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Friday, May 13, 2016

Climate Central: Increasing Tornado Extremes and Variability

Increasing Tornado Extremes
by Climate Central, May 11, 2016

The impact of climate change on tornadoes is still an active topic of study. Early research suggests that the warming earth will provide more energy to produce the storms that generate tornadoes, but less shear to give the necessary spin. Observations over the past few decades have yielded a couple of interesting trends. First, while there are now fewer days with tornadoes in a given year, there are more tornadoes on the days when they do occur. Second, the variability in the number of tornadoes in a month has been increasing.
2014 NOAA study examined trends in the tornado record for a possible climate change signal.
Since the early 1970s, the nationwide average annual number of days with at least one (E)F1 or stronger tornado has dropped from 150 to 100. Yet, there has been an increase in the number of days with a very high number of tornadoes. In the 1970s, the average number of days with more than 30 (E)F1 or stronger tornadoes was less than one. In the last decade, that number had jumped to 3.
Only two years in the record had more than 750 (E)F1+ tornadoes – 1973 and 2011. But even during the active year of 1973, only 2 of those days had more than 30 tornadoes that rated (E)F1 or stronger. In 2011, that number ballooned to 9. Further, 2011 had as many days with more than 30 (E)F1+ tornadoes than the entire period between 1961 and 1981. 
To look for individual state trends, Climate Central compared the average number of tornado days in each state from the first decade of the analysis to the last decade. The core of Tornado Alley (KS, NE, OK, TX) has seen the average number of days with more than one EF1+ tornado drop more than 50% during that 50-year time span.
Another trend we are seeing in the long-term tornado record is the increased variability of tornado counts from year-to-year. In the last 25% of the NOAA study period (19992013), the record for the largest number of tornadoes within a calendar month has been set 6 times. During that same period, the record for the fewest number of tornadoes within a calendar month has been set 5 times and tied twice. In effect, 13 monthly extreme records have been established or tied during the final quarter of the study period.
Not surprisingly, there are several months in which the minimum tornado record count was zero. But when the months with no tornadoes are excluded from the analysis, the study found more monthly records have been established in the most recent 15 years versus the first 45 years of the study period. Those extremes are balanced between records highs and record lows.

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