What happened
The aircraft was operating an en route segment between Las Vegas, Nevada, and Minneapolis, Minnesota, at Flight Level 330. Prior to the incident, the flight had experienced smooth conditions for a minimum of 30 minutes. Suddenly, the crew encountered clear air turbulence (CAT). In response, the flight crew activated the fasten seat belt sign.
At this time, a flight attendant was attempting to secure themselves into a folding seat located in the rear cabin. The sudden turbulence threw the attendant from the seat, resulting in a back injury. Following the incident, the pilot executed an unscheduled landing at Denver, Colorado, without further complications.
Weather reports indicated that no forecast for CAT existed at FL330 at the time, although chop had been reported at lower altitudes. Subsequent reports from two other aircraft crews confirmed mountain wave activity and a significant airspeed loss of 30 to 60 knots in the vicinity. The National Weather Service subsequently issued an advisory effective between 1110 and 1300 MST, warning pilots of locally severe turbulence with mountain wave activity between FL310 and FL450 caused by strong westerly winds aloft.