On April 3, 1974, a "Super Outbreak" of tornadoes struck the United States and Canada, causing extensive damage over an 18-hour period and killing 319 people. At the time it was the largest tornado outbreak for a 24-hour period in history. In a span of eighteen hours between April 3 and April 4, 148 tornadoes were sighted in thirteen U.S. states and one Canadian province. Severe weather watches were set in place throughout the region earlier that day, and St. Louis was pummeled with hailstones the size of baseball. In the afternoon a system of tornadoes began to develop in Illinois and the Appalachians, later shifting to the Ohio Valley. Particularly hard-hit were the Kentucky cities of Brandenburg and Louisville; the Ohio cities of Cincinatti and Xenia; Windsor, Ontario; and the states of Indiana and Alabama. In addition to the 319 killed, over 5,000 were injured.