Midwestern U.S. Severe Weather Climatology

April 11, 2023 // Article by: Steven Weinstein

Now that we are well into April, it's time to start thinking about and preparing for severe weather season. Severe weather poses risks to life and property each year. Therefore, this had us wondering, what is the climatology behind severe weather across the Midwest? How many wind, hail, and tornado days does the Midwest see per year, what are the averages, seasonality, and timing of such events, and what are some other statistics behind these meteorological hazards that impact and threaten the region? Let's take a deeper dive into the climatology surrounding severe weather across the Midwest, and briefly explore some of the more extreme severe weather events and outbreaks that have affected the region.

When talking about the Midwest, the rough coverage area examined includes areas all along the I-70 corridor, and more specifically the cities of Chicago, Indianapolis, Cincinnati, Columbus, and Pittsburgh, the counties those cities reside in and immediate surrounding counties. Now you may be wondering what makes a severe day? Well, there are three types of severe weather: wind, hail, and tornadoes. While tornadoes are relatively self explanatory, you may also be curious as to what makes a severe wind or hail day? For the purposes of this climatology study, severe wind and hail days were based on the Storm Prediction Center (SPC) reports, with the criteria for that being winds of at least 58 mph or greater, and severe hail that is 1"+ in diameter. Lastly, a "severe day" is considered as any day across the footprint that had at least one 1 wind, hail, and / or tornado report.

Analyzing some basic counts since 2017, it is evident that wind is by far the most common severe type. Specifically, these cities and surrounding areas have seen 316 total wind days and nearly 1800 total wind reports, 105 hail days with nearly 300 total hail reports, and 47 tornado days which corresponds to nearly 100 total reports. Out of the cumulative 334 severe days that have occurred since 2017, the Midwest typically sees around 56 severe days per year on average. In terms of extremes, the Midwest has seen as much as 71 severe days per year in 2017 and as little as 45 severe days in a year, which occurred in 2022. In terms of frequency of events, wind days are most common with nearly 96% of all severe days since 2017 including at least one wind report. This drops off pretty quickly for hail and tornadoes, however, with ~31% of all severe days seeing at least one hail report, and only 14% of all severe days seeing a tornado. The highest amount of tornado days since 2017 occurred in 2021 with a total of 10, and the least amount of tornado days occured in 2019 with just 3 such events. Moreover, since 2017, 92 total tornadoes have been spawned across the Midwest, with over a third of those having occurred in or around the Chicago area, making this region somewhat of a hotspot.

Courtesy of WeatherWorks Inc.

Looking at the seasonality and timing of day of severe weather events across the Midwest, we see that severe weather peaks in the late spring and into the summer, with around 11 severe days on average occurring in June, which is followed up by 10.5 severe days in July. However, May and August also sees its fair share of severe weather with each month averaging around 10 severe days. In fact, even the lowest number of severe days that have occurred in July since 2017 is still greater than the maximum number of severe days corresponding to the months of October through March. Unsurprisingly, the bulk of significant severe weather days (i.e., days with either 25+ reports, hail of at least 1.5" in diameter, 5+ hail reports, and / or a tornado) occurs during the months of May through July. Additionally, as a general theme, the bulk of all severe weather occurs from May - August with a rather sharp decline in severe weather through the month of September and a significant increase in severe weather through the month of April.

Meanwhile, out of the 334 total severe days since 2017, roughly 78% of them occurred during the afternoon to evening timeframe (~12 - 10 PM), with the remaining 22% of all events falling during the nighttime (~10 PM - 6 AM) and morning (~6 AM - 12 PM) timeframe. While this may be somewhat intuitive as when you typically think of severe weather you probably associate its occurrence with the daytime, you may be wondering what the cause for this is? Essentially, this is due in large part to the presence of the sun during the daytime, which is crucial in heating up the earth and providing enough instability for thunderstorm formation. However, it is worth noting that it can be those more rare nighttime events that prove to be the most hazardous and life threatening, often catching people off guard as they are asleep.

Courtesy of WeatherWorks Inc.

Meanwhile, analyzing the climatology of where severe weather is most prevalent reveals that Southern OH (in and around Cincinnati) is a hotspot, with an average of 22 severe weather days per year. However, Illinois (particularly in and around the Chicago area) comes in at a close second with an average of 18 severe weather days per year, followed by the Pittsburgh area at 17 days and the Central OH / Columbus area at 14 days. This leaves Central Indiana (in and around the Indianapolis region) in last with an average of 13 severe weather days per year. Moreover, breaking down the severe weather types, we found that wind days peak in June with 64, along with hail days with 24. Tornado days peak May - June, with 10 tornado days having occurred in both months since 2017.

While this gives you a general idea about severe weather climatology across the Midwest you may be thinking, what about the significant and higher end severe weather events? Well since 2017, we have had 9 severe weather days that have produced 25 or more wind reports across the Midwest zones, and only 2 that have produced 50 or more combined (wind, hail, tornado) reports. Additionally, while the vast majority of tornadoes across the region are weak with wind speeds of 85 mph or less, more violent tornadoes are not that uncommon. In fact, since 2017, a significant EF-2 rated tornado or higher (i.e., a strong tornado that often produces considerable damage with wind speeds of 111 - 135 mph) has occurred is most of the Midwest zones, with the Chicago area, Cincinnati area, and Pittsburgh areas having seen 1 such event. For the Cincinnati / Southern OH region, this one day was the Memorial Day Tornado Outbreak of May 27th, 2019 that produced a violent EF-4 twister near Dayton, OH. Another such notable event during the period of record we analyzed was the widespread wind producing "derecho" storm on August 10th, 2020 which led to 40 wind reports in and around Chicago, Indianapolis, and Cincinnati, and even more widespread damage outside of these immediate metropolitan areas, that also included many hail and tornado reports. 

We hope these stats give you a better idea of the when and where behind the occurrence of Northeast severe weather events. Also, you may be able to better assess your risks to these severe weather hazards, and thereby be more prepared before the next severe weather event strikes!