On March 9, 2024, at 1331 eastern standard time, Southwest Airlines flight 4318 encountered turbulence during initial descent into Baltimore Washington International airport (BWI), Baltimore, Maryland. The flight was a regularly scheduled passenger flight from Northwest Florida Beaches International Airport (ECP), Panama City, Florida to BWI. As a result of the turbulence, one flight attendant sustained serious injuries. The airplane was not damaged, and the flight continued to BWI without further incident. The flight deck crew reported that they were aware of, and had received, pilot reports of turbulence at lower altitudes on arrival into BWI. They had coordinated with the cabin crew during their preflight briefing about securing the cabin early and reiterated this again (as well as asking them to take their seats afterward) just prior to starting a standard terminal arrival (the RAVNN SIX) into BWI. As they began the arrival, they learned of a pilot report of severe turbulence at flight level (FL) 250 from an airplane on arrival into nearby Ronald Reagan Washington National Airport (DCA), Arlington, Virginia. While the cabin crew were securing the cabin and galleys, the airplane encountered severe turbulence as it descended through FL270. The captain immediately made a public address announcement to the cabin for the flight attendants to take their seats. Flight attendants B and C were ?tossed around?.sent into the air? before landing on the floor of the aft galley. Flight attendant C sustained a hairline fracture to the left arm. A non-revenue Southwest Airlines flight attendant who was seated in the rear of the airplane rendered assistance to flight attendant C. The captain declared a medical emergency and received an expedited approach into BWI. The first officer coordinated with airline operations and arranged for medical personnel to meet the airplane at the gate. A post-accident review of weather records revealed that there were four pilot reports of moderate to severe turbulence from FL160 to FL250 in an area to the southwest of the accident location. A high-resolution rapid refresh numerical model computed for the time and location of the accident revealed that conditions were conducive for moderate clear air turbulence from FL250 to FL280. Infrared and visible satellite imagery depicted a transverse wave cloud pattern (often observed in turbulent conditions) over the accident area. The cloud temperatures were consistent with cloud tops near 33,000 ft above mean sea level. A graphical Airmen?s Meteorological (G-AIRMET) information Tango (turbulence), issued by the National Weather Service, valid at the time of the accident advised of occasional moderate turbulence between FL180 and FL380 for much of the mid-Atlantic and northeast states. Graphic turbulence guidance products predicted areas of moderate turbulence over central Virginia moving northwest between 1300 and 1400 eastern standard time, at FL240. Probable Cause: An encounter with clear air turbulence during descent.