April 27 marked the pinnacle of what was actually a multi-day outbreak spanning April 25-28, with 358 confirmed tornadoes in 21 states. With total damages around $11 billion, it was the costliest outbreak in U.S. history.
What made the outbreak so devastating was not as much the number of tornadoes, but the number of highly destructive tornadoes. As the website USTornadoes.com documents:
On April 27, 15 violent tornadoes rated EF-4 [on 0-5 scale] or higher (including four EF-5s) struck the states of Mississippi, Alabama, Tennessee and Georgia. Alabama was the hardest hit, with 9 violent tornadoes touching down there, and 11 total crossing within its boundaries. The top two deadliest tornadoes in April since modern records began hit that day.
Prior to the 4 EF-5 twisters on April 27, 2011, there had only been two confirmed EF-5 tornadoes since the implementation of the Enhanced Fujita (EF) scale in February 2007.
Alabama T.V. meteorologist James Spann, who earned widespread recognition for his outstanding coverage of the outbreak, worked with his station (ABC 33/40) to produce the following dramatic and moving video:
Video from ABC 33/40: Alabama’s generational tornado outbreak of April 27, 2011. Death toll was 252, so much suffering.
The outbreak was triggered by a powerful jet stream carving a deep trough in the western U.S. before charging East towards the South Central U.S. Warm, moist air out ahead of the jet collided with cooler, drier air behind it setting up a volatile, unstable atmosphere.
According to the Weather Channel’s Dr. Greg Forbes, the combined paths of all the tornadoes from the outbreak totaled over 2,600 miles - greater than the distance from Atlanta to San Francisco.
Northern Virginia-Maryland effects
On April 27 and 28, 19 tornadoes touched down from northern Virginia to northern Maryland. Most of these tornadoes were not destructive, but small spin-ups, rated EF-0 or EF-1. In the Washington, D.C. metro area, tornado watches were in effect for nearly 24 consecutive hours. 38 tornado warnings were issued during this time frame.