Violent thunderstorms kill 3 in North Carolina; extreme heat continues in Southeast

Posted by: Dr. Jeff Masters, 11:48 AM GMT on July 02, 2012

Violent severe thunderstorms swept through Eastern North Carolina Sunday afternoon and evening, killing three people--one in a collapsed building, and two due to a falling tree. The deadly thunderstorms were fueled by the extreme heat affecting the Southeast, coupled with unusually high levels of moisture. The extraordinary heat and moisture caused high levels of atmospheric instability rarely seen.

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Fortunately, there was very little wind shear Sunday, so the upper-level winds were not able to induce the kind of twisting force needed to generate tornadoes. Sunday's powerful storms brought more power outages and damage to a region still suffering the impacts of Friday's rare derecho event, which killed 14 people and left power outages that still affect at least 2 million people. According to insurance broker Aon Benfield, the storm initially knocked out power to 2.5 million people in Virginia, the largest non-hurricane related power outage in state history. A derecho is a fast-moving, long-lived, violent thunderstorm complex that usually develops along the northern edge of a very hot air mass, in conjunction with an active jet stream. Friday's derecho was one of the largest and most destructive in U.S. history, and compares to the May 8, 2009 derecho that swept across the nation from Kansas to Kentucky, killing six and causing $500 million in damage.

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While the temperatures are generally expected to be a few degrees cooler than what was seen Friday and Saturday, it is not the extremity of the temperatures in a heat wave that result in the highest heat stress to vulnerable people, but rather the length of time very high temperatures last. A multi-day period of exceptionally hot weather often causes high mortality. Yesterday's airmass was exceptionally humid, which greatly increases heat stress, since the body cools less efficiently in humid conditions. Aberdeen, Mississippi recorded a temperature of 104° with a dewpoint of 84° at 3 pm EDT Sunday, resulting in ridiculously high heat index of 136.