Abstract
On several significant tornado days, a distinct combination of near surface (< 1 km AGL) speed and directional shear has been noted in pre-tornadic vertical wind profiles. This combination consists of intense speed shear followed by intense directional shear between the surface and 1 km AGL, which appears as a sickle-shape on a hodograph. A composite sickle-shaped hodograph was constructed using several notable tornado outbreak days. The composite hodograph was used to examine the historical upper-air database for the 1995-2004 period in order to determine the relative frequency of the sickle-shape. Furthermore, the occurrence of the sickle-shape was related to severe weather documented within spatial and temporal proximity of the sounding site. Over the 10-year dataset, the sickle-shape was found to occur at most 4% of the time. It was also found that the sickle-shape occurs less than 0.5% of the time in the presence of minimally supportive kinematics and thermodynamics. On those days when the sickle-shape was accompanied by minimally supportive kinematics and thermodynamics (0-6 km AGL shear ≥ 20 m s-1 and CAPE ≥ 500 J kg-1), it was found that severe weather and tornadoes were more likely to be observed in proximity to the sounding site. The results show that the presence of the sickle-shaped hodograph and minimally supportive thermodynamics and kinematics is related to an increased likelihood of tornadic and significantly tornadic events.