(with contributions from Steve Keighton)
Late in the evening on Tuesday, 7 October 2014, a high-end EF1 tornado struck Mercer County West Virginia in the WFO Blacksburg, VA CWA. The Mercer County tornado produced significant tree damage, minor damage to a few homes and barns, and significant damage to a mobile home that injured two residents (Fig. 1). The tornado was on the ground from 1139 PM EDT to 1149 PM EDT between Matoaka and Spanishburg, in far southern West Virginia, around 10 miles from the Virginia border (Fig 2.). The tornado had a path length of 5.5 miles and estimated maximum wind speeds of 105 MPH.
Other severe weather was observed in the eastern Ohio Valley and central Appalachians on Tuesday and Tuesday night including an EF2 tornado near Raleigh, West Virginia (see SPC storm reports). The Raleigh tornado occurred in the WFO Charleston, WV CWA, about an hourly earlier but only 10 miles north of the Mercer County Tornado.
While a thorough case study and analysis of the forecast process and guidance is unavailable at this time, we thought we would share some of the SPC mesoanalysis products near the time of the tornado touch down between 0339 and 0349 UTC on 8 October. Figure 3 is the 0330 UTC regional reflectivity radar composite. The storms of interest are located across far southern West Virginia, in a broken line with embedded shallow supercells.
Radar imagery at a time when the tornado was on the ground is shown in fig. 4 from KFCX (Blacksburg, VA) at 0342 UTC on 8 October. Base reflectivity data at 0.5 degrees is shown on the left with a high reflectivity core noted northeast of Matoaka. The 0.5 degree storm relative velocity imagery is shown in fig. 4 to the right with a small and compact circulation noted southeast of Arista.
Strong deep layer flow was noted over the Ohio Valley and Appalachians with the 0-6 km bulk shear analyzed by SPC between 60 and 70 kts (Fig. 5). The air mass across the southern Appalachians was only weakly unstable (Fig. 6) with the SBCAPE values analyzed around 100 J/Kg (left) and MUCAPE values between 100-250 J/Kg (right) at 0300 UTC. These bulk shear and instability values are consistent with High-Shear Low-CAPE (HSLC) events.
A key outcome of a recent CSTAR project is a new experimental parameter called SHERB to identify the potential for tornadoes and significant severe wind events in HSLC environments. The SHERB parameter is a product of the 0-3 km shear magnitude, the 0-3 km lapse rate, and the 700-500 hPa lapse rate normalized so that the optimal SHERB threshold is 1. SHERB values above 1 are more likely to be associated with significant severe convection than non-severe convection. The SPC mesoanlysis of the SHERB parameter from 0300 UTC (Fig. 7) indicated a region of elevated threat with SHERB values ranging between 1.0 and 1.25 across the central Appalachians.
SPC composite convective parameters including the Significant Tornado Parameter and the Supercell Composite Parameter are shown in Fig. 8. These composite parameters suggest the greatest significant tornado or supercell threat was located across the Tennessee Valley and the southern Appalachians, near but not coincident with the location of the tornado. Additional insight into the mesoscale environment can be gleamed from the SPC Mesoscale Discussions issued at 1950 UTC and 2359 UTC on 07 October.
Thanks to Phil Hysell and Robert Stonefield from WFO Blacksburg who conducted the Mercer County storm survey and provided the photos.