Southwest Airlines flight 1118 encountered moderate turbulence while descending through 12,000 ft for landing at Pittsburg International Airport (PIT) Pittsburg, Pennsylvania, and a flight attendant fractured her wrist. The flight crew reported that while descending on the JESEY4 RNAV arrival into PIT they observed low cumulus clouds with tops at about 14,000 ft. The captain notified the flight attendants to take their seats due to the possibility of turbulence. As they descended through the cloud layer at about 12,000 ft the flight encountered an area of moderate turbulence for about 20-30 seconds. After receiving the notification from the captain to take their seats. The “B” and “C” position flight attendants (FAs) moved to their respective jumpseats and were attempting to fasten their safety harnesses when the turbulence event occurred. The “B” position FA was thrown into the air and impacted the jump seat. The “C” position FA was thrown into the air and impacted the aft lavatory fracturing her wrist. She described the turbulence as “like nothing I have ever felt almost like something hit the back of the airplane.” The flight crew stated that they were not notified of the injury until after the plane had landed and they were walking to their accommodations. At the time of the turbulence encounter there were no active significant meteorological information (SIGMETs), convective SIGMETs, airman’s meteorological information (AIRMETs), center weather advisories (CWAs), or pilot reports (PIREPs) of any significant turbulence other than occasional light turbulence/chop over the area. Eddy dissipation rate (EDR) is a universal measure of turbulence rate. Recorded data from the airplane revealed that about 1929 Coordinated Universal Time (UTC), or 1529 local, at an altitude of about 9,970 ft the EDR indicated 0.40 which equates to moderate turbulence. According to the flight crew, neither the clouds nor the turbulence were depicted on their Weather Services International (WSI) Pilotbrief weather application or displayed on their airplane weather radar.