There have been only 16 recorded tornadoes this year, the fewest since 1963, according to State Climatologist Harry Hillaker.
Hillaker told the Gazette in Cedar Rapids (http://bit.ly/We0BfF) that this summer's extreme dry conditions have helped keep tornadoes at bay.
"It's been an extremely quiet year," he said. "Fewer thunderstorms means there's fewer opportunities to get tornadoes."
Over the last 33 years, Iowa has seen an average of 47 tornadoes annually. There were 49 reported during another drought year in 1988, including an outbreak of 25 on Mother's Day. The annual totals vary widely, from 16 to 120.
Hillaker said the tornadoes that have hit Iowa this year were fairly weak, with the three most powerful rated F3 on the enhanced Fujita scale, carrying winds of 158 to 206 mph. The scale was changed in 2007 to more accurately reflect the potential damage caused by different wind speeds. The previous model overestimated the wind speeds required to cause specific levels of damage.
The extreme drought throughout the Plains and the rest of the country sapped the moisture required to trigger a tornado, said Roger Vachalek, a meteorologist with the National Weather Service in Johnston.
"If you have no moisture, you're not going to get thunderstorms," Vachalek said. "And if you don't have thunderstorms, you're not going to have tornadoes."
Hillaker said Iowa likely experienced more than 16 tornadoes in 1963 — meaning this year is even more unusual — but record-keeping wasn't as complete as it is now.
No tornado has touched down in Iowa since May 24.
Peak tornado season in the upper Midwest is late spring to early summer, although tornadoes have been known to occur in every month of the year. A weaker jet stream in the fall makes conditions less likely for tornado formation.
"As always, it can change in a very short time," Hillaker said.
Information from: The Gazette, http://www.gazetteonline.com/
Copyright 2012 The Associated Press.