Comments on “Double Impact: When Both Tornadoes and Flash Floods Threaten the Same Place at the Same Time”

[...]our goals in this comment are to 1) discuss the importance of storm (and supercell) motion in potentially helping to anticipate TORFF events; 2) provide examples contrary to the notion that tornadic storms are necessarily fast moving, as well as discuss briefly the characteristics of slow-moving tornadic supercell environments; and 3) clarify the relationship of CAPE with rainfall and precipitation efficiency. 2. [...]Zeitler and Bunkers (2005) provided an example in which a supercell produced an (enhanced Fujita scale) EF12 tornado and flash flooding, and the hodographs indicated slow supercell motions of around 5 m s−1 (10 kt; refer to their Figs. 9-12). [...]the individual cells were moving relatively fast, but they were “training” over the same area for an extended period. [...]the low-level clockwise curvature of the hodographs resulted in 0-3 km AGL storm-relative helicity (SRH3) of 250-300 m2 s−2, which is quite favorable for tornadic supercells (e.g., are common with modest midlevel lapse rates and CAPE of 1500-2000 J kg−1 (versus steep midlevel lapse rates with drier environmental air aloft that result in large CAPE). [...]large CAPE can be associated with low precipitation efficiency. In addition to buoyancy, supercells have another source of vertical accelerations [i.e., the dynamic perturbation vertical pressure gradient force from the mesocyclone; Weisman and Klemp (1984)], augmenting the effect from buoyancy on the updrafts, resulting in very high rainfall rates even if at low efficiency (Doswell et al. 1996; Smith et al. 2001). [...]Nielsen et al.