On July 19 and 20, 2018, an unexpected strong tornado outbreak affected Iowa and nearby areas. The event was triggered mostly by three supercells that produced several tornadoes across southeastern Iowa east and southeast of Des Moines. Two of the tornadoes reached EF3 intensity, affecting Pella and Marshalltown. These were the strongest tornadoes to hit Iowa since June 22, 2015, when another EF3 tornado struck near Lovilia. Of the 36 injuries, 13 occurred in Pella, 22 in Marshalltown, and one the following day in Indiana. One indirect injury also occurred in Marshalltown. The event was also notable due to the Table Rock Lake duck boat accident that killed 17 people and injured 7 others at Table Rock Lake, Missouri.
Meteorological history The tornado outbreak, as well as the other severe weather on July 19, was either poorly forecasted or essentially unforecasted; the outlook from the Storm Prediction Center that day had much of eastern Iowa under a 2% risk area for tornadoes. The outbreak was produced by a closed 700 mb low moved east-southeast over Minnesota and Iowa, with cool air aloft sliding southeast above very warm and moist air at the surface, resulting in strong instability for the system. A surface warm front was also analyzed to branch off an occluded front that was moving through northern Iowa, which generated low-level wind shear that produced conditions favorable enough to spawn tornadoes. However, while the overall environment was well analyzed, what was not well taken into account was the unusual attributes of how the ingredients would come together that afternoon. The system exhibited attributes of a cold-core low, which is typically seen for lows closed at 500 mb rather than 700 mb since these lows are stronger aloft than at the surface. The lower-than-normal altitude of this low led to colder air flowing at a much lower altitude than what is typically over dewpoints in the 70s°F (20s°C), which is moist for this type of event. These unusual environmental aspects were not picked up well by the HRRR model, and while it did hint at the development of two supercells in the eastern part of the state, the initiation of storms occurred at around 2 pm CDT in central Iowa, two to three hours earlier and further west than expected. The larger than normal CAPE and low-level instability allowed these storms to rapidly intensify and become tornadic quickly, which allowed these storms to quickly evolve into supercells that became rooted in the favorable environment.
As the event began, several weak supercells formed over central Iowa and moved eastward, producing multiple weak tornadoes. However, one supercell produced two simultaneous EF2 tornadoes in Bondurant. Shortly afterwards, a tornado watch was issued for the region from 3:10–10 pm CDT. However, a significant tornado outbreak was still not expected as the threat for tornadoes was only increased to 5% at the 20:00 UTC outlook, indicating the threat of only a couple of tornadoes. At 21:00 UTC (4 pm CDT), a large Storm-Relative Helicity (SRH) and low-level wind shear were reported across the region. Around this time, the southernmost storm evolved into a large tornadic supercell and moved southeastward, producing several tornadoes, including an EF3 tornado that passed near Pella. The Bondurant supercell then weakened, but another strong supercell developed to its north and absorbed it as it moved eastward. Fueled by an interaction with an outflow boundary from the southern supercell, the storm produced a large, wedge EF3 tornado that directly struck the town of Marshalltown. Afterwards, the northern supercell and the storms north of it weakened, but the southern supercell continued southeastward and eventually transitioned into a small squall line.
As the system trekked eastwards into July 20, damaging winds and hail became the main threat, with a 30% chance of strong winds across much of Arkansas, Tennessee and Kentucky, and a 45% chance of damaging hail across central Tennessee and Kentucky. However, a 10% tornado risk was also issued for western Tennessee and eastern Kentucky, with the threat of tornadoes stretching from western Arkansas to southern Michigan. An EF1 tornado in Indiana caused an injury to a camper, who was flipped over. However, that was the only casualty, as no tornadoes that day were stronger than EF1 intensity.
Confirmed tornadoes
July 19 event + List of confirmed tornadoes – Thursday, July 19, 2018EF#LocationCounty / ParishStateStart Coord.Time (UTC)Path lengthMax width EF0WNW of CeylonMartinMN17:45–17:56A tornado was reported in the area with trees downed. EF0S of Clarion (1st tornado)WrightIA19:30–19:37A tornado damaged crops. EF0S of Clarion (2nd tornado)WrightIA19:35–19:47A tornado was photographed to the south of Clarion. Only crop damage occurred. EF0NE of WoolstockWrightIA19:42–19:45High-resolution satellite imagery confirmed a tornado over agricultural areas. EF0Northeastern AnkenyPolkIA19:48–19:49Several trees, fencing, and pieces of playground equipment were damaged by this weak tornado. EF2N of BondurantPolkIA19:50–20:02This low-end EF2 tornado occurred simultaneously with the tornado below. A well-built metal outbuilding had most of its roof removed and sustained the collapse of two walls. Numerous trees were snapped or uprooted, and severe crop damage occurred along the path. EF2BondurantPolkIA19:51–20:00This low-end EF2 tornado, which developed as a twin to the tornado above, moved through residential areas in Bondurant. One home had its second story blown off, a few homes had major roof damage, and other homes sustained less severe damage. An unanchored detached garage was swept away, and vehicles were overturned as well. EF0W of Story CityStoryIA19:52–19:57This rope tornado remained over open country, causing only crop damage. EF0N of BlairsburgHamiltonIA19:56–20:05A weak tornado north of Blairsburg damaged crops. EF0NW of Prairie CityJasperIA20:18–20:24This was the first tornado produced by the Pella supercell. It remained over open farm fields, causing no damage. EF0NE of RolandStoryIA20:22–20:30High-resolution satellite imagery confirmed a tornado with damage to crops. EF0SE of Prairie CityJasperIA20:28–20:34This was the second tornado produced by the Pella supercell. Crops and trees were damaged along the path. EF0NE of CollinsStoryIA20:35–20:40High-resolution satellite imagery confirmed a tornado with damage to crops. EF1E of MonroeJasper, MarionIA20:45–20:52This was the third tornado produced by the Pella supercell. A machine shed lost a small part of its roof, and trees and crops were damaged. EF0E of OtleyMarionIA20:56–20:58A brief tornado caused little to no damage. EF3Eastern PellaMarionIA21:01–21:24The fourth produced by the Pella supercell, this intense cone tornado clipped the eastern edge of Pella, causing major structural damage to large factory buildings at the Vermeer plant complex, where a celebration for the company's 70th anniversary was being held. Reinforced masonry exterior walls were bowed in and collapsed at this location, metal support beams were severely bent, and large amounts of roofing was peeled from the buildings and scattered. Numerous vehicles, semi-trailers, and pieces of machinery were thrown and mangled at the plant, some of which were found piled atop each other and wrapped in sheet metal. Outside of town, barns were destroyed and a wide swath of corn was flattened in farm fields. A two-story farm home was shifted off of its foundation, and an addition to the south side of the house was destroyed. A few other homes sustained less severe damage. Thirteen people were injured. EF0E of ClemonsMarshallIA21:11–21:13High-resolution satellite imagery confirmed a tornado with damage to crops. The same storm produced the EF3 Marshalltown tornado. EF0NE of PellaMahaskaIA21:15–21:18A satellite tornado to the Pella EF3 tornado damaged crops. This was the fifth tornado produced by the Pella supercell. EF3W of Marietta to MarshalltownMarshallIA21:24–21:47See section on this tornado – There were 22 direct injuries and one indirect injury. EF0S of OskaloosaMahaskaIA21:50–21:53High-resolution satellite imagery confirmed a tornado with damage to crops. This was the sixth tornado produced by the Pella supercell. EF1NNE of Keosauqua (1st tornado)Van BurenIA23:07–23:15Twin tornadoes formed near Keosauqua, with this being the first one. A large historic stone barn was destroyed. Trees, a cornfield, and a small farm outbuilding were damaged. This was the seventh tornado produced by the Pella supercell. EF1NNE of Keosauqua (2nd tornado)Van BurenIA23:09–23:11Twin tornadoes formed near Keosauqua, with this being the second one. Several large farm outbuildings and equipment were destroyed. Corn crops were damaged. This was the final tornado produced by the Pella supercell.
July 20 event + List of confirmed tornadoes – Friday, July 20, 2018EF#LocationCounty / ParishStateStart Coord.Time (UTC)Path lengthMax width EF0NW of CamdentonCamdenMO09:32–09:33Numerous trees were uprooted and tree limbs were snapped. Power lines were downed as well. EF0SW of BremenMarshallIN17:53–17:55A barn sustained damage to its cinder block exterior wall, corn was flattened, and some tree branches were broken. EF0WNW of WautomaWausharaWI17:55–17:56A small tornado snapped branches off of several pine trees and tossed them into the air. EF0N of SalemWashingtonIN18:03–18:05A weak and intermittent tornado downed a few tree limbs and small fences. Multiple metal roof panels were ripped from a barn, while its overhead garage doors were blown in and the entrance door was ripped off. A house sustained considerable damage to its siding and roof, a small play shed was destroyed, and corn was flattened in a field. EF1S of CorydonHarrisonIN18:04–18:20A multiple-vortex tornado impaled a wooden two-by-eight plank into a concrete grain silo, extensively damaged four large barns, and caused significant tree damage. One tree fell on a garage, several homes sustained substantial roof damage, a pole barn was completely destroyed, and a truck and horse trailer was twisted and moved . Corn crops were flattened, a camper was tossed and flipped over, and single-wide trailers were flipped over as well. EF0N of La FontaineWabashIN19:14–19:15A brief tornado ripped the roof from an outbuilding and snapped or uprooted trees. EF1MoonvilleMadisonIN19:50–19:52A barn was destroyed, a storage shed was flipped onto its side, and trees in town were damaged. EF0ESE of SweetwaterMiami-DadeFL20:22–20:24Trees, mailboxes, street signs, canopies, and basketball hoops all sustained minor damage from a brief tornado that eventually became a waterspout over a large retention pond before lifting. EF0SE of EdmontonMetcalfeKY23:03–23:05A carport was overturned and two metal outbuildings lost roofing material. A double-wide mobile home was shifted off its foundation, with its windows broken and shingles ripped from the roof. Numerous trees were snapped. EF1NE of PricevilleHartKY02:56–02:59A large barn and several large hay bales were destroyed. Numerous trees were snapped along the path.
Marshalltown, Iowa