On December 18, 2022, Hawaiian Airlines flight HA35 encountered severe turbulence in a cloud-free region without warning. We simulated this incident using the Model for Prediction Across Scales (MPAS) with a convective permitting grid. We found that the turbulence formed due to the Kelvin-Helmholtz instability (KHI) generated by strong vertical wind shear. At low altitudes, deep convection caused a decrease in wind speed in both upstream and downstream regions. At upper levels, the jet descended and accelerated after flowing over the convection, which acted like a barrier and produced a situation similar to a downslope windstorm. The low Scorer parameter above the jet and the self-induced critical level created the locally enhanced descending jet stream, which destabilized the flow through KHI.