On August 7, 2003, at 1449 mountain daylight time, a McDonnell Douglas DC-9-81, N814ME, registered to and operated by Midwest Airlines, Inc., as flight 9, encountered severe turbulence 5 miles north of the Gill VORTAC, Greeley, Colorado. The 2 pilots, 1 flight attendant and 110 passengers were uninjured; however, 2 flight attendants were seriously injured and 3 passengers received minor injuries. The scheduled domestic passenger flight was being conducted under the provisions of Title 14 CFR Part 121. Instrument meteorological conditions prevailed and an IFR flight plan had been filed. The flight originated at San Francisco, California, at approximately 1255. According to the captain, during cruise flight at 33,000 feet, the aircraft encountered moderate to severe turbulence. The flight was deviating to the south of course to avoid weather. Upon breaking out of a cloud layer, the captain noted a cumulous cloud ahead that could not be avoided. Upon entering the cloud, the airplane encountered severe turbulence. Turbulence had not been forecasted. The seat belt sign had been illuminated for approximately 20 minutes.