Mark Johnson: Where are the tornadoes? Lack of twisters in US sets new record low

CLEVELAND - It's giving world-famous tornado Chaser Reed Timmer a chance to catch up on his house cleaning. He hasn't been very busy so far this spring chasing twisters.

You see, tornado numbers in the United State are down. Way down. In fact, it's the lowest tornado total ever recorded in a 12-month period for our country.

From May 2012 to April 2013, the United States recorded a paltry 197 tornadoes of EF-1 strength or higher (EF = Enhanced Fujita Scale of Tornado Damage). That smashes the previous record low in a one-year period of 247 twisters from June 1991 to May 1992.

What's even more remarkable, this record low follows the record high number of tornadoes in a 12-month period set just two years ago. Between June 2010 and May 2011, the U.S. confirmed 1,050 tornadoes.

We have also seen the number of tornado fatalities drop to near record levels. From May 2012 to April 2013, seven people died in tornado-related events. That's second only to the 12-months from September 1899 to August 1900. During that time, five tornado fatalities were reported.

So, why the big drop? Last summer's persistent drought in our nation's bread basket kept most tornado-producing weather systems at bay. Now, this spring, we have a stubborn pool of cold air that continues to pour in from the north and settles in across the Tornado Alley in Plains States.

Patrick Marsch of the Storm Prediction Center in Norman Oklahoma played around with the numbers a bit. He found that the odds of a tornado surplus, like the one we had two years ago, is likely to happen once every 1,066 years. Astoundingly, he found that a yearly tornado drought, like our current one, would occur about once every 28,571 years.

"As you can see, the current 12-month tornado drought is still a much rarer occurrence." he said.

Here's what Harold Brooks, from the National Severe Storms Laboratory said in the official government press release:

The 12-month period from May 2012 to April 2013 was remarkable for the absence of tornado activity and tornado impacts in the United States.

We can start by looking at the number of EF1 and stronger tornadoes during that period. A final count is available through January 2013 and we have a pretty good estimate of how many occurred in February through April, although final numbers won't be available until July. Although the 12 month total may change a little bit with the final data, it's unlikely to change enough to affect the results here.

From May 2012-April 2013, the estimate is that there were 197 tornadoes rated EF1 or stronger. Where does that stack up historically? Well, we have pretty good data back to 1954. During that time, the previous low for (E)F1 and stronger tornadoes in a 12 consecutive calendar month period was 247, from June 1991-May 1992. The next lowest (ignoring the overlapping periods, such as April 2012-March 2013) was 270 from November 1986-October 1987. The lowest non-overlapping 12 month counts on record from 1954-present are:

1) 197 tornadoes from May 2012-April 2013 (preliminary)
2) 247 tornadoes from June 1991-May 1992
3) 270 tornadoes from November 1986-October 1987
4) 289 tornadoes from December 2001-November 2002
5) 298 tornadoes from June 2000-May 2001

This apparent record was set less than two years after the record for most EF1+ tornadoes in a 12-month period was set, with 1050 from June 2010-May 2011. The time series showing the evolution of the number of (E)F1+ tornadoes since 1954 is included above. The number of (E)F1+ 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.

The death toll from May 2012-April 2013 was 7. National Weather Service official statistics go back to January 1950, but we can extend that by using the work of Tom Grazulis from the Tornado Project, who has collected tornado fatality information back into the 17th century. The data are reasonably good back to 1875, but it's still possible that there are some missed fatalities, particularly as we go back farther in time. So, where does 7 fatalities in 12 consecutive calendar months stack up? Again, here are the lowest totals, going back to 1875, for 12 consecutive months, with the starting month. (For overlapping periods, such as April 2012-March 2013 and May 2012-April 2013, only the lowest period is listed.)

1) 5 deaths from September 1899 thru August 1900
2) 7 deaths from May 2012 thru April 2013
3) 8 deaths from August 1991 thru July 1992
4) 12 deaths from November 1909 thru October 1910
     12 deaths from May 1940 thru April 1941

Mother Nature corrects any extreme with the opposite extreme within a few years.

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