Forecasting tornadic thunderstorm potential in Alberta using environmental sounding data. Part II: Helicity, precipitable water, and storm convergence

Sounding parameters are examined to determine whether they can help distinguish between Alberta, Canada, severe thunderstorms that spawn significant tornadoes (F2-F4), weak tornadoes (F0-F1), or nontornadic severe storms producing large hail. Parameters investigated included storm-relative helicity (SRH), precipitable water (PW), and storm convergence. The motivation for analyzing these parameters is that, in theory, they might affect the rate of change of vertical vorticity generation through vortex stretching, vortex tilting, and baroclinic effects. Precipitable water showed statistically significant differences between significant tornadic storms and those severe storms that produced weak tornadoes or no tornadoes. All significant tornadic cases in the dataset used had PW values exceeding 22 mm, with a median value of 24 mm. Values of PW between 19 and 23 mm were generally associated with weak tornadic storms. Computed values of storm convergence, height of the lifted condensation level, and normalized most unstable CAPE did not discriminate between any of the three storm categories. The SRH showed discrimination of significant tornadoes from both weak tornadic and nontornadic severe storm groups. The Alberta data suggest that significant tornadoes tended to occur with SRH > 150 m(2) s(-2) computed for the 0-3-km layer whereas weak tornadoes were typically formed for values between 30 and 150 m(2) s(-2). Threshold values of SRH were lower than those suggested in studies based on storm observations throughout much of the United States.