One week after a violent severe weather outbreak across the Southern and Southeastern United States, thousands of disaster response personnel and inspectors remain on the ground to provide emergency relief and security, perform damage assessments, clear debris, and plan recovery efforts. The death toll from the events of April 25–April 28 is currently estimated at 354 across seven states, and thousands have been left homeless when entire neighborhoods were flattened to the ground. It is the second deadliest severe thunderstorm outbreak in U.S. history, after the Tri-State tornado outbreak of 1925.
Based on preliminary assessments, the Mississippi Emergency Management Agency reported 2527 homes damaged, including 993 severely damaged or destroyed, and 104 damaged businesses, including 62 severely damaged or destroyed. The Virginia Department of Emergency Management reports initial estimates of 443 damaged structures, including 137 destroyed or severely damaged. In Georgia an EF-4 tornado touched down in Cartoosa County with winds of 175 mph, damaging or destroying 75–100 homes.
In Alabama, the worst affected state where 38 of 67 counties have been declared as disaster areas, the insurance commissioner has indicated that losses could reach those from Hurricane Ivan (2004) in his state. Information compiled by the American Red Cross points to as many as 5000 properties destroyed in Tuscaloosa alone, a town particularly hard hit on April 27 by a long duration EF-4 tornado with a track length of 80 miles.
The Storm Prediction Center (SPC) of the National Weather Service (NWS) has reported more than 475 eyewitness tornado sightings over the four day period from April 25–April 28. Of these, more than 200 tornado touchdowns have been confirmed by NWS damage surveys, including two EF-5s (which are characterized by wind gusts of over 200 mph).
“These two—the Smithfield tornado in Mississippi and the Hackleburg tornado in Alabama—both occurred on April 27, marking the first time in more than 20 years that two EF-5s occurred on the same day,” said Dr. Tim Doggett, principal scientist at AIR Worldwide . “A total of 12 EF-4 tornadoes (with gusts of 166–200 mph) have been confirmed, and many of these—including the exceptionally damaging Tuscaloosa-Birmingham tornado that remained on the ground for 90 minutes—had path lengths exceeding 70 miles.”
Last week, after search and rescue efforts had largely ceased, an AIR post-disaster survey team visited the worst-hit areas of Alabama around Tuscaloosa and Birmingham to assess building performance, wind speeds, and damage footprints. (AIR will make additional survey findings available in the weeks ahead.)
AIR engineers were particularly interested in the relative performance of commercial structures in the center of the tornado’s track and those on the periphery. Structures located on a tornado’s periphery can be severely damaged as a result missile impacts (i.e., airborne debris) and strong inflow winds, with the result that the overall damage footprint is effectively widened.
“In areas of Tuscaloosa affected by the EF-4 tornado, large commercial structures were reduced to rubble,” continued Dr. Doggett. “Many properties closer to the periphery of the tornado sustained significant damage to their roofs and openings (large plate glass windows and doors). With the building envelope breached, many sustained subsequent structural damage.”
The AIR team also visited the city of Cullman, north of Birmingham, where much of the historic district was left in ruins. The area was home to older commercial buildings and churches, many of which experienced significant roof damage and structural failure.
Dr. Doggett commented, “The two Birmingham suburbs most severely impacted by this same EF-4 tornado-Pratt City and Pleasant Grove-were also surveyed. As one would expect from a tornado of this strength, the damage in these mostly residential areas was near total along the direct path, leaving only slabs of cement foundation in many cases. Along the periphery, the downing of large trees played a significant role in weakening homes, making them susceptible than would have been otherwise; many sustained significant roof damage and structural failure.”
This event once again demonstrates that the losses from severe thunderstorms can be truly catastrophic, and that the risk must be managed—and modeled—as any other catastrophe peril.
AIR will continue to monitor developments from this devastating severe thunderstorm outbreak and will provide updates as warranted.