By Kevin Skow, Meteorologist Intern
Overall, the 2015 Iowa tornado season has been relatively quiet up through the end of August. A preliminary total of 30 tornadoes have been recorded for the year throughout the state, which is below the average of 46 tornadoes typically seen in a given year. Iowa’s tornado season historically peaks in the months of May and June, though late season outbreaks can occur well into November. With the bulk of the season behind us, here is a recap of some notable tornadoes to strike central Iowa this year.
May 10: Carroll and Calhoun Counties
The first significant tornado of the season to hit central Iowa occurred on the late afternoon of May 10. A small and compact supercell overrode a warm front and, aided by high low-level instability and shear, produced when has been up to this point the longest-tracked tornado of the season for Iowa. This very visible EF1 tornado touched-down at 7:10pm northwest of Lidderdale in northern Carroll County and tracked to the NNE. The broad but thankfully weak tornado made a direct hit on the town of Lake City at 7:32pm, damaging the roofs of a number of homes and businesses, including the school. The tornado continued churning northeastward across rural Calhoun County, hitting several farmsteads along its way, but damage was relatively light. The dying tornado passed just to the west of Rockwell City shortly after 7:50pm and finally dissipated northwest of town at 8:00pm. It travelled for 23.5 miles during the 50 minutes it was on the ground.
The tornado quickly lofted dust and other debris high enough to be sampled by the Doppler radar in Des Moines, 70 miles away and at a radar beam height of 5,000 ft. This shows up as the yellow and green shaded region on the correlation coefficient radar image at right, also provided with a velocity image (left) to show the location of the tornado. This tornadic debris signature reached up to 16,000 ft into the storm! Videos shot of the tornado, as well as satellite imagery of the track in the days afterwards, showed that this tornado was multi-vortex in nature, with the worst damage concentrated in small streaks.
Mid-May to Mid-June
The middle part of May to the middle of June, typically the peak of Iowa’s tornado season, was only characterized by a few weak, but somewhat rare, early morning tornado episodes. The first event early on the morning of May 17 was also responsible for producing a downburst that derailed an 80-car train near Osceola. The five short-lived tornadoes that occurred afterwards were weak but did damage a few farmsteads in Madison and Dallas counties.
Another early morning tornado event took place between 1 and 2 am on June 7 across Webster, Boone, and Story counties. Four very brief tornadoes touched down along the leading edge of a squall line, one of which moved through far southern Ames and caused tree (photo on right) and light building damage. A squall line was also responsible for strong EF1 tornado on the afternoon of June 20 near Eddyville.
June 22: Marion, Lucas, and Monroe Counties
The streak of weak tornadoes came to an end on June 22. A powerful supercell spawned a tornado over far southern Marion County shortly after 5:00pm, which then clipped northeastern Lucas County before strengthening and tracking into northwest Monroe County. While it thankfully stayed out over rural areas of the county during its half hour long life, this now 500 to 600 yard wide EF3 tornado did make a direct hit on one farmstead, completely destroying the house and several outbuildings (photo). Thousands of trees were destroyed by this rain-wrapped tornado as it traversed the hilly and wooded regions of Monroe County on its 11 mile track. The tornado lifted eight miles northwest of Albia, but a second EF2 tornado developed just to the west of town and hopscotched through the southwest part of the city, heavily damaging several businesses and homes.
August 2: Adair and Adams Counties
Following a quiet July, a cold front swept through Iowa on the afternoon of August 2. Thunderstorms erupted along the leading edge of the cold front, aided by over 6000 J/Kg of surface based CAPE. Despite the lack of low level or deep wind shear, the cold front generated enough localized spin along its forward flank to produce a very picturesque tornado just after 6:20pm when this broad circulation was stretched by a developing thunderstorm updraft in southern Adair County. With little in the way of steering winds near the surface, the tornado first drifted southwestward for about a mile, then interacted with another storm outflow boundary and turned southeast and strengthened. Widening to 300 yards, the tornado struck the hamlet of Williamson and inflicted EF1 damage to several buildings. Scouring its way southward, the tornado paused for several minutes two miles south of Williamson before turning to the northeast and dissipating. See the satellite image below that shows this unusual path of this tornado. In the 36 minutes the tornado was on the ground, it traveled only 7.5 miles.