On September 16, 2006, about 2030 central daylight time, a Boeing 737-700, N793SA, operated by Southwest Airlines as flight 2877, was not damaged when it encountered turbulence while in cruise flight near Hastings, Nebraska. The 2 airline transport rated flight crew members, 1 flight attendant, and 137 passengers were not injured. One flight attendant sustained serious injuries, and another flight attendant sustained minor injuries. Instrument meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that departed Mc Carran International Airport (LAS), Las Vegas, Nevada, destined for Albany International Airport (ALB), Albany, New York. The scheduled air carrier flight was conducted under 14 CFR Part 121. According to the flight crew, the airplane was in cruise flight at 37,000 feet, and was approaching an area of thunderstorm activity. The seat belt sign was illuminated, and the flight attendants were asked to return to their seats. The Captain reported that he responded to a traffic collision alert by descending, and making a right turn. He then attempted to fly the best route through the weather; however, the airplane encountered moderate turbulence for about 30 seconds. According to a flight attendant, the Captain informed the flight attendants to be seated; however, before he could return to his seat, he and another flight attendant were repeatedly thrown from the floor to the ceiling, by severe turbulence. The other flight attendant sustained a fractured ankle and a head injury. The flight continued to ALB and landed without further incident. Southwest Airlines further reported that the traffic alert and collision avoidance system (TCAS) resolution advisory (RA) called for a descent for 10 seconds, and for a "Do Not Climb" for 17 seconds. Shortly after the TCAS reported "Clear of Conflict," the airplane encountered turbulence. The airplane's vertical acceleration during the event varied from 1.75 to 0.3gs. The lateral and longitudinal accelerations varied from +/- 0.2 gs, and +/- 0.1 gs; respectively. Data from the airplane's flight data recorder was downloaded by Southwest Airlines, and provided to the Safety Board for review. The data indicated that the upset was consistent with the airplane's encounter with turbulence.