Meteorological Synopsis
A powerful spring-time low pressure system developed across the North American Interior Plains on April 1. While moving into the Mississippi and Ohio Valley areas, a surge of very moist air intensified the storm further while there were sharp temperature contrasts between both sides of the system. NOAA officials were expecting a severe weather outbreak on April 3, but not of the extent which ultimately occurred. Several F2 and F3 tornadoes had struck portions of the Ohio Valley and the South in a separate, earlier outbreak on April 1 and 2, and this earlier storm system included three killer tornadoes in Kentucky, Alabama, and Tennessee. The town of Campbellsburg, northeast of Louisville, was hard-hit in this earlier outbreak, with a large portion of the town destroyed by an F3. Between the two outbreaks, an additional tornado was reported in Indiana in the early morning hours of April 3, several hours before the official start of the outbreak.
On Wednesday, April 3, severe weather watches already were issued from the morning from south of the Great Lakes, while in portions of the Upper Midwest, snow was reported, with heavy rain falling across central Michigan and much of Ontario. St. Louis, Missouri was pounded by a very severe thunderstorm early in the afternoon which, while it did not produce a tornado, did include damaging baseball-sized hailstones.
By the early afternoon, numerous supercells and clusters of thunderstorms developed and the outbreak began quickly, with storms developing in central Illinois and a secondary zone developing near the Appalachians across eastern Tennessee, central Alabama, and northern Georgia. The worst of the outbreak shifted towards the Ohio Valley between 4:30 pm and 6:30 pm EDT where it produced four of the six F5s over a span of just two hours when three powerful supercells traveled across the area—one in central and southern Ohio, a second one across southern Indiana and Ohio, and a third one in northern Kentucky.
During the evening hours, activity again began to escalate farther to the south, with several violent tornadoes crossing the northern third of Alabama. Activity also spread to central Tennessee and eastern Kentucky, with numerous tornadoes, most of which were concentrated in the Cumberland Plateau region. Additional supercells developed across northern Indiana and southern Michigan producing additional violent and/or killer tornadoes between 6:00 pm and 10:00 pm EDT including the Windsor, Ontario tornado. Michigan was not hit as hard as neighboring states or Windsor, with only one twister which hit near Coldwater and Hillsdale causing any fatalities, all in mobile homes; however, thunderstorm downpours caused flash floods, and north of the warm front in the Upper Peninsula, heavy snowfall was reported.
Activity in the south moved towards the Appalachians during the overnight hours and produced the final tornadoes across the southeast during the morning of April 4.
A 2004 survey for Risk Management Solutions, citing an earlier Dr. Ted Fujita study, found that three-quarters of all tornadoes in the 1974 Super Outbreak were produced by 30 'families' of tornadoes; i.e., multiple tornadoes spawned in succession by a single thunderstorm cell. Note that most of these tornadoes were not associated with squall lines. These were long lived and long track supercells.
Read more about this topic: Super Outbreak