High Quality Content by WIKIPEDIA articles! The May 2004 Tornado Outbreak Sequence was a series of tornado outbreaks that affected much of southern Ontario, the Central and Southern United States from east of the Rockies to the Mid-Atlantic States from May 21 to May 31, 2004. Particularly hard hit were the central Plains from Missouri to Iowa and the Ohio Valley. The Central Plains were hit by two significant outbreak on May 22 and May 24, the first outbreak which produced a very large and violent tornado in Hallam, Nebraska. The Ohio Valley was affected by one of the largest tornado outbreaks ever during the Memorial Day weekend on May 29–30.