Forecasting Tornadic Thunderstorm Potential in Alberta Using Environmental Sounding Data. Part I: Wind Shear and Buoyancy

Abstract

This study investigates, for Alberta, Canada, whether observed sounding parameters such as wind shear and buoyant energy can be used to help distinguish between thunderstorms with significant (F2?F5) tornadoes, thunderstorms with weak (F0?F1) tornadoes, and nontornadic severe thunderstorms. The observational dataset contains 87 severe convective storms, all of which occurred within 200 km of the upper-air site at Stony Plain, Alberta, Canada. Of these storms, 13 spawned significant (F2?F5) tornadoes, 61 spawned weak (F0?F1) tornadoes, and 13 had no reported tornadoes yet produced 3 cm or larger hailstones. The observations suggest that bulk shear contained information about the probability of tornado formation and the intensity of the tornado. Significant tornadic storms tended to have stronger shear values than weak tornadic or nontornadic severe storms. All significant tornado cases had a wind shear magnitude in the 900?500-mb layer exceeding 3 m s?1 km?1. Combining the 900?500-mb shear with the 900?800-mb shear increased the probabilistic guidance for the likelihood of significant tornado occurrence. The data suggest that buoyant energy alone (quantified by the most unstable convective available potential energy) provided no skill in discriminating between tornadic and nontornadic severe storms, or between significant and weak tornadoes.