July 19th, 2018 was a day of fascination for some, and devastation for others. Nineteen confirmed tornadoes touched down on that Thursday afternoon in Des Moines, Iowa. Two of the tornadoes, both EF-3 rated, touched down in Marshalltown and Pella, damaging structures and injuring people in both cities.
At 2:50 P.M, twin EF-2 tornadoes touched down in Bondurant, Iowa. Here is the remarkable video captured from an Iowa DOT camera.
Shortly after these tornadoes lifted, a new cell quickly formed to the south, and at 3:18, a tornado touched down near Prairie City, Iowa. Multiple tornadoes spun through Iowa cornfields as the storm intensified. In total, there were four tornadoes verified in Prairie City, Monroe, and Otley. The storm continued to gain strength before putting down an EF-3 tornado north of Pella. This tornado touched down at 4:03 PM and stayed on the ground for an unusual 20 minutes. Miraculously, this tornado missed the heavily populated north side of Pella by less than a mile. As the tornado headed southeast, the town was spared for the most part, but extensive damage was done to Vermeer Corporation, one of Pella’s largest businesses. Below is the incredible video I shot myself of the tornado behind Vermeer just minutes before taking a direct hit.
The Pella tornado was not the only damaging tornado that afternoon. At 4:24 PM, a supercell rapidly intensified west of Marshalltown and headed straight for downtown. This EF-3 tornado was on the ground for over 15 minutes and cut right through the north part of the city. Here is the footage of the tornado tearing through the city.
— Trenten (@tfinders34) July 19, 2018
Although the EF-3 tornado missed the majority of Pella, Vermeer Corporation took a direct hit. Of the corporation’s six plants in Pella, two were heavily damaged. One year later, Vermeer has begun construction on its brand new plant to replace the destroyed buildings. The new plant is planned to be completed towards the end of 2020, a bit more than a year from now.
A similar EF-3 tornado impacted downtown Marshalltown that afternoon as well. The damage really hit home for many people as a great number of historic buildings in the town were destroyed. Around thirty buildings in the downtown area have already been or are yet to be torn down.
There were hundreds of people in Vermeer’s building at the time, including visitors from foreign countries. Not to mention, all of the usual residents in the center of Marshalltown at the time. Thanks to twenty minutes of lead time from the National Weather Service and well-coordinated protocol, not a single life was lost on July 19th. On top of that, emergency management was on the scene within minutes, taking a few people with only minor injuries to the hospital.
While there were several other damaging tornadoes throughout Iowa, the most notable damage was done to Marshalltown and Pella. A collection of damage photos can be found at the National Weather Service website.
July tornadoes are very rare as far south as Iowa, though not unheard of. Even looking at 500mb layer wind maps from three days out, it was quite obvious that a remarkable amount of wind shear for mid-summer would be in place over central Iowa. Ahead of the outbreak, the SPC issued a slight risk for east central Iowa, including a 2% tornado risk. A triple point was set up over north-central Iowa through the afternoon, a warm front stretching to the southeast, and a cold front stretching south along I-35. The first matter of forecasting the storms would be to verify that the morning rain would clear and there would be enough instability to fuel supercells. There was morning rain as forecasted, but the clouds were fully cleared in Central Iowa by 11 A.M. This allowed plenty of time for the sun to heat up the humid summer air.
The next important matter was whether or not these storms could first initiate in the warm sector, then move the correct direction and speed to latch onto the warm front. It became apparent by midday that these supercells were going to ride the warm front for quite some time, and the SPC upgraded the tornado risk to 5%, as well as issue a tornado watch. By mid-afternoon over 3000 j/kg of surface-based cape as well as 200-300 m2/s2 of storm-relative helicity was present in the warm sector.
Low-topped supercells initiated near the triple point and quickly produced several brief tornadoes. Shortly after 2:30 a supercell initiated on the warm front and dropped several tornadoes near Bondurant. Over the next half hour, another cell developed to the south, combining with the Bondurant storm. As the supercell cycled, it took a right turn, aligning its motion with the warm front and extending its lifespan by hours. The supercell moved southeast at a slow 25 mph, putting down several more tornadoes. One of these was the Pella tornado. This particular supercell lived nearly six hours, crossing the length of half the state.
The Marshalltown supercell was a lower-topped and shorter-lived supercell that rode the triple point for about an hour. With limited instability and time, it was truly incredible that this storm produced a tornado of that size and strength. The storm moved straight east, meaning it crossed the warm front an hour after beginning, and quickly died out.
July 19th will be a day that Meteorologists and people of Iowa will remember forever. It will be both a testament to our much-improved tornado warning system as well as evidence of how far from perfect our severe weather forecasting still is.