A severe weather outbreak producing nearly 400 severe wind reports along with more than 10 tornado reports, moved across the southeastern U.S. on 30-31 January 2013. The event can be categorized as a high shear-low CAPE event across the Carolinas and Virginia. Examples of an experimental parameter called the SHERB, which was developed as a part of a Collaborative Science, Technology, and Applied Research (CSTAR) project between NC State University and numerous NWS offices in the Southeast, will be briefly shown below.
On 30 January, an upper-level trough approached the Mississippi and Tennessee Valleys as a strong 100 to 125 kt mid-level jet moved across the Tennessee and Ohio Valleys. An associated cold front was moving slowly eastward across the region and this feature combined with large scale ascent lead to the development of a large squall-line that moved across the region. The atmosphere across the region was characterized by 60 to 70 kts of deep layer shear along with surface based CAPE values of 500 J/kg across most of the region with the exception of southern AL, southern GA, and Florida where surface based CAPE values ranged between 500-1000 J/kg . The wind profile along with sufficient instability suggested that there would be a considerable wind damage threat along the squall line with the threat of isolated tornadoes confined to the southern extent of the line.
The Severe Hazards in Environments with Reduced Buoyancy parameter (SHERB) was designed through a series of statistical tests as a tool for determining the potential severity of convection within high shear, low CAPE (HSLC) environments. The SHERB was developed by comparing the environments of HSLC significant severe convection to environments associated with unverified severe thunderstorm or tornado warnings issued on a day when no severe reports were gathered in that respective CWA.
The SHERB parameter is a product of the 0-3 km shear magnitude, the 0-3 km lapse rate, and the 700-500 mb lapse rate, with both lapse rates normalized by their optimal values (i.e., where they most skillfully discriminate between HSLC significant severe convection and nulls) and the 0-3 km shear magnitude normalized such that the optimal threshold for the SHERB is a value of 1. In other words, the potential for significant severe convection increases when the SHERB is above 1 in HSLC environments. A climatology has shown that approximately 75% of HSLC significant severe events in our CSTAR region between 2006-2011 occurred with a SHERB ≥ 1, while only about a quarter of non-severe convective events were associated with a SHERB ≥ 1. The SHERB should be used as a guidance tool to assess the potential strength of convection, not to forecast the occurrence of convection.
The evolution of the SHERB parameter along with other parameters, radar imagery and damage reports can be seen in the example from 01 UTC on 31 January and in the imagery shown below. These figures contain the SHERB (top left), MUCAPE (top center), severe weather reports with wind plotted in yellow and tornado plot in red (top right), 700-500 mb lapse rate and 0-3km shear (bottom left), and regional radar imagery (bottom right). The SHERB (shown in the top left) performed particularly well during the 22-02Z period in highlighting the growing potential for HSLC significant severe convection across the Carolinas. While the locations of the damage reports are scattered somewhat along the line, the SHERB values are coincident with the enhancement of the reflectivity values along the line.