WRAL WeatherCenter Blog

Tornadoes in Our State

Posted November 20, 2006

Although North Carolina is located in a geographic region that favors fewer tornadoes than some other parts of the United States, we are not immune by any means from their formation, as we were reminded, with unfortunate and tragic results, by the funnel that devastated a part of northeastern Columbus County last Thursday morning. The tornado there covered about a seven mile path, with the first mile or so at F3 strength on the current Fujita intensity scale (which will be replaced in 2007 by the "Enhanced Fujita Scale") across a 300 yard width, and the remainder of the path at F1 intensity and about the length of a football field across.

The National Weather Service office in Wilmington, which issued severe thunderstorm and tornado warnings for the cell that spawned the deadly tornado, has established a page on its web site with a path map and damage descriptions covering the storm, in addition to links to the official public report on the tornado and to NEXRAD radar loops showing both the reflectivity and radial velocity field associated with the system. Those loops clearly show a well-defined rotating supercell thunderstorm, with a classic "hook echo" reflectivity signature and a strong mesocyclone indicated by a "velocity couplet." This couplet is visible in the form of a semi-circular bright green area that develops with a smaller bright red area immediately adjacent to it. These colors indicate strong wind flow toward the radar (green) and away from the radar (red) very close to one another, indicative of a strong rotation.

The same deep low pressure area aloft and strong cold frontal approach that triggered the tornado-producing storm was responsible for very strong winds and wind shear through the lower half of the atmosphere Wednesday night into Thursday morning over much of our state, and there were quite a few reports of thunderstorm wind damage in the form of trees down, power outages and some spotty damage to structures, along with a few reports of funnel clouds. That there were no other confirmed tornado touchdowns is probably due to the fact that while dynamics were very strong supporting these thunderstorms (the large wind shear values helped promote formation of cells with rotation, and the high winds speeds meant that thunderstorm downdrafts could carry strong gusts to the surface), instability was weak, with the greatest instability toward the leading edge of the entire complex of storms, where the tornado cell occurred. Had a similar overall scenario developed with greater instability over a larger region, we may well have seen an outbreak of several tornadoes, not unlike these cases from 1992 and 1984.
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