Forecasting tornado potential in Alberta using environmental sounding data

This study investigates, for Alberta Canada, whether observed sounding parameters can help distinguish between severe thunderstorms that spawn significant tornadoes (F2-F4), weak tornadoes (F0-F1), or severe non-tornadic hail storms. The observational data set of 87 severe convective storms occurring within 200 km of the upper air site at Stony Plain, Alberta, consisted of 13 significant tornadoes, 61 weak tornadoes, and 13 severe non-tornadic storms (3 cm or larger hailstones). Parameters investigated included wind shear, buoyant energy, storm-relative helicity (SRH), and precipitable water. Additionally, composite soundings for each of the storm categories were constructed and examined. The results suggest bulk shear, storm-relative helicity, and precipitable water contained information about the probability of tornado formation and the intensity of the tornado. Environments of significant tornadoes generally had stronger bulk shear values compared to those of weak and non-tornadic storms. All significant tornado cases had a wind shear magnitude in the 900-500 mb layer exceeding 3 m s^-1 km^-1. Significant tornadoes tended to occur with 0-3 km SRH >150 m² s^-2, whereas weak tornadoes were typically formed with 0-3 km SRH between 30 and 150 m² s^-2. Significant tornadoes were generally associated with precipitable water values in the 23-32 mm range compared to 19-25 mm for weak tornadoes and non-tornadic storms. A combination of the 900-500 mb shear with the 900-800 mb shear and also the 900-500 mb shear with precipitable water increased the probabilistic guidance for the likelihood of significant tornado occurrence. The data suggest that buoyant energy alone provided no skill in discriminating between the three storm categories. Composite soundings showed little differences in the thermodynamic structures between the three groups. Composite hodographs showed a distinct separation of the significant tornadoes which exhibited a profile having strong low level veering, similar to that for supercell storms. Weak tornadoes and non-tornadic storm composites had weak or no veering and comparatively lighter winds.