Late-May 1998 Tornado Outbreak and Derecho

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.

The derecho was the most violent line of thunderstorm observed on earth during the 1998 calendar year according to the National Weather Service review shortly after the year was over. It was also the climax of an unusually heavy derecho season in the North-Central United States and adjacent parts of Canada and the Northeast U.S., and this storm combined the characteristics of the two major forms of derecho, the serial and progressive derecho. At various points of its evolution, it displayed textbook and/or record manifestations of supercell and derecho-related phenomena such as the right-mover supercell, evolution of supercells into a linear mesoscale feature which rapidly became a derecho, cumulonimbus with overshooting top and dome, bow echo, bookend vortices, regular and rotor downbursts, gust front, gustnado, rear inflow notch, classic derecho radar signature, effects of infrasound and atmospheric electricity, haboobs, and wind effects on bodies of water including seiches and exposure of bottoms of water features by the wind. The disturbance which was originally the derecho finally disappeared off the coast of Norway more than a week later.