On August 4, 2009, at about 1715 CDT, Pinnacle Airlines flight 2871, a Bombardier CL 600-2B19, N8543F, from McGhee Tyson Airport (TYS), Knoxville, Tennessee to Detroit Metropolitan Airport (DTW), Detroit, Michigan, encounter severe turbulence during cruise at FL300. Of the 50 passengers and 3 crew members onboard, one flight attendant (FA) received serious injuries and one passenger received minor injuries. According to the flight crew, the flight was going in and out of clouds and was using the weather radar and visual observations to detect cloud formations and precipitation. The turbulence lasted about 10 seconds after entering a cloud formation. There was no radar indications of thunderstorms or precipitation when the turbulence was encountered. No PIREPS had been reported for the area. The FA was unable to continue her duties and directed a passenger on how to use the intercom to communicate with the flight crew. The captain declared a medical emergency and the flight diverted into Louisville International Airport (SDF), Louisville, Kentucky. Emergency services personnel met the airplane at the gate and the passenger and flight attendant were transported to a local hospital. The passenger was released with minor injuries but the flight attendant was diagnosed with a fractured vertebrae.