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Terry’s Take: Record breaking Iowa tornado drought

Posted on: 4:46 pm, May 17, 2013, by , updated on: 06:44pm, May 17, 2013

Terry Swails Weather Blog

Here’s a remarkable fact, the last time a tornado occurred in Iowa was on May 24th, 2012 in Fayette County near the town of Hawkeye. As of Friday, May 17th , it’s been 358 days since this tornado occurred. That sets a new record for the most consecutive number of days since the last tornado in the state of Iowa. The previous record occurred between the dates of May 5, 1955 and April 26, 1956. Should the streak continue through May 23rd, an entire year would pass without a tornado, something that’s virtually unheard of in Iowa.

PARKERSBURG, IOWA EF5 TORNADO MAY 2008

PARKERSBURG, IOWA EF5 TORNADO MAY 2008

Obviously the streak is impressive in its own right. However, reporting standards in the 1950s were not nearly as robust and comprehensive as they presently are. The lack of digital technology, cell phones, and other communication devices along with education, storm chasing, and widespread spotter networks have increased observing and reporting of tornadoes today. Therefore, the likelihood of having a tornado occur and not be observed (or even observed but not reported) was much greater in the 1950s than what would be expected today.  That makes the current streak even more impressive!
The following graph shows the number of consecutive days without a tornado in Iowa extending from the 1950-1951 cessation period to the 2012-2013 cessation period. The cessation period is considered the time from when the last tornado occurred in first year to the time when the first tornado occurred in the next year. Therefore, in the 1950-1951 (labeled just 1951 in the chart), there were 335 days between the last tornado of 1950 and the first tornado of 1951.

CONSECUTIVE DAYS WITHOUT A TORNADO IN IOWA

CONSECUTIVE DAYS WITHOUT A TORNADO IN IOWA

This record is partly due to the fact that the end of the tornado season in 2012 was so early. In fact, it was the 2nd earliest ending to any tornado (May 24th, 2012) season behind only 1955 (May 6th, 1955). Please see the chart below to see how the ending of the tornado seasons have varied over the years. The average date for the last tornado in any given year from 1950-2012 was September 7th.

AVERAGE START TO THE IOWA TORNADO SEASON

AVERAGE START TO THE IOWA TORNADO SEASON

2012 also had the fewest tornadoes since 1963 when only 15 occurred. However, tornado numbers early in the period of record may suffer given some of the technological and reporting deficiencies as discussed above. The fewest tornadoes during the period of record was 4 which occurred in 1950 which was the first year records were published nationally for tornadoes. The chart below shows the number of tornadoes in Iowa since 1980.

IOWA TORNADOES BY YEAR SINCE 1980

IOWA TORNADOES BY YEAR SINCE 1980

As I write this, the potential for severe weather and tornadoes is increasing later this weekend so it is possible that the streak could come to an end in the next few days.  Let’s hope that’s not the case!

Average number of tornadoes (1980-2012) year-to-date through May 15th is 14
Average number of tornadoes (1980-2012) for the entire year is 47
Earliest beginning of a tornado season (1950-2012) is January 24, 1967
Earliest beginning of a tornado season (1980-2012) is March 6, 1992
Earliest ending of a tornado season (1950-2012) is May 6, 1955
Earliest ending of a tornado season (1980 – 2012) is May 24, 2012
Latest beginning of a tornado season (1950-2012) is June 7, 1978
Latest beginning of a tornado season (1980 – 2012) is June 1, 2010
Latest ending of a tornado season (1950-2012) is December 14, 1975
Latest ending of a tornado season (1980-2012) is December 2, 1982