EF3 Tornado — Marshall, Iowa

2018-07-19 · near Marietta, Marshall, Iowa

Event narrative

This tornado developed northwest of Marshalltown west of the little hamlet of Marietta. The tornado traveled southeast on the western side of the Iowa River producing extensive crop and tree damage. The tornado strengthened quickly and took on a wedge appearance as it entered northwest Marshalltown. The tornado tracked more easterly as it entered town producing widespread EF1 and EF2 damage. The tornado went directly through the business district of Marshalltown with EF2 and EF3 damage to many structures including the county courthouse which lost the spire off the clock tower. The tornado continued east through housing areas before encountering the Lennox plant on the east side of town, producing EF3 damage once again. The tornado began to weaken rapidly after this point and dissipated near power plant located on the east side of town. The tornado produced a total of 22 direct injuries. An additional indirect injury resulted during the clean-up of storm debris when a tree fell on a man.

Wider weather episode

A setup for some rotating storms was in place over much of central and southern Iowa. A low pressure system situated northwest of the state slid to the northeast throughout the day, eventually moving into western and central Minnesota. Given its fairly mature frontal situation, an occluded front dropped into northwest Iowa with the warm front extending E/SE from central Iowa and the cold front dropping back to the SW. At the surface within the warm sector observations were favorable with temperatures into the mid 80s and dew points in the low to mid 70s. Looking in more detail, lifted condensation levels were quite low with values under 750 meters, MUCAPE values were modest in the 1000-3000 J/kg range, effective bulk shear was in excess of 40 kts, and effective storm relative helicity was around 200 m2/s2 to name a few. Resulting composite indices correctly highlighted an increased chance for supercells and tornadic potential as seen with supercell composite values in excess of 12 and significant tornado parameter values in the 3 to 5 range in the afternoon. All of which taking into account the general setup, understandably unable to account for storms potentially tapping into additional shear and helicity generated from rooting themselves along outflow boundaries.

The result was a sizable tornado outbreak across Iowa, including more than a dozen confirmed tornadoes. The tornadoes of most note occurred in the NE Des Moines Metro (specifically Bondurant), Pella, and Marshalltown. The Bondurant area actually experienced two tornadoes, which were concurrently on the ground, but fortunately only one actually caused damage (EF2) within town. The Pella tornado tacked on the north side of town, directly hitting the Vermeer plant, causing EF3 damage. The Marshalltown tornado entered the town from the NW and tracked into the downtown area before exiting and weakening east of town. Widespread damage was experienced in Marshalltown including to the Lennox plant and the County Courthouse. All in all, while numerous injuries were experienced and millions of dollars of damage, no direct fatalities were reported from the tornadoes.

View location on OpenStreetMap → (42.0829, -93.0275)

Source: NOAA Storm Events Database, event_id 766075. Narrative written by the NWS forecast office that issued the report.