Reading the article, it looks like there was barely any warning, because it was the type of turbulance that is extremely difficult to detect. Additionally, it seems the dead man had his seat belt on; he died of, as far as they can tell, a fear heart attack, as the plane essentially fell off a midair cliff, dropping 1800 meters (6000 feet) in three minutes. That's over 10 meters (30 feet) a second).