August 2020 in the Midwest: Historical Line of Thunderstorms

August is the last month of meteorological summer and it was pretty typical in terms of temperature across the Midwest, but there were a few anomalous warm spots. For example, Indianapolis, Dayton, Cincinnati and Columbus all observed average temperatures, while Chicago came in 4.5 degrees above normal. In terms of precipitation, it was quite drastic across the Midwest where Chicago and Indianapolis recorded only 1" and 2” of rain, respectfully, for the entire month. This is 1 - 4” less than what they typically see for this time of year, while Dayton, Cincinnati and Columbus observed the exact opposite with average to slightly above normal total rainfall. With regards to severe weather, it was lackluster in terms of the number of days of severe wind gusts, hail or tornadoes; but the big exception was a historical Derecho (long duration severe line of storms), which plowed through the Midwest on August 10^th. This created over 700 severe wind reports across the Midwest.

Derecho blasts through the Midwest on August 10th with damaging winds. Courtesy College of DuPage.

The first week of the month started off rather cool and wet with temperatures 5 – 8 degrees below normal (upper 70s to lower 80s) thanks to multiple disturbances pushing through the Midwest. These disturbances led to 1-2” of rain for Indianapolis, Dayton, Cincinnati and Columbus, while Chicago remained on the outskirts with only 0.10” of rain. Then, the patterned flipped on the 9^th and 10^th as a powerful low pressure system moved through the region. This system started off as a cluster of thunderstorms in Nebraska before moving east into Iowa. As these storms trekked into Iowa, they intensified greatly thanks to favorable atmospheric mechanisms, which turned them into a dangerous line of storms. These storms continued to mature and pummeled Chicagoland between 3 and 6 PM before weakening as it hit Indianapolis. In fact, the line of storms was severe from 9 AM to well into the evening. The storms were classified as a derecho, which is a fancy term for a long-lived severe wind event over a great distance. Over 700 severe wind reports were observed from Omaha, NE through IL with 200 focused in northern IL and northwest IN alone. Some gusts were even as high as 140 mph! In addition to the winds, this derecho also produced 15 separate and destructive tornadoes causing even more damage. Unfortunately, 4 people lost their lives on August 10^th due to damage this derecho caused.

After this historic storm, much warmer temperatures and drier conditions pushed into the region between 11^th – 13^th with afternoon highs mainly in the upper 80s to lower 90s. The warmer temperatures continued into the 14^th and 15^th as a disturbance quickly pushed through the region. It produced 0.50 – 1.0” of rain primarily in Dayton, Cincinnati and Columbus, while Chicago and Indianapolis remained on the outskirts once again. Then, a warm and dry stretch followed for Chicago between the 16^th and 22^nd with no measurable precipitation. It was mainly precipitation free for the rest of the Midwest too, but a quick inch of rain fell from a low-pressure system on the 18^th.

The remainder of the month remained above average with temperatures in the 80s and 90s with rounds of severe weather on the 23^rd, 24^th, 25^th and 28^th. The most widespread days were on the 24^th and 28^th with damaging wind reports of 55 - 65 mph in Indianapolis, Dayton, Cincinnati and Columbus. This led to a few downed trees and powerlines. Overall, Chicago and Indianapolis only saw 8 days of measurable precipitation throughout the month of August, with most of their rainfall falling on the historic August 10^th derecho.