On August 27, 2019, at 0650 am mountain daylight time, Frontier Airlines as flight 461, an Airbus A320, N328FR, encountered turbulence while on descent to the Denver International Airport (KDEN), Denver, Colorado. As a result of the turbulence, one flight attendant was seriously injured. There were no other injuries to the remainder of the passengers and crew onboard and the airplane was not damaged. The flight was operating under 14 CFR Part 121 as a regularly scheduled passenger flight from the Minneapolis/St. Paul International Airport (KMSP), Minneapolis, Minnesota to KDEN. Visual flight conditions prevailed at the time of the accident. According to the flight crew, as the flight was approaching 15,000 feet, the captain, who was the pilot monitoring, identified a single cumulus cloud in front of them with the tops at about 15,000 feet, and informed the first officer (FO) of possible turbulence. The FO reduced airspeed to 250 knots in anticipation of the turbulence and the captain advised the flight attendants to take their seats. The airplane then entered moderate turbulence for about 10 seconds. According to the flight attendants (FA), very shortly after being notified by the captain, the turbulence was encountered. The aft flight attendant was returning to her jumpseat when the turbulence occurred and she was thrown to the floor. After the encounter, she was helped back to her jumpseat by another FA and they informed the flight crew. The flight was met at the gate by medical personnel and the FA was transported to the hospital and diagnosed with a broken left ankle. An NTSB Weather Study indicated likely mountain wave conditions between 11,000 and 14,000 feet with cloud formation near the accident site and points eastward, and cloud tops near 15,000 ft.