Southwest Airlines Flight 2782, a scheduled domestic passenger flight from Dallas Love Field Airport (DAL), Dallas, Texas, to Tulsa International Airport (TUL), Tulsa, Oklahoma, encountered moderate turbulence during descent. As a result, a flight attendant sustained a serious injury. According to the flight crew, all flight attendants (FAs) were briefed prior to departure that no cabin service would be conducted due to forecast turbulence. They were instructed to remain seated for the duration of the short 39-minute flight. Following passenger boarding and prior to pushback, the captain made a public address (PA) announcement advising passengers that, due to current and forecasted weather conditions, the FAs would remain seated for safety reasons and cabin service would not be provided. The flight crew reported that before descent, at approximately FL250 (25,000 feet), the captain made a descent PA announcement advising passengers to remain seated due to expected turbulence during the descent into Tulsa. The flight crew reported that they chimed (pressed the ATTEND call button once) the cabin between FL190 and FL180 to indicate final descent. Per Southwest Airlines procedures, the 18,000-foot chime directs flight attendants to secure the cabin for arrival and make passengers final announcements. The policy specifies that if turbulence prevents a compliance check or walkthrough, the flight attendants are required to stop duties and secure themselves in a jumpseat. The flight crew reported the high-low chime occurred earlier than the standard FL180 due to anticipated weather and received the normal confirmation from the cabin crew. The flight crew were set up for the ILS approach to runway 36R and began their descent for the approach. They inquired and received ride reports from Tulsa approach, who indicated light to moderate turbulence and heavy rain reported from a regional jet. During the descent into Tulsa, at approximately 15,000 feet mean sea level (MSL), the airplane encountered moderate turbulence while operating in instrument meteorological conditions (IMC). Despite prior instructions from the flight crew, the FAs in the “C” and “B” position left their jumpseats during final descent. The “C” FA had to use the lavatory and the “B” FA got up to prepare the cabin for landing. FA “B” stated it was an automatic response to prepare the cabin even though she remembered afterwards being briefed to remain seated. FA “B” was standing in the aft galley and making an announcement to the passengers when she experienced two pronounced jolts of turbulence. The turbulence caused the flight attendant to become airborne and subsequently impact the cabin floor with significant force, resulting in serious spinal injuries. The “C” FA notified the flight crew that the “B” FA was injured. The flight crew requested medical assistance before arrival, and emergency personnel met the aircraft at the gate. The injured FA was subsequently transported to a local hospital.