North Carolina has a long history of deadly nocturnal (nighttime) tornadoes with many of these tornadoes occurring during the fall secondary severe weather season. Just last year, during the evening of 16 November 2011, a tornado killed two people in Davidson County. On 15 November 2008, several tornadoes touched down after midnight in the Coastal Plain killing two people.
A study by Walker Ashley and others (Ashley et al. 2008) examined tornadoes from 1950 to 2005 and found that 27.3% of all tornadoes across the nation were nocturnal, while 39.3% of tornado fatalities occurred at night. In this study, nocturnal or nighttime tornadoes were defined as tornadoes with start times in local standard time that occur between the local sunset and sunrise. During the 55 year period, tornadoes were nearly 2.5 times as likely to kill during the overnight hours as those during the daytime. Even more alarming, the study found that North Carolina was ranked first with the greatest percentage of killer nocturnal tornadoes. They noted that 81% of all tornado deaths in North Carolina during this period occurred at night while only 28% of all tornadoes in North Carolina during this period actually occurred at night.
The NWS Raleigh includes the previously mentioned statistics in some of our Call To Action statements and public safety messages. Given the incredible tornado outbreak of 16 April 2011 that produced 30 tornadoes and killed 24 people, we were interested in how some of these statistics might have changed.
Student volunteer Dan Brown examined all of the tornado events in NC from 2006 through 2011, extending the previous database to include all tornadoes from 1950 to 2011 for a total of 61 years. We made every effort to follow the same methods as the Ashley et al. study. The new data set from 2006 through 2011 included 230 new tornadoes resulting in 24 new daytime fatalities and 14 new nighttime fatalities.
The combined dataset that stretched from 1950 to 2011 includes a total of 1132 tornadoes with 48 fatalities occurring during the day and 85 at night. The daytime tornado count more than doubled from 18 to 42 during the 6 years of new data. Based on these totals, 67% of all tornado deaths in North Carolina from 1950 to 2011 occurred at night while the percentage of nighttime tornadoes remained steady at only 28%. To put it another way, for every 1,000 nocturnal tornadoes there would be 265 fatalities while for every 1,000 daytime tornadoes, there would be 60 fatalities.
Ashley, W.S., A.J. Krmenec, and R. Schwantes, 2008: Vulnerability due to Nocturnal Tornadoes. Wea. Forecasting, 23, 795–807.