1Most severe tornado damage; see Fujita scale 2Time from first tornado to last tornado
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 37 injuries, 13 occurred in Pella, and 23 in Marshalltown, and one the following day in Indiana. 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 the severe weather that day 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. This was due to an inaccurate analysis of a closed low over the northern part of the state . At the time, the low was analyzed as being closed at the 500 mb level. However, reanalysis of the event revealed that the low was closed at only 700 mb. 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 on July 19 (4 pm CDT), a large Storm-Relative Helicity (SRH) and low level wind shear was reported across the region. The dewpoints were also in the 70s°F (20s°C), which is moist for this type of event.[4] Around this time, the southern-most 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 wind 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.[5] However, that was the only casualty, as no tornadoes that day were stronger than EF1 intensity.
This low-end EF2 tornado occurred simultaneously with the tornado below. A well-built metal outbuilding had most of its roof removed and sustained collapse of two walls. Numerous trees were snapped or uprooted, and severe crop damage occurred along the path.[11]
A low-end EF2 tornado 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.[12]
The fourth produced by the Pella supercell, this large cone tornado clipped the eastern edge of Pella, causing major structural damage to large factory buildings at the Vermeer plant complex. 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.[22][23]
A large and intense multiple-vortex wedge tornado moved directly through downtown Marshalltown, prompting the issuance of a tornado emergency and causing widespread major damage. Numerous brick businesses in the downtown area were severely damaged, some of which sustained total destruction of their upper floors. Streets in this area were littered with bricks and structural debris, and the Marshall County Courthouse had the top portion of its clock tower ripped off. A large office building had much of its facade ripped off, and sustained loss of exterior walls on multiple floors. Homes in residential areas of town were heavily damaged and a few were destroyed, along with numerous vehicles and detached garages. A Lennox International plant had significant roof loss and collapse of multiple reinforced exterior walls, with many cars tossed and damaged in the parking lot. Numerous trees, light poles, signs, and power poles were downed throughout the town as well. Twenty-three people were injured.[26]
A large historic stone barn was destroyed. Trees, a corn field, and a small farm outbuilding were damaged. This was the seventh tornado produced by the Pella supercell.[28]
A 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.[33]
A 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 50 ft (15 m). Corn crops were flattened, a camper was tossed and flipped over, and single-wide trailers were flipped over as well.[34]
Trees, 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.[37]
A 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.[38]
As part of the storm system as a whole, a duck boat tour sank in Table Rock Lake, Missouri, at approximately 7 pm CDT. Of the 31 people on board, 17 of them died and 7 were injured. Nine of them were in a single family. Wind gusts were reportedly in excess of 60 miles per hour (97 km/h).[40] A wind gust in Branson, Missouri reached 74 miles per hour (119 km/h).[41]
Impact and aftermath
Pella had 14,000 power outages while Bondurant, which got hit by an EF2 tornado, had 1,800 outages.[42] Despite the damage and injuries, though, no one died due to tornadoes, which is likely credited to timely warnings once the tornadoes actually did touch down.[43] Following these tornadoes, several counties in Iowa were declared disaster zones by Kim Reynolds.[44]Marsalltown would suffer even more destruction just over two years later when a powerful derecho produced widespread destruction across the city. Several buildings in downtown Marshalltown were condemned and demolished in the following years as a result of both storms.[45]
^"Minnesota Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office Twin Cities, Minnesota. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF2 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF2 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF3 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF2 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF3 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Des Moines, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Davenport, Iowa. 2018. Retrieved March 11, 2019.
^"Iowa Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Davenport, Iowa. 2018. Retrieved March 11, 2019.
^"Missouri Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Springfield, Missouri. 2018. Retrieved March 11, 2019.
^"Indiana Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Northern Indiana. 2018. Retrieved March 11, 2019.
^"Wisconsin Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Green Bay, Wisconsin. 2018. Retrieved March 11, 2019.
^"Indiana Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Louisville, Kentucky. 2018. Retrieved March 11, 2019.
^"Indiana Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Louisville, Kentucky. 2018. Retrieved March 11, 2019.
^"Indiana Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Northern Indiana. 2018. Retrieved March 11, 2019.
^"Indiana Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Indianapolis, Indiana. 2018. Retrieved March 11, 2019.
^"Florida Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Miami, Florida. 2018. Retrieved March 11, 2019.
^"Kentucky Event Report: EF0 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Louisville, Kentucky. 2018. Retrieved March 11, 2019.
^"Kentucky Event Report: EF1 Tornado" (Report). National Centers for Environmental Information. National Weather Service Weather Forecast Office in Springfield, Missouri. 2018. Retrieved March 11, 2019.
Damage totals in US dollars, unadjusted for inflation. Data from the NOAA National Centers for Environmental Information (NOAA NCEI).
1. This tornado has an incomplete crop damage total from NCEI for part or all of its track.
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