Before takeoff, the aircrew received weather data that included a forecast for a severe thunderstorm watch. While en route, the captain elected to leave the seat belt sign on for the entire flight. The flight was routine until a descent was started into the destination. The captain stated that he was not receiving any radar returns, but based on his observation, he thought that the weather might become bumpy. He announced on the public address system that during the descent, choppy air may be encountered. He directed the flight attendants to discontinue cabin service, secure the cabin, and return to their stations. Also, he requested all passengers to remain seated with their seat belts securely fastened. The "A" flight attendant secured the galley and took her seat. However, the "B" flight attendant was going to her seat when turbulence was encountered. At that time, she lost her balance, hit the ceiling, and then landed on the floor with an injured neck and right foot.