Simulating the storm environment responsible for Nepal's first observed tornado

Abstract

A high-resolution numerical forecast model was used to simulate the meteorological conditions leading up to the March 31st, 2019 severe weather event that produced Nepal's first-ever observed tornado. The sparse meteorologic observations in the region capturing the storm environment limit the ability to anticipate another similar situation should the particular set of conditions present themselves again. This study presents a multifaced view of the storm environment through 1) a synoptic perspective provided by the Global Data Assimilation System (GDAS) reanalysis dataset and 2) a trio of progressively higher resolution one-way nested simulations (12 km–4km–1km) driven by GDAS boundary conditions to more closely examine the storm-scale environment. GDAS data and numerical simulations revealed moderately strong instability throughout the region with CAPE values between 1000 and 2000 J kg−1 K−1 and lifted index values between −4 and −7. Vertical wind profiles featuring little directional shear and moderate velocity shear yielded shear-based convective indices that suggested slight potential for rotating supercell thunderstorms. Within this environment, the 1-km simulation produced strong, rotating convection in nearly the same location and at nearly the same time as the observed tornadic storm. Lastly, an assessment of the limited number of observed historical tornadic events in the region showed that with amply convective available potential energy, the 2019 Nepal tornado environment stood out for the limited vertical directional wind shear present.

Keywords

• Tornado
• Nepal
• WRF Model