Incident Overview
An aircraft traveling from Las Vegas, Nevada, to Minneapolis, Minnesota, at flight level 330 (FL330) encountered clear air turbulence (CAT) after at least 30 minutes of smooth flight conditions. The flight crew responded by activating the 'Fasten Seat Belt' sign. A flight attendant, who was attempting to secure themselves in a rear cabin folding seat, was thrown from the seat and sustained a back injury. The pilot subsequently performed an uneventful non-scheduled landing at Denver, Colorado.
Turbulence Encounter
According to the crew, there was no forecast for clear air turbulence at FL330, although 'chop' had been reported at lower altitudes in the area. Later, aircrews from two other aircraft operating in the vicinity reported mountain wave activity and a loss of airspeed ranging from 30 to 60 knots.
Aftermath and Weather Advisory
The National Weather Service issued an advisory effective from 1110 to 1300 Mountain Standard Time, warning pilots of locally severe turbulence associated with mountain wave activity between FL310 and FL450. This condition was attributed to strong westerly winds aloft.