On 28 May 2021, a Boeing 767-300 climbing over central South Korea with extensive cloud including embedded cumulonimbus present and near an active transverse Jetstream axis below the intended cruise altitude suddenly encountered severe turbulence as it re-entered cloud on a resumed climb. A serious injury occurred to one of the cabin crew who was unable to return to her crew seat and secure herself because of the flight crew’s short notice that turbulence risk would increase from moderate to severe. Opportunities for further improvement in both aircrew turbulence risk prevention procedures and pilots’ turbulence risk response training were identified.