Terry’s Take: The Iowa tornado season (or lack of it)
The 2013 Iowa severe weather season started out as 2012 ended, on a quiet note. With unseasonably cold air trapped across the region, news headlines were instead dominated by our extended winter and late season rain and snowstorms well into early May. There were a smattering of severe hailstorms from late March through the first half of May, but no tornadoes during these first two months of Iowa’s climatological “tornado season.” This tornado drought, coupled with the earliest end to Iowa’s tornado season on record in 2012 (May 24), enabled Iowa set a record for the most consecutive days between tornadoes on May 15. The old record was 355 days set between May 5, 1955 and April 26, 1956. Iowa almost made it a full year without tornadoes, but a severe weather outbreak on May 19 solidified the 2012-2013 record at 359 days.
On May 19 a supercell thunderstorm over southwestern Iowa morphed into a squall line to the southwest of Des Moines and produced three brief tornadoes along its leading edge in Dallas and Story counties (radar image on left). These tornadoes were rated either EF1 or EF0 and impacted several homes and farms along their paths. A line of thunderstorms lifting north across eastern Iowa dropped a pair of EF1 tornadoes south and west of the Davenport area on May 30, damaging the towns of Conesville and Buffalo. The following day a rain shower over northeastern Iowa produced a short-lived landspout tornado in Floyd County. Landspouts (also known as “cold-air funnels”) are a special type of tornado that form under weak storms and rarely touch down or produce any damage.
The next severe weather outbreak took place on June 12 across northern Iowa, with one storm producing five tornadoes over the span of an hour across Wright and Franklin counties. The town of Belmond was grazed by two tornadoes, including a strong EF3 that severely damaged several homes and businesses. The storm that spawned these tornadoes was known as a “cyclic supercell” or a storm capable of generating multiple tornadoes, possibly at the same time. This was the case with the Belmond storm, where two tornadoes were occurring at the same time at several points during its lifecycle.
The last two tornadoes recorded this season came in late June, with an EF1 squall line tornado tracking through northern Muscatine and a brief landspout touchdown taking place in Webster County. High pressure built into the region as July set in, putting an end to the summer tornado season. While tornadoes are rare in Iowa past the end of meteorological summer, they are not unheard of. One of the more recent late season outbreaks in recent memory was the Woodward/Stratford tornadoes on November 12, 2005.
Should no additional tornadoes touch down in Iowa, the15 tornadoes observed this year would be the lowest since modern record keeping began in 1980. This would eclipse the previous record of 16 tornadoes, set in 2012. The tornado season began on May 19 and ended on June 26, a span of 38 days. This would also be the shortest tornado season on record, beating the old record of 40 days set in…2012. The 2012 and 2013 tornado seasons produced the lowest back-to-back yearly totals in modern history (16 and 15, respectively).
Here is some additional information on the June 24th EF1 Muscatine, Iowa tornado which produced one fatality. http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=dvn&storyid=95633&source=0