Southwest Airlines flight 2392, operating between Chicago Midway International Airport (MDW), Chicago, Illinois and General Edward Lawrence Logan International Airport (BOS), Boston, Massachusetts encountered moderate turbulence while in descent to BOS. One flight attendant sustained a serious injury and three sustained minor injuries during the turbulence encounter. The aircraft continued to BOS and landed without further incident. The captain was the pilot monitoring and the first officer was the pilot flying. Prior to the flight, the flight crew reviewed the dispatch release and weather forecasts, which indicated turbulence during departure and two weather systems enroute. The flight crew instructed the flight attendants (FA) to remain seated until the turbulence subsided. About 20 minutes into the flight, the flight crew coordinated with the FAs on timing of cabin service and agreed they would be seated if the flight encountered turbulence. While briefing the BOS approach, the flight crew observed that the weather conditions had deteriorated. The flight crew prepared for a Category III (CAT III) instrument landing, which would require a transfer of controls from the first officer to the captain. The flight crew did not anticipate turbulence during descent into BOS. While descending through 14,000 ft, the airplane encountered unexpected moderate turbulence, however, the flight crew recalled that nothing was depicted on the airplane’s weather radar at that time. The flight crew attempted to deviate from their course to avoid further turbulence, but they continued to encounter light to moderate turbulence. At the time of the turbulence encounter, the FAs were preparing the cabin for landing. Two FAs, in the aft galley, were thrown into the air and then onto the floor. One of the forward FAs and a passenger assisted both injured FAs to passenger seats in the last row, then the forward FA took an aft jumpseat for landing. The flight crew requested emergency medical personnel meet the airplane at the gate. The landing was uneventful and both FAs were subsequently transported to the hospital. One FA was treated and released and the other diagnosed with a fractured left foot. The two other FAs reported minor injuries. A National Weather Service Convective Significant Meteorological Information (SIGMET) was valid for an area about 75 miles west of the event location. SkyPath, a crowdsourced turbulence tracking tool for pilots, did not depict turbulence in the vicinity of the flight. The flight recorded a moderate turbulence event in SkyPath. The hourly meteorological aerodrome report (METAR) for Rhode Island T.F. Green International Airport (PVD), Warwick, Rhode Island, located about 4 miles east of the turbulence encounter, indicated that the cloud ceiling had descended from 11,000 ft above ground level (agl) to about 9,500 ft agl.