May 30, 1998 Spencer, SD 6 killed 150 injured; town largely destroyed: F4, 1/2 mile wide
This was the deadliest tornado in South Dakota history.
A 1/2-mile wide tornado destroyed most of the town of Spencer. Many structures in the town were completely leveled. Debris was scattered over a large area, and very far downwind; one receipt blown from Spencer was found about 80 miles away after the storm.
The same weather system responsible for this event moved eastward and spawned dozens of additional tornadoes in Pennsylvania and New York State on May 31; Saratoga County, New York was especially hard hit by a ½ mile wide F3 tornado; it injured 68 people and did $60 million in damage. A half dozen other tornadoes did $20million in other locations but amazingly no one was killed in any of these twisters.