Low-level Jet Underestimated in a Possibly Tornadic Environment in Alberta by Numerical Weather Prediction and Reanalysis

Some of the most destructive atmospheric phenomena are tornadoes formed in supercell thunderstorms. Supercell thunderstorms require large instability and strong deep layer wind shear. A tornado from a supercell thunderstorm also requires strong low-level wind shear perpendicular to the storm motion, and this can be the most difficult ingredient to diagnose. We investigated a case from 13 July 2024 in eastern Alberta, where the low-level jet was drastically underestimated by numerical weather prediction models and poorly represented in reanalysis data. We sampled the core of the low-level jet using measured 10 meter surface winds from nearby weather stations and clear air Doppler radar velocity returns. We also attempted to reconstruct it using theoretical profiles based on the surface wind and the log wind profile. We found that all the model data we used (RDPS, HRDPS, NAM), and all the reanalysis data we used (NARR, ERA5) suggested a low-level jet of 5-10 knots at the surface increasing to 10-18 knots about 500 meters above the surface. However, estimates from radar and surface winds indicate 8-18 knots at the surface, and 20-35 knots at 500 meters above. The discrepancy in low-level winds can have a major impact on the low-level wind shear and helicity – with the actual values nearly double the modelled values. The significant tornado parameter combines normalized values of instability, deep layer wind shear, low-level storm-relative helicity, and low-level moisture. In this case, recalculating the helicity in the significant tornado parameter with real values raised it from 1.4 to 2.8. Although a tornado did not happen, a very intense and severe supercell developed, producing very strong low-level rotation, at least 5 cm hail, and possible funnel clouds.