A flight from Mexico City to Miami experienced a descent due to severe turbulence caused by thunderstorms. An aircraft encountered turbulence at approximately 50 miles from Key West. The aircraft’s airspeed was 270 kts, and it was passing through a cumulus cloud. One passenger sustained a serious injury, and four additional passengers and four flight attendants received minor injuries. A weather forecast revealed a large area of thunderstorms with levels 2 to 5, accompanied by a convective SIGMET. Passengers reported no prior warnings of turbulence, except for the seat belt sign.A flight from Mexico City to Miami experienced a descent due to severe turbulence caused by thunderstorms. An aircraft encountered turbulence at approximately 50 miles from Key West. The aircraft’s airspeed was 270 kts, and it was passing through a cumulus cloud. One passenger sustained a serious injury, and four additional passengers and four flight attendants received minor injuries. A weather forecast revealed a large area of thunderstorms with levels 2 to 5, accompanied by a convective SIGMET. Passengers reported no prior warnings of turbulence, except for the seat belt sign.