On March 1, 2004, at 1422 central standard time, a Boeing 757-200, N516UA, operated by United Airlines as flight 1212 encountered moderate turbulence while in a descent 20 miles west-southwest of Janesville, Wisconsin. One flight attendant suffered a broken ankle and tibia during the encounter. The captain, first officer, 4 flight attendants, and 169 passengers were not injured. The airplane was not damaged. The 14 CFR Part 121 scheduled, domestic, passenger flight was operating in instrument meteorological conditions (IMC) at the time. The flight was on an instrument flight rules (IFR) flight plan. The flight originated from the Portland International Airport (PDX), Portland, Oregon, at 0915 pacific standard time. The flight continued to its planned destination of Chicago International Airport (ORD), Chicago, Illinois, where it landed at 1450. The captain reported the autopilot was engaged from the time they leveled off after departure until reading the final approach fix at ORD. He reported that when they were about 50 nautical miles (nm) west of Mason City, Iowa, they were cleared direct to Janesville (JVL), Wisconsin. He stated they noticed lowing visibilities at lower altitudes and cumulus clouds building in the distance so they turned on the radar and kept it on until landing. The captain reported they were in instrument conditions between flight level (FL) 260 down to 10,000 feet. The captain reported that when they were about 70 nm west of JVL, they were cleared direct to JVL which was a course of about 090 degrees. He reported that when they were about 60 nm west of JVL they turned to a heading of 120 degrees to avoid a green cell that had a 1nm yellow diameter. The captain stated they passed south of the cell in smooth air. He reported that when 20 nm west of JVL they were cleared direct to the KRENA intersection. The captain reported the seat belt sign was turned on as they descended through FL180. He reported they encountered about 15 seconds of moderate turbulence as they passed through 16,000 feet. He reported there were no weather returns in the area and they were back in smooth air after the encounter. The first officer recounted essentially the same events as the captain. However, he reported that he informed the flight attendants to take their seats as soon as they began to encounter the turbulence. He reported he was getting ready to call the flight attendants after the turbulence to make sure they were alright, when the cockpit received a call from the back stating that one of the flight attendants injured her ankle. The first officer reported that they requested paramedics meet the flight upon landing. The injured flight attendant reported she was in the aft galley when they encountered the turbulence. She stated she tried to get in the jumpseat, but was unable to because of the turbulence. The injured flight attendant stated she heard another flight attendant tell a passenger who was in the aft galley to sit on the floor, so she decided to do the same. She stated that as she was sitting down the "plane dropped" and she fell. Once the turbulence subsided, the other flight attendant and a passenger helped her off the floor and into a seat. A flight attendant in the front of the airplane reported she was going to her jumpseat to make a public address announcement when they encountered the turbulence. She stated she flew into the air and as she came back down her arm caught on the jumpseat pulling it open. Another flight attendant in the front of the airplane was restrained by a passenger during the turbulence encounter. None of the flight attendants recalled having any advance warning of the turbulence. Rapidly building convective activity was evident along the upper Mississippi Valley at the time of the accident. Convective tops averaging FL250 in the area were moving north-northeast at 60 knots. Severe Weather Advisory 15S valid from 1731 universal coordinated time (UTC) to 2301 UTC covered the route of flight from the Mississippi River to ORD. The severe thunderstorm watch included a forecast for hail, surface wind gusts to 60 knots, maximum cloud tops to FL450, and a storm motion vector to 240 degrees at 35 knots. Convective SIGMET 25C was valid until 2255UTC. The SIGMET called for an area of embedded thunderstorms moving from 190 degrees at 20 knots. The thunderstorms had tops to FL300. Data from the digital flight data recorder (DFDR) shows the airplane experienced vertical acceleration fluctuations from a minimum of 0.437 g's, to a maximum of 1.994 g's, and back to 0.576 g's in less then 3 seconds.