Dr. Jeff Masters ·

Above: The tornado that swept through Marshalltown, Iowa on Thursday, July 19, 2018. Image credit: Benjamin Fuller.

One of the most destructive tornado outbreaks of this relatively quiet 2018 tornado season hit Iowa on Thursday, causing severe damage and up to 17 injuries. Marshalltown (population 28,000), about 50 miles northeast of Des Moines, suffered the loss of the iconic 16-foot cupola on top of the Marshall County Courthouse, which fell 175 feet to the ground. A preliminary analysis showed the town had up to 43 minutes of lead time before the tornado hit. Damage photos I’ve seen suggests EF3 damage in some portions of the town, but a detailed storm survey will have to be done to determine exactly how strong the twister was.

Figure 1. Marshalltown, Iowa suffered the loss of this iconic 16-foot cupola on top of the Marshall County Courthouse, which fell 175 feet to the ground during the July 19, 2018 tornado. Image credit: AP Photo/Charlie Neibergall.

The Marshalltown tornado was just one of many tornadoes that raked Iowa on Thursday. NOAA’s Storm Prediction Center (SPC) logged 28 preliminary reports of tornadoes on Thursday--27 in Iowa and 1 in Minnesota. Minutes before the Marshalltown tornado hit, a separate tornado heavily damaged Bondurant, Iowa (population 3,900), then demolished two buildings at the Vermeer Corp. plant, a farm and construction equipment manufacturer near Pella. Seven people were injured there. See the weather.com write-up on the tornadoes for more details.

Figure 2. Drone image of damage to the Vermeer plant iin Pella, Iowa, after a tornado hit on July 19, 2018. Image credit: Colter Fisher.

One of the biggest severe weather outbreaks of 2018 expected on Friday

The storm system that spawned Thursday’s severe weather has even more favorable conditions for generating severe weather on Friday, according to the latest forecasts from NOAA’s Storm Prediction Center. The continuing severe weather outbreak is associated with a sprawling low-pressure system that was centered over southern Wisconsin on Friday morning. The “warm sector” on the southeastern flank of this low, encompassing much of the Ohio and Tennessee Valley regions into the mid-south, will see very unstable air, due to the strong heating of the surface by the summer sun and a very moist surface airmass (dewpoints in the 70s). A strong jet stream aloft, with its winds changing speed and direction with height, will contribute plenty of twisting motion (50 – 65 knots of shear) to the atmosphere. This shear will allow thunderstorms that develop during the afternoon to become rotating supercells capable of spawning tornadoes.

The full smorgasbord of severe weather is possible today, including damaging straight-line wind gusts, large hail and tornadoes. SPC has put up a “Moderate Risk” area for the center of the action—their second-highest level of severe weather concern. The risk of large hail is the main causative factor for the “moderate risk” designation; SPC was giving the following risks of the various types of severe weather for Friday afternoon:

Tornado: "enhanced risk"

Damaging straight-line thunderstorm winds: "enhanced risk"

Large hail: "moderate risk"

Figure 3. WU depiction of severe weather risk areas for Friday, July 20, 2018, as issued by NOAA/NWS/SPC on Friday morning.

On the whole, a quiet tornado season

As of July 18, the U.S. had seen only seven EF3 tornadoes, and no violent EF4 and EF5 tornadoes. This year is just the third year since 1950, along with 2000 and 2005, without a violent EF4/EF5 touchdown in the first half of the year. So far, only four tornado deaths have been reported in the U.S. in 2018, and we have a shot at breaking the record for the fewest tornado deaths in a year—15, set in 1986. A big test will come on Friday, since the current tornado outbreak is likely to rank as one of the top three outbreaks in 2018 for number of tornadoes. The top two tornado outbreaks so far in 2018:

1) 1) The biggest tornado outbreak of 2018 came on April 13 – 15, when 70 tornadoes touched down in Arkansas, Louisiana, Missouri, Texas, Mississippi, Georgia, Florida, South Carolina, Virginia, Ohio, and North Carolina. Most of the tornadoes hit Arkansas and Louisiana during the evening hours of April 13. The most significant tornadoes were an EF1 that caused a fatality in Red Chute, Louisiana, early on April 14, an upper-end EF2 tornado that impacted eastern sections of Greensboro, North Carolina on April 15, causing 17 injuries, and a significant EF3 tornado that impacted areas from Lynchburg to Elon, Virginia, causing severe damage and at least 10 injuries.

2) 2) A total of 40 tornadoes were confirmed in a May 1 - 3 outbreak in the Great Plains, none of which resulted in injuries or fatalities.