. Since we expanded our county warning area on 1 October 1995, we have had 180 tornado events, of which 45 occurred in the months from October through March. In other words, 25% of our tornado events have occurred in the “cool season,” which is somewhere in the neighborhood of three per year. Of the cool season tornadoes, 25 of them have been associated with QLCSs, although we have not identified the nature of the environment in all of the events. That translates to one or two QLCS tornado events per year. Twenty of the tornado events occurred without a warning in effect. Clearly, we have room for improvement.
Not all the tornadoes that occur in HSLC environments happen in the cool season. Two tornado events occurred in the Spring of 2010, one near Fair Play, South Carolina, on 8 April, and the other near Abbeville, South Carolina, on 3 May. The station climatology shows that occurrence of QLCS tornadoes in HSLC environments can be irregular. Prior to the Fair Play tornado on 8 April, the most recent event was back in 5 January 2007. This suggests that the study of HSLC severe weather might need to include a period longer than three years to capture enough tornado events.
Identification of QLCS tornado failures in HSLC environments might be more problematic because of the nature of most tornadoes in this environment. The typical QLCS tornado will be very short lived, with a track less than a mile long (and often much shorter than that) and less than 50 yards wide. The small damage path makes discovery difficult in rural areas. Thus, some events that appear to be a classic QLCS in an HSLC environment, such as the one over Laurens County, South Carolina, on 24 January 2010, that do not produce a tornado might actually have been tornadic. Damage might have occurred in a small area too remote to be seen.
To recap, recent high profile events include…
The Liberty, Moore, and Gastonia tornadoes on 5 January 2007. Fifteen people were injured while waiting in a line of vehicles to pick up children at Liberty Elementary School.
The Fair Play tornado on 8 April 2010
The Abbeville County (Buck Stand) tornado on 3 May 2010.
A recent null event occurred on 24 January 2010.