High Quality Content by WIKIPEDIA articles! The Late-May 1998 Tornado Outbreak and Derecho was a historic tornado outbreak and derecho that began on the afternoon of May 30 extending throughout May 31, 1998, across a large portion of the northern half of the United States and southern Ontario from southeastern Montana east and southeastward to the Atlantic Ocean. The initial tornado outbreak, including the devastating Spencer tornado, hit southeast South Dakota on the evening of the May 30. The Spencer tornado was the most destructive and second deadliest tornado in South Dakota history. 13 people were killed; 7 by tornadoes and 6 by the derecho, and damage was estimated at least $500 million in damage. Over two million people lost electrical power, some for up to 10 days.