News•January 11, 2013

U.S. Sets Record for Days Without a Deadly Tornado

While the contiguous U.S. suffered through its second-most extreme weather year on record since 1910, with a litany of extreme events from scorching heat waves to parching drought affecting the country from coast-to-coast, there was one type of deadly and damaging severe weather that was notably missing: tornadoes. According to statistics from the Storm Prediction Center in Norman, Okla., 2012 had the fewest tornadoes since 1954, using adjusted figures to account for changes in tornado detection and observation methods over time.

Chart of the adjusted tornado counts for 2012 compared to other years. The count for 2012 given in this chart is close to the final official total.
Click to enlarge the image. Credit: NOAA/SPC.

The total number of tornadoes during 2012 was 936, although the number could be adjusted slightly before it is finalized, said Harold Brooks, a tornado expert at the National Severe Storms Laboratory in Norman. The last time there were fewer than 1,000 observed tornadoes in a single season was in 2002, Brooks said.

In addition, the U.S. has broken a record for the longest streak of consecutive days without a tornado-related fatality. The last death as a result of a tornado was recorded on June 24, which means that through Jan. 11, there have been 201 straight days without a tornado death. That eclipses the old record of 197 days that occurred between August 1986 and February 1997, Brooks said.

“When you don’t have as many tornadoes that tends typically to lower the fatalities . . . and there wasn’t that much that got hit in the last six months of the year,” Brooks said.

The 2012 tornado season was a bipolar one, having started fast and furious, only to peter out starting in the summer. Through April 14, 2012, the year was running ahead of 2011 in terms of the number of tornadoes and fatalities, Brooks said. Then a strong, persistent dome of high pressure set up across the Central U.S., and the dry, sinking, stable air mass associated with it inhibited thunderstorm and tornado formation. After an active spring, it was as if Mother Nature flipped a switch and shut down the tornado season.

Tornado damage in Tuscaloosa, Ala. from a massive tornado that struck there in 2011.
Credit: NOAA.

Tornado activity during six of the past nine months of the year was well below average. The month of July, for example, saw the fewest number of tornadoes since modern records began in 1954, with more tornadoes occurring in Canada than in the U.S.

Because tornadoes require a particular combination of ingredients in order to form, the number of tornadoes is highly dependent on the prevailing weather patterns, and tornado statistics tend to fluctuate considerably from one year to the next.

During 2012, the same weather pattern associated with the record heat and drought also stifled tornado activity by keeping a hot, dry, and stable air mass in place across Tornado Alley. The heart of Tornado Alley was where the drought was most intense. For example, Nebraska had its driest year on record last year, and extreme drought conditions were present in Oklahoma, Missouri, Iowa, and other states where spring and summer twisters are typical.

The difference between the 2011 tornado season, which was one of the deadliest and most active on record since modern, reliable records began in the early 1950s, is especially striking. There were 1,692 tornadoes in 2011, which was well above the annual average of 1,300, and 553 tornado deaths, which was the fourth-largest death toll since 1925. In 2012, only 68 fatalities occurred, which is below the annual average of 91.

In order for tornadoes to form there needs to be an ample supply of warm, moist air, strong winds aloft (this is necessary to create wind shear, the most critical ingredient of all), and a trigger mechanism to spark thunderstorms, such as a cold front.

This year, there were often times when some of these ingredients were present, but not all of them, or not all in the right amounts. There were some noteworthy severe weather events this summer, including the derecho event that caused heavy damage to the Ohio Valley and Mid-Atlantic states.

As for tornado potential in 2013, Brooks said the upcoming weather patterns do have the potential to generate some tornadoes as Arctic air clashes with warmer, more humid air across the South, but it’s unclear if any major outbreaks will occur.