By Verisk | March 28, 2022

The U.S. experiences more than 1,200 tornadoes each year—a greater number than any other country. Most cross open land but any buildings and vehicles in their path can be seriously damaged. But when tornadoes pass through urban areas, where property is concentrated, the damage can be significant.

On March 21 and 22, 2022, a huge storm system was responsible for more than 60 tornadoes in five states in the southeastern U.S. Widespread damage was caused in Texas and Oklahoma before the system moved through Louisiana, where two tornadoes struck the New Orleans area. One, estimated to have been an EF-1 tornado with maximum winds of 90 mph, struck Lacombe in St. Tammany Parish on the north shore of Lake Pontchartrain. Its path was just over 12 miles long and about 100 yards wide and it damaged homes and trees in the area. The other, now confirmed by the National Weather Service (NWS) to have been an EF-3 with estimated wind speeds of about 160 mph, struck the eastern suburbs of New Orleans at about the same time.

Tornadoes come in various forms and tend to vary in intensity regardless of their shape, size, and location. Weaker tornadoes generally last about 10 minutes and travel only short distances, but the strongest can last for several hours and can travel many miles. Stronger tornadoes tend to be larger than weak ones and longer track tornadoes also tend to be stronger than those that have a short track. New Orleans’ EF-3 was described by the NWS as a ”very narrow, intense tornado.” Its 11-mile path took it over the Mississippi River, through the Lower 9th Ward and parts of St. Bernard Parish, and on to New Orleans East in Orleans Parish.

The tornado caused significant damage in areas impacted by both Hurricane Katrina in 2005 and Hurricane Ida in 2021. The worst damage occurred to the east of the Lower 9th Ward in Arabi. A total of about 150 homes suffered significant damage. There was widespread damage to windows, walls, and roofs, and some homes were shifted from their foundations. Power was lost, and vehicles were overturned—among them a school bus blown 100 yards from its parking spot. One life is known to have been lost.

Tornado wind loads differ from hurricane wind loads in two distinct ways:

  1. Tornadoes are characterized by a central vortex (i.e., a rapidly rotating column of air around a region of low pressure). The central vortex leads to greater upward vertical forces on structural components and higher suction pressures.
  2. The width of a tornado wind field is significantly smaller than that of a hurricane. Tornadoes have diameters on the scale of hundreds of meters and are produced from a single convective storm (i.e., a multi-cell or supercell severe thunderstorm). A tropical cyclone, however, has a diameter on the scale of hundreds of kilometers and comprises several to dozens of convective storms.

A direct hit by a tornado usually causes catastrophic damage to residential buildings and major damage to engineered structures. Roofs, wall covers, and windows are often the first part of a building to be damaged. The high suction pressure induced by tornadoes causes uplifting of roof systems, which leads to progressive damage to the rest of the building. Tornado winds can also peel off unsecured slates, roll metal roofs, and damage windward overhangs and eaves. In addition, tornado-borne flying debris could also cause breaching of windows and create internal pressurization, which exacerbate the uplifting of a roof system. Failure of the roof system weakens lateral support of the walls and compromises the main wind-resisting frames, thus contributing to their collapse as well.

Commercial buildings are, on average, less vulnerable than residential structures or automobiles but exhibit a broader damage distribution due to wide variations in construction practices and design. Older commercial buildings with reinforced concrete or brick exteriors are more resilient than modern commercial structures, particularly those with a great deal of glass.

Louisiana gets an average of 37 tornadoes each year according to the NWS and despite folklore that tornadoes don’t strike coastal areas, the vicinity of New Orleans has seen many. Twisters as strong as Tuesday’s, however, are rare in the area; according to the NWS, since 1980 there have been 59 EF-3 or stronger tornadoes in the state as a whole. Fortunately, on this occasion the twister passed through suburban areas rather than the more densely developed city center. In the past tornadoes have struck other major cities too—Chicago in 1876; Washington, D.C., in 1888; and Detroit in 1997 to give just three examples. Where will be next?

 Be proactive and own your risk with Verisk severe thunderstorm models.

Categories: Severe Thunderstorm

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