U.S. Experiences Early Season Severe Thunderstorm and Tornado Outbreak: AIR

Alabama Tornado 2012According to catastrophe modeling firm AIR Worldwide, just two days after a series of tornadoes caused significant destruction in parts of the Ohio Valley and the Central Plains, a massive storm system on March 2 spawned dozens of tornadoes across 11 states. Preliminary data from the National Weather Service (NWS) indicate that more than 100 tornadoes touched down during this latest violent outbreak. Across the southern Ohio Valley and parts of the Southeast are widespread reports of houses ripped off their foundations, roofs blown off, downed power lines and trees, and cars thrown about. The worst affected states are Indiana and Kentucky.

“The outbreak was caused by a strong, spring-like low pressure system that moved out of the southern plains and over the Great Lakes on Friday, March 2,” said Dr. Tim Doggett, principal scientist at AIR Worldwide. “However, this storm system was accompanied by a strong cold front that swept eastward from Kansas to the Appalachian Mountains, before finally moving offshore on March 3. Ahead of the front, warm, humid air was drawn northward from the Gulf of Mexico, providing the fuel for the widespread outbreak of severe thunderstorms.”

“Coupled with this unstable air mass was a potent jet stream disturbance that interacted with the surface low pressure as the system tracked over the southern Ohio Valley. The result was very favorable conditions for multiple large, rotating supercell thunderstorms and long track tornadoes.”

Dr. Doggett continued, “This early season storm system differs somewhat from more typical April or May outbreaks, when higher surface temperatures and dew points are higher provide more moisture for the storms to feed on. However, the winds associated with this surface low pressure system and jet stream aloft were stronger, driven by stronger, more winter-like temperature contrasts across the front. The mixture of necessary ingredients was more than sufficient to spark a significant outbreak of tornadoes, hail, and severe straight-line winds.”

According to AIR, as a result of this recent outbreak, to-date seasonal activity in 2012 is currently higher than normal. However, it should be noted that a few quiet weeks of activity would correct the anomaly and return activity to normal levels. Additionally, these counts represent preliminary tornado reports and, as is often the case in large outbreaks, the final number of actual tornado tracks will likely be lower as multiple reports of the same tornado by different individuals are common. While a more accurate count of discrete events will not be known until later in the year, the preliminary reports do provide a good means for determining the impacted area and the event severity.

Damage assessments from the National Weather Service are ongoing and it will take weeks to develop a more complete picture of the destruction. Residential structures in the affected area are typically of wood-frame construction, which are more vulnerable to high winds and windborne debris than masonry structures. 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. Light-metal structures are the most vulnerable to high winds and can suffer severe to complete damage from tornadoes categorized as EF-2 or higher.

According to AIR, for all types of structures, roofs are often the first part to be damaged by tornado winds, as once a single shingle is removed, neighboring shingles can easily be penetrated and lifted. Tornado winds can peel off unsecured slates, roll metal roofs, and damage windward overhangs and eaves. In the direct path of the tornado, failure of the roof system weakens lateral support of walls, contributing to their collapse. On the periphery of the track, lower damage ratios are expected.

According to data provided by NOAA’s Storm Prediction Center (SPC) as of March 5, the NWS received 128 tornado reports, 290 hail reports, and 454 straight-line wind reports on March 2. An additional 10 tornado, 2 hail, and 29 straight-line wind reports were associated with the storm system as it exited the coast on Saturday.

More than a dozen tornadoes were reported on March 2 in Indiana, with the most severe damage occurring in southern portions of the state. Preliminary NWS survey results found damage consistent with an EF-4 tornado in Jefferson and Clark counties. This tornado ravaged the town of Henryville (estimated population of 1905 in 2010), where homes, businesses, and schools were heavily damaged. Less than 10 miles to the northeast, officials report that the unincorporated town of Marysvillle (estimated population of 2000) was completely destroyed by the tornado. According to the Indiana Department of Homeland Security, 98 homes have been reported as damaged in the state (including 19 destroyed), mostly in the counties of Jefferson, Scott, Ripley, and Washington.

In Kentucky, preliminary reports indicate that some 30 tornadoes touched down, and officials have recorded damage in at least 40 counties. Worst affected is the eastern portion of the state in the foothill communities of the Appalachian Mountains. A tornado tore a path of 34 miles through Morgan and Menifee counties, causing extensive damage. Another tornado touched down in Laurel County and traveled 6.3 miles, causing severe damage in the town of East Bernstadt.

Significant damage has also been reported in Tennessee, where officials estimate that 100 homes were damaged by a tornado in the town of Cleveland, and in southern Ohio, where preliminary reports indicate that eight tornadoes touched down, with one given a preliminary EF-3 rating.

AIR is continuing to monitor the situation and will provide updates if warranted.

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