The Use of Convection-Allowing Models to Improve Threat Assessment Associated with Nocturnal Supercells

At night, issuing warning for damaging winds is a difficult challenge for operational meteorologists. When nocturnal damaging winds do occur they can cause significant damage and injuries. The danger is enhanced at night as many people do not receive the warning, especially if the event was not forecast. Beyond the difficulty in predicting nighttime convective mode, the development of the nocturnal inversion is assumed to decrease the probability of severe wind gusts. This enhances forecaster uncertainty as to whether damaging winds may occur.

Supercell thunderstorms can be accompanied by both significant hail and wind, defined here as greater than 4.4 cm (1.75 in) and greater than 33.4 m s-1 (65 kts). An analysis of storm reports associated with nocturnal supercells over the northern and central Plains of the United States showed that some events produced only significant hail (hail-only) while other events produced both significant hail and wind (hail-and-wind). Convective-allowing models (CAMs) predict 10-m wind speeds as part of their output and have been shown to produce stronger winds with some convective cells. They also provide the maximum gust for each hour. This study ran simulations of 11 hail-only supercells and 10 hail-and-wind supercells using the WRF-ARW with 3 km grid spacing and explicit convection. All models are initialized at 1200 UTC of the previous day in order to simulate guidance that would be available to forecasters prior to the development of nocturnal convection. Parameters associated with modelled convection including hourly maximum updraft helicity, hourly maximum wind speed and hourly maximum wind gust were compared between the two groups. Model soundings were also analyzed to determine the pre-convective environment prior to the development the supercell within the model. Results showed that hourly maximum wind speeds and wind gusts were stronger in simulations in which damaging winds were observed than those where only hail was observed.