Last winter, the Southern states were ravaged by a record number of February tornadoes from a single U.S. storm system. After the event, the National Oceanic and Atmospheric Administration's National Weather Service Storm Prediction Center collected more than 130 reports of tornadoes, more than 250 reports of straight-line winds and 128 reports of hail--including several reports of hailstones the size of softballs.
While the actual number of tornadoes was likely fewer than the number of tornado reports (80 is a more credible number), such details provided little comfort to the property owners whose homes and business were reduced to rubble.
The cause of the outbreak was a strong, developing low-pressure system that moved out of east Texas. As the system moved into the Mississippi Valley, cold air from the Plains was drawn southward. It clashed with the warm, humid air ahead of the system, forming a cold front. The resulting cold front caused widespread thunderstorm formation in the middle of unstable air feeding in from the Gulf of Mexico. A dynamic jet stream located above the system provided the wind shear that fueled the numerous tornadoes.
Some of the tornadoes were strong EF3 and violent EF4 twisters on the Enhanced Fujita Scale. In Tennessee, a single tornado maintained contact with the ground for about 40 miles, according to the National Weather Service. (The typical path of a tornado is two miles long.) In Arkansas, according to the National Weather Service, a tornado with 200 mph winds cut the longest tornado track since 1950--a reported 123 miles in length.
SURVEYING THE DAMAGE
AIR sent a team to survey the tornado damage on Feb. 7, 2008. The team visited some of the hardest hit regions, including Memphis, Tenn., where a tornado ravaged a shopping mall, and Jackson, Tenn., where dormitories and academic buildings at Union University suffered extreme damage. The team also visited sites in Hohenwald and Centerville, Tenn., as well as the town of Abbeville in Mississippi.
Of the sites AIR visited, the worst damage occurred in Jackson at Union University--home to 3,000 students. An EF4 tornado (100 yards wide with a two-mile track) tore directly through the center of the 288-acre campus, damaging 17 buildings, tearing the roof off a main academic hall and destroying 80 percent of the dormitories.
Dormitories were constructed of either wood frame, wood frame with concrete-block side walls or brick veneer. All had asphalt shingle roofs. Not surprisingly, many of the wood frame dorms sustained damage exceeding 90 percent, and one--Jelks (Figure 1)--was completely destroyed.
However, the wood-frame dorms with concrete-block side walls fared slightly better, as did dorms with brick veneer (Adams and Ellis in Figure 2). Once the building envelope is breached, however, there is little chance for contents to survive. Contents were entirely destroyed in nearly 80 percent of the dormitory facilities.
As a reminder that construction matters even in the face of tornadic winds, damage to two masonry academic buildings across from the dorms, Jennings Hall and White Hall, was largely restricted to their roofs. Of the two, Jennings Hall suffered the more significant damage, with much of its roof entirely torn away (Figure 3).
Vehicles are particularly vulnerable. The tornado also damaged more than 1,000 cars on the campus--tossing many of them hundreds of feet from dormitory parking lots into a nearby field (Figure 4). Total damage to the university from February's tornado outbreak has been estimated at $47 million.
Nearby, in the university neighborhood, the EF4 tornado caused major damage at the intersection of US-45 and Channing Way. As expected, wood-frame and light metal commercial buildings, including an Exxon gas station and several restaurants, suffered tremendous damage, and trees and utility poles in the area were knocked down.
Once again, however, two masonry banks fared better. The tornado ripped off parts of their roofs and, in the case of Bancorp South (Figure 3), blew out doors and windows, but walls remained intact. This is a fairly typical damage pattern. The high winds passing over the roof act like the air moving over an airplane wing: it creates uplift, which tends to raise the roof vertically. Winds are then able to enter, pressurizing the building and blowing out windows that are not already broken by airborne missiles.
Southwest of Union University, in southeast Memphis, the AIR survey team visited the Hickory Ridge Mall, where an EF2 tornado (100 yards wide with a 10-mile track) caused extensive roof damage, most notably to Sears and Macy's. Minor damage to the mall's masonry and stucco exterior was observed, while two steel and glass entrances were more severely impacted. In the mall parking lot, nearly 100 cars were totally destroyed by the tornado.
The same tornado caused substantial damage to buildings up to nine miles from Hickory Ridge Mall, including several commercial structures. About a half-mile from the mall premises, the survey team visited several buildings owned by DSC Logistics (Figure 5).
Two of these buildings--made of tilt-up walls with highly vulnerable corrugated metal roofs--sustained more than 90 percent damage, including contents. A few miles away, buildings belonging to Sharp Solar Panels Manufacturing Facility and Katt Worldwide Logistics suffered severe damage (Figure 5).
In a local strip mall within the same 10-mile radius, large glass windows were blown out of several stores, which also incurred roof and contents damage. In a nearby industrial park, several light metal commercial buildings--including the Hardy Bottling Plant, Memphis Oaks Distribution Center and all three buildings belonging to TCI Tire (Figure 5)--were completely destroyed or suffered extensive damage. The Hardy Bottling Plant lost 120,000 pounds of anhydrous ammonia, which evaporated into the atmosphere.
Elsewhere, in parts of rural Tennessee southwest of Nashville, a tornado obliterated a few small, poorly constructed homes, barns and garages. All that remained at the site of one home was a TV, splintered wood from the structure's frame and insulation.
In the Brushy community in south Centerville, single-family wood-frame houses sustained severe or complete damage from an EF2 tornado. The twister took a severe toll on wooden or corrugated metal roofs in the area. To the south, near the city of Hohenwald, an EF1 tornado caused tremendous damage to more wood frame houses and trailers. Altogether, about 525 homes were destroyed in Tennessee, according to one report.
In Mississippi, residential areas of Abbeville in Lafayette County were hit by an EF3 tornado (1000 yards wide with an 8.5-mile track). The tornado leveled His Harvest Ministries Church (Figure 6)--a wood-frame building with a corrugated metal roof. The same tornado completely destroyed two single-family wooden-frame houses with wood shingle roofs and caused significant damage to single-family homes with brick veneer walls and asphalt shingle roofs (Figure 6).
It has been several years since the United States experienced such damaging tornado activity so early in the year and, indeed, 2008 has gotten off to a very fast start.
As the number and value of properties continue to increase in areas of high hazard, so do losses. And while individual event losses from severe thunderstorm systems are not generally as high as from tropical cyclones, AIR estimates that average annual aggregate losses from severe thunderstorms are roughly comparable to average annual aggregate losses from hurricanes. The Super Tuesday tornado outbreak of February 5 serves as yet another reminder of the importance of actively managing severe thunderstorm risk.
SHIRAJ KHAN, Ph.D., is a research engineer at AIR Worldwide Corp.
June 1, 2008
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