In a good news, bad news situation, the United States now sees fewer days per year with tornadoes than it did decades ago. But the number of days with multiple tornadoes is up.
Between 1954 and 2013, the annual number of tornadoes with wind gust speeds greater than 138 kilometers per hour, called EF1+ tornadoes, stayed steady on average (top graph), researchers report in the Oct. 17 Science. It’s the timing that has changed: Since the 1970s, twisters have clustered in fewer days (bottom graph). Now, a given year will see an average of three days with more than 30 tornadoes, compared with the average of one day or less from a few decades ago.