The captain reported that while flying at 16000 feet in visual flight rules conditions with scattered precipitation, air traffic control (ATC) advised of an area of turbulence that was located approximately 25 to 30 miles from their present position. The flight crew was advised of a heading to fly which led the airplane into a cloud that was not depicted on the weather avoidance radar. “In this cloud we encountered moderate to severe turbulence….” According to the American Eagle Safety/Flight Service Trip Report (FSTR) which is an attachment to this report, “[flight attendant] Alvarez was finishing her beverage service and they hit turbulence. She flew into the air twice and cart came down on her right leg. She was on her way to the galley to stow cart.” The captain also reported that the autopilot which was on at the time, did not disengage. A passenger who is an emergency medical technician (EMT), attended to the passenger and was assisted initially by the first officer. A medical emergency was declared and the flight continued to the destination airport and landed uneventfully. The first report to ATC occurred at 1448 local, when, while approaching the JUNUR intersection, a flightcrew member declared a medical emergency and advised the controller that the flight had encountered turbulence 50 miles back. Probable Cause: The in-flight encounter with moderate to severe turbulence in clouds resulting in the serious injury to the flight attendant. A contributing factor in the accident was the poor in-flight planning by the captain for his allowing beverage service to continue after being advised by ATC of an area of turbulence that was approximately 4 minutes ahead of their position.