Yesterday's EF2 tornadoes in Rutherford and Burke counties of western North Carolina (see photos above) injured 10-15 people (one critically) and came as a bit of a surprise. This case is a good example of how difficult it is to forecast winter-time tornadoes. Instability (CAPE) is often small (around 500 J/kg or less), but wind shear and low-level storm-relative helicity (SRH) are often large (> 200-300 m2/s2); that makes it hard to assess when CAPE and SRH combinations are truly strong enough to support significant tornadoes. It's just as important to assess the strength and focus of the accompanying synoptic setting and how the CAPE/shear environment fits with that.
Wednesday's system (1/11/12) had been a mid-level cut-off low over Texas the previous day (1/10/12, see NAM 500 mb vorticity charts above), and was being pushed rapidly into the Carolinas by a larger wave at 500 mb digging into the Central Plains. Note that although Wednesday's upper system over the Southeast was no longer a closed low, it was now a tight and dynamic short wave with a strong vorticity max moving into the western Carolinas (red area and "X", see arrows), focusing upward forcing. This strong upper wave and vorticity max had also caught up to the associated surface front (blue curved line), addiing to the strong dynamic focus in the western Carolinas area. (The previous day, the same upper vorticity max located in Texas had lagged the surface front in Mississippi by some distance.)
The fixed-layer Sig Tor Parameter at 2100z (STP at 4 pm EST, see SPC mesoanalysis chart above) suggested a marginally favorable environment (> 1.0) for tornadoes over NW South Carolina inching into SW North Carolina, just ahead of the strong upper vorticity center seen earlier and the surface front, near the "triple point" where fronts met (see surface inset). These dynamic factors and environment provided enough focus for the south end cell of a broken arc of low-topped storms (see visible satellite image above) to become a damaging tornadic supercell as it moved into North Carolina.
A similar setting the day before over Mississippi, but with somewhat less focus and slightly less favorable STP values (not shown), did not generate any tornadoes, illustrating how subtle and difficult winter tornado settings with small CAPE can be to assess, in contrast to days like the Joplin tornado where CAPE exceeded 4000 J/kg. Again, an awareness of how synoptic features come together or focus into a particular area (e.g., strong short wave and vorticity max, frontal orientation relative to those features, along with prefrontal environment) is important. Typically, STP or EHI values slightly larger than 1.0 aren't impressive, but the surface and upper-air dynamic focus in this case suggested more attention.
A 2200 UTC RUC-estimated sounding at Shelby NC, about 20 minutes before the first tornado, is shown in the last graphic above. Notice that the CAPE was small (< 500 J/kg), but the SRH moderate to strong (260 m2/s2). With all the CAPE "compressed" below 400 mb (approximately 24,000 ft MSL), rapid supercell updraft accelerations may have been enhanced in low-levels, even though total CAPE was relatively small, helping with tilting and stretching of streamwise vorticity associated with SRH in the environment.
Jon Davies - 1/12/12