On May 7, 2003, about 1615 eastern daylight time, a Boeing 767-300, N650UA, operated by United Airlines as flight 188, was not damaged during an encounter with turbulence near New Market, Virginia. The 2 pilots, 5 flight attendants, and 140 passengers were not injured; however, 1 flight attendant was seriously injured. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the flight destined for Dulles International Airport (IAD), Dulles, Virginia. The scheduled passenger flight departed San Francisco International Airport (SFO), San Francisco, California, at 0849 Pacific daylight time, and was conducted under 14 CFR Part 121. According to a representative of the airline, when the flight was 90 miles west of Dulles, at flight level 210, thunderstorm cell activity was noted on the weather radar. The flight was given clearance to descend to 11,000 feet, and during the descent, an area of "moderate turbulence" was encountered, which lasted about 20-30 seconds. During the encounter, a flight attendant fell and twisted her ankle. Upon arrival at Dulles, the flight attendant was transported to a local hospital and diagnosed with a triple fracture to her right ankle. Weather reported at Dulles Airport, at 1621, included winds from 130 degrees at 4 knots, 10 miles visibility, thunderstorms and light rain, a few clouds at 900 feet, scattered cumulonimbus clouds at 2,300 feet, a broken cloud layer at 4,000 feet, and another broken cloud layer at 14,000 feet. The temperature was reported as 75 degrees Fahrenheit, the dew point was 64 degrees Fahrenheit, and the barometric pressure was 29.81 inches Hg.