Severe Thunderstorm Outbreak of Nov. 9-11

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Posting Date: November 13, 2002, 12:00:00 AM

From Saturday evening November 9 through Monday, November 11, 2002, an outbreak of severe thunderstorms produced a high number of damaging tornadoes, hailstorms, and windstorms from Louisiana to Pennsylvania. The Storm Prediction Center of the National Weather Service (NWS) lists 68 reports of tornadoes, 163 of hailstorms, and 281 of straight-line windstorms for Sunday, November 10, alone. Current preliminary reports suggest 72 tornadoes affected 12 states between Saturday evening and Monday morning. The exact number of discrete events behind these reports will not be known until the data have been analyzed further. The first storms struck in western Tennessee on Saturday night, November 9. Preliminary estimates from the National Weather Service (NWS) survey team are still sparse, but suggest there were four F2 tornadoes and one F1 tornado in that state. About 40 homes were damaged in Bells, TN. On Sunday, at least five tornadoes struck rural Van Wert County in northwest Ohio, and left a 100-mile swath of widespread damage to homes and businesses. One of those has been classified as a violent F4 tornado with winds of 207 to 260 mph (333 to 418 kph). The last recorded F4 tornado in the area, in February 1992, also occurred in Van Wert County. A second tornado was rated F3, the rest were rated F0 or F1. Also on Sunday, the NWS has verified at least 3 tornadoes in middle Tennessee, two rated at F1 and one at F3. Tens of thousands of people lost electricity. On Sunday evening, at least 7 additional tornadoes were reported in northern Alabama, at least 3 of which were rated F3. Two small towns in Walker County, Carbon Hill and Nauvoo, bore the brunt of the storms. Tornado, wind, or hail damage was also reported in Indiana, Illinois, Georgia, Florida, and Mississippi, and South Carolina. In all, at least 36 people were reported killed, and more than 200 were injured. Meteorologists attribute the rare outbreak to a storm front caused by the collision between a jet stream of cold air moving west to east and strong surface winds moving north from the Gulf of Mexico. The AIR severe thunderstorm modeling team has collected the available information on the overall thunderstorm system, or macroevent, which, in these early stages, consists primarily of the location of reports of individual microevents (tornadoes, hailstorms and straight-line windstorms) and, in the case of tornadoes, estimates of Fujita intensity. In general, exact wind speeds associated with localized events are not known, nor is hailstone size. Furthermore, there is duplication in these early reports and the data must be cleaned to avoid double counting. For example, there may be two or more reports of hail in close physical and temporal proximity to each other; the likelihood is that this represents one hailstorm, not two. Estimating losses from severe thunderstorm events in real time is an extremely challenging task. Microevents are typically of short duration – sometimes lasting only minutes – and more often than not occur out of the range of weather stations or anemometers. When no measured wind speeds are available, scientists and engineers must estimate wind speeds from observed damage. The data gathering and analysis process usually takes several months, after which the data enters the official SPC database. Despite the difficulties surrounding the data in its current form, the AIR team has cleaned the data of what appear to be duplicate microevents, and has run the severe thunderstorm model using certain assumptions regarding model parameters. Three scenarios have been posted on the Scenarios page of this web site. In addition, an event set is available for download by CATMAP/2 and CATRADER clients.

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Posting Date: November 13, 2002, 12:00:00 AM