United Airlines flight 194 encountered convective turbulence while climbing through 19,000 ft after departure from San Francisco International Airport (SFO), San Francisco, CA, and a flight attendant was seriously injured. Upon being notified of the injury, the flight crew declared an emergency and diverted to Denver, Colorado for medical assistance. The flight crew reported that before departure, the purser (lead flight attendant) was asked to have the flight attendants remain seated until the 10,000 ft signal due to weather conditions at the departure airport. After departure, airspeed fluctuations and turbulence were encountered with a smooth ride once the airplane was above a cloud layer. About 12,000 ft the captain gave the 10,000 ft signal that it was safe for the flight attendants to begin the initial service. When the flight was about 32 miles northeast of SFO the flight crew established communication with Oakland Air Route Traffic Control Center (ARTCC) while climbing through flight level (FL) 175 [17,500 ft]. The controller advised them to expect occasional light turbulence up to FL 260 and instructed them to climb to FL350. Shortly thereafter, as the airplane climbed through 19,000 ft, the flight encountered unexpected turbulence for about 5 seconds. The flight crew then received a call from the cabin advising them that a flight attendant had fractured her leg. At the time of the turbulence event, one of the flight attendants was in the aft galley preparing her cart for the initial service when she felt “violent up and down jolting turbulence”. She attempted to move towards the nearest jump seat when another “intense bump” occurred. She was thrown into the air and impacted the floor fracturing her leg. Airmen’s Meteorological Information (AIRMETs) were in effect at the time of the accident for moderate turbulence below 12,000 ft and moderate turbulence from FL220 to FL360. A convective Significant Meteorological Information (SIGMET) was issued at 1455 PDT and in effect at the time of the accident for the area with areas of thunderstorms forecast with tops to FL290. A convective SIGMET implies severe or greater turbulence, severe icing, and low-level windshear. Numerous pilot reports (PIREPs) reporting moderate or greater turbulence were issued for central California (including the accident area) within two hours of the accident time.