A flight attendant was seriously injured when the airplane encountered convectively induced severe turbulence in cruise flight at 38,000 feet. The operator’s meteorology department had properly forecast the convective activity in the vicinity of the accident site and the flight crew indicated that they were aware of the forecast thunderstorm activity. About 19 minutes before the event, the flight received clearance to deviate around an area of scattered thunderstorms. The monitoring pilot then called the lead flight attendant (LFA) on the interphone and advised her to make sure all the flight attendants took their seats and remained seated until further notice as the flight deviated around some areas of weather and that it should not last much longer than 15 minutes. The pilots reported back on course and believed they were clear of any weather and were discussing allowing the flight attendants to resume their duties when they encountered a brief area of severe turbulence without warning. Both pilots indicated that they were clear of any clouds and were well clear of any echoes when the turbulence occurred. The LFA reported that although it had been ?bumpy? most of the flight, when she received the call from the flight deck, it was smooth, and the seat belt sign was on. She walked to both galleys and told the flight attendants to be seated for the next 15 minutes. According to the LFA, about 12 to 13 minutes later, the turbulence encounter occurred. The two flight attendants in the aft galley misunderstood the LFA’s instructions and believed they had 15 minutes before they were to be seated. Both were standing, completing preparation of their crew meals, and were knocked to the floor during the event. One sustained serious injuries and the other minor injuries. Flight data recorder data indicates that the airplane experienced a vertical acceleration of -0.4 G to 1.8 G, or 2.2 G total change, which corresponds to severe to extreme turbulence. Probable Cause: The failure of the two flight attendants in the aft galley to understand the lead flight attendant’s instructions, which resulted in them not being seated when the flight encountered convectively induced severe turbulence. Contributing to the accident was the convectively induced turbulence.