Significant tornadoes that occur in environments of relatively weak detectable low-level and deep-layer shear (e.g., 0-3 km storm-relative helicity less than around 100 m²/s² and boundary-layer to 6 km shear of only 20-30 kts) continue to be documented. Recent examples of damaging tornadoes in this category are the Jackson, Nebraska tornado on 8/17/01 (rated F2), the Myrtle Beach tornado on 7/6/01 (rated F2), and one of the Lamar, Colorado tornadoes on 5/29/01 (rated F3). Although these tornadoes were associated with storms that at times exhibited supercell characteristics, the shear environments appeared notably weaker than accepted shear guidelines typically associated with supercells and supercell tornadoes from operational research.

Examination of the sub-synoptic environments for several events of this type, including the aforementioned Jackson, Nebraska and Myrtle Beach, South Carolina tornadoes, reveals some common features. These include a well-defined pre-existing surface convergence boundary, large convective available potential energy in low-levels (e.g., CAPE below 3 km AGL), and storm motion that deviates considerably to the right of the mean wind. While the mechanisms for mesocyclone or tornado development in such cases are quite unclear, the combination of these factors appears to contribute to a scenario and environment supportive of tornadoes. The potential for large low-level parcel accelerations (suggested by large low-level CAPE) in the vicinity of boundaries may be a factor. Also, if storm motion is such that there is an increase in midlevel storm-relative flow for a storm, this may aid in storm organization and longevity.

Based on several cases with similar ingredients, this paper will document some detectable features that may help in diagnosing notable tornado potential in relatively weak shear environments that appear benign according to established supercell tornado forecast guidelines.