Low Number of Tornadoes in 2013 Linked to Absence of Strong Storm Systems
By by Jillian MacMath, AccuWeather.com Staff Writer
August 22, 2013, 8:58:10 AM EDT
Though the severe weather season of 2013 will likely become known for its destruction in the southern Plains, the number of tornadoes to occur this year bears a stark contrast to average.
Through Aug. 18, 2013, there have only been 716 preliminary local storm reports of tornadoes in the United States, compared to the 2005-2012 average of 1,221 for the date.
To catch up to the average of those years, 2013 would have to double its current amount by the end of December, but experts say that's not likely.
United States Annual Trend of Local Storm Report Tornadoes
"The chance of catching up to normal or average tornado numbers through the end of the year is low," Storm Prediction Center Warning Coordination Meteorologist Gregory Carbin said.
"However, a combination of an increase in tropical cyclone landfalls and the possibility of a late autumn tornado outbreak could turn the curve, so to speak."
According to Carbin, the low number of storms this year is what's keeping the number of tornadoes at bay.
"Since tornadoes require strong shear, the low numbers are a result of a lack of shear and that is tied to the lack of strong/intense weather systems," Carbin said.
Wind shear is the change in direction or speed of wind with altitude. Strong wind shear creates rotation in the atmosphere.
"So, in a sense, the lack of powerful storm systems, especially during the late winter/early spring, is why the tornado numbers are lower than average," he said.
Despite significant tornado events this year such as Moore, Okla., where a massive EF-5 tornado ripped apart nearly everything in its path, it's stronger tornadoes that have been less frequent this year.
Through June 2013, tornadoes rated EF-2 and stronger were running about 37 percent of normal.
Despite the low numbers, 2013 is certainly not the lowest year on record. June of 2013 yielded more than 500 tornadoes. About one-third of the past 60 years have yielded fewer than 500.
Through June, the states showing the greatest below average numbers include Florida and Texas. Oklahoma, Arkansas and Tennessee have seen above average numbers through June.
Regionally, the the northern Plains and Upper Midwest states of South Dakota, Minnesota, Iowa and Illinois were running below average.
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