NewsMay 3, 2013

U.S. Sees Record-Low Tornadoes and Tornado Deaths

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Andrew Freedman

By Andrew Freedman

Just two years after the U.S. experienced one of the deadliest and busiest tornado seasons on record, it appears that Mother Nature has done a complete reversal, setting an apparent record for the fewest tornadoes during any 12-month period, going all the way back to 1954.

The National Severe Storms Laboratory (NSSL) in Norman, Okla., estimates that, between May 2012 and April 2013, there were just 197 tornadoes ranked EF-1 or stronger on the Enhanced Fujita scale. That beats the previous 12-month low, which was 247 tornadoes from June 1991 and May 1992.

2013 tornado activity compared to the long-term average.
Click on the image to enlarge.

According to NSSL statistics, the apparent record follows less than two years after a different kind of record was set, that for the most EF-1 or stronger tornadoes in a 12-month period. Between June 2010 and May 2011, there were 1,050 such twisters. 

Tornado researcher Harold Brooks wrote on NSSL’s blog that the death toll from tornadoes during the past 12 months was also flirting with a record low, with seven tornado fatalities during the period. That's the lowest 12-month tornado death toll on record dating to 1950, but research has shown that the 12-month period starting in September 1899 may have been even quieter, with five fatalities.

The U.S. did set a record for the longest streak of days without a tornado-related fatality — at 220 days — between June 24, 2012 and Jan. 26, 2013. And July 2012, which was the hottest month on record in the U.S., saw the fewest tornadoes on record for any July. 

The drought that enveloped the majority of the lower 48 states during the past year has contributed greatly to the paucity of tornadoes, since the dry conditions have robbed the atmosphere of the water vapor that fuels severe thunderstorms. Other tornado ingredients, such as strong upper-level winds and atmospheric wind shear, have also been missing.

A time series showing the evolution of the number of EF1+ tornadoes since 1954. The number of EF1+ tornadoes in the 12 months beginning with the time on the x-axis is plotted for every month starting in January 1954 and ending in May 2012, the most recent point.
Credit: Harold Brooks, NSSL.

This spring, a persistent dip, or trough, in the jet stream across the West, Midwest, and South Central states has squelched widespread severe thunderstorm activity, instead resulting in record cold and record-breaking spring snowfall. April 2013 contrasts sharply with April 2011, which set the record for the most tornadoes of any month on record, with 358. The weather pattern then, though, consisted of a westerly air flow across the U.S., with low-pressure systems that brought slow-moving cold fronts that collided with warm, moist air from the Gulf of Mexico, yielding major severe weather outbreaks.

Since tornado seasons vary considerably from one year to the next due to natural variability, it is unclear that the absence of tornadoes during the past 12 months has anything to do with global warming, just as it's unclear if the 2011 tornado outbreaks were connected to it, either.

Tornadoes are complicated beasts, affected not only by moisture and temperature but also by wind shear and other factors. So far, there’s simply not enough information to say anything definitive about the future of tornadoes under climate change. Studies of how the environment that gives rise to severe thunderstorms and tornadoes may change as global warming continues shows that the number of thunderstorm days may increase in parts of the U.S. — owing to an upward trend in heat and humidity — but wind shear may decrease, which could curtail tornado numbers.

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