A terrible tornado season

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  • Tornado frequency trends since 2005. Image: NOAA / Storm Prediction Center

    The line of supercell storms approaching Joplin, Missouri, that spawned a devastating tornado on 22 May 2011. Image: NOAA

    The trail of destruction wrought by the Joplin tornado on 22 May, the deadliest twister in the USA since at least 1950. Image: NOAA

    Frequency of tornado watches issued by NOAA's Storm Prediction Center. Image: NOAA / Storm Prediction Center

    The track of the Massachusetts tornado stands out clearly in this natural-colour image from the Landsat satellite. Image: NASA/Landsat Project Science Office

  • A terrible tornado season
    07.06.2011 07:55

     

    The fearsome outbreaks of tornadoes in the USA this spring and early summer have wrought damage in the billions of dollars and caused the loss of hundreds of lives.

    The EF-5 (Enhanced Fujita Scale) tornado that hit the city of Joplin, Missouri, on Sunday May 22 was alone responsible for an estimated 141 deaths (as of 5 June 2011). That made it the deadliest single tornado since official records began in 1950, and probably the eighth worst in US history.

    There were relatively few tornadoes through March but as the first graphic on the left shows, the frequency accelerated at an unprecedented rate through April, when 875 tornadoes were reported - a new record by far for any month, including May which is usually the most active month.

    The previous record for April was 267 tornadoes in 1974, while in May the highest number recorded was 542 in 2003.

    A major tornado outbreak on 26 April 2011 viewed from above. Courtesy of NOAA Visualizations.

    Most of the USA's twisters occur across the Plains, the Southeast and the Midwest but tornadoes have been seen in every state. The fourth image on the left shows geographical ten-year frequencies. Rarely do they break out west of the Rockies but they do move across the Northeast from time to time.

    One such outbreak struck Massachusetts on the first day of June. Two tornadoes were spotted, one of EF-3 strength that flipped cars and severely damaged or destroyed buildings not far from your correspondent's family's home and school in Westfield. Its track farther east is shown in the vivid Landsat satellite image on the left, fifth down.

    Tornadoes of this power are not common in Massachusetts and might be expected to occur every 15 years or so. Only two EF-4 tornadoes have been noted in the state but one of those was New England's deadliest. It struck the town of Worcester, MA., on 9 June 1953, killing 94 people.

    However, there is rarely a year when a weaker twister, EF-2 or lower, is not seen. In fact, the first tornado ever recorded in the USA might have been that witnessed by the Governor of Massachusetts on 5 July 1643.

    There is speculation that this might have been a severe "micro-burst" rather than a twister, though, so the dubious honour might fall to the town of Rehoboth, also Massachusetts, in August 1671.

    By: Stephen Davenport