May 2020 in the Midwest: Heavy Rain Inundates the Month

Just like April, the month of May was extremely erratic in terms of temperatures with the only constant throughout the month being numerous rounds of rain, severe storms and flooding. A few atmospheric mechanisms caused the continued cold and stormy weather, especially during the first half of the month. The first was blocking (high pressure) over Alaska and Greenland which encouraged cold air to move southward. This blocking pattern persisted due to the strong progression of the MJO (Madden Julian Oscillation) across the western hemisphere. Another piece to the puzzle was the continued influx of atmospheric moisture due to an active subtropical jet stream. With these mechanisms in place, storms had sufficient moisture and time to evolve which led severe weather and flooding. This pattern finally eroded during the second half of the month, which allowed warmer air to surge northward. However, the continued influx of moisture maintained numerous periods of flooding and severe weather. Overall, this set-up helped to produce average to just below average temperatures (0-2 degrees below normal) across the Midwest. In terms of precipitation, the added moisture this month led to above average rainfall with areas like Chicago receiving more than double their normal amount!

Wet and cold weather continued from the end of April to the start of May with temperatures in the 60s along with a few showers. These showers were a product of leftover flooding rains that caused several rivers across Chicago to reach minor to moderate flood levels. Then, a warm front pushed in on the 2nd and the 3rd which helped warm afternoon temperatures into the 70s and lower 80s (highest in Ohio). Thanks to the added moisture, the front sparked periods of showers and rain with amounts totalling 0.50” in spots. Then, a cold front moved into the region on the 3rd and into the 4th. This front was coupled with a lake breeze in Chicago, which made temperatures crash more than 20 degrees in just a few hours. Significantly colder temperatures and blustery northeast winds remained entrenched across the region on the 5th - 8th. This stretch of weather was not completely dry as nuisance showers remained scattered across the Midwest. The cold stretch peaked on the morning of the 9th, as temperatures plummeted into the upper 20s to lower 30s! Based on historical weather data, it has been 18 years since Chicagoland has dropped below the freezing mark this late in the spring. In addition, records fell in Cincinnati (old record 30F 1947) and Indianapolis (old record 29F 1947) with temperatures of 28 degrees and 27 degrees respectively. This cold snap created the 4th coldest start to May on record. After this chillly stretch, a line of storms blew through the Midwest on the 10th and 11th. Damaging winds resulted from Chicago to Indianapolis with pea to penny sized hail. However this line strengthened through Dayton, Cincinnati and Columbus and led to down trees from 65 mph winds, along with 1” diameter hail. 

Drier conditions followed these storms on the 13th with temperatures mainly in the 60s before another moisture laden system slowly pushed through the Midwest on the 14th & 15th. Thanks to temperatures in the upper 70s and lower 80s, strong to severe thunderstorms developed mainly in areas around Aurora, IL and northwest of Columbus. These storms mainly produced 60 mph wind gusts which led to downed trees and powerlines; however, the biggest impact from this system was the flooding rain. Within 48 hours, 2.50 - 5.00” of rain fell across Chicago (significantly less east) with around 3.50” falling on the 14th alone, which is more than their normal amount for the month. With all this rain falling in such a short time span, flash flooding and river flooding was observed across Chicago. To add insult to injury, another system moved into the region on the 17th – 19th , which dropped another 2.00 - 4.00” of rain across the Midwest. This reinvigorated the river and flash flooding across the Midwest, with the hardest hit areas mainly confined to Chicago due to the previous saturated grounds. As a result of the excessive rain, rivers across northeastern Illinois rose to moderate to major flood levels! This system also produced an EF-0 tornado just southwest of the Columbus metro on the 18th with winds up to 85 mph that damaged a barn and uprooted multiple trees.

This system finally pushed out of the area on the 20th to 21st with only a few showers remaining east of Chicago, and temperatures in the 50s to lower 60s. The pattern finally broke on the 23rd into the 27th as warmer air surged in. Temperatures into the upper 80s, but a new system caused another round of severe weather. Two tornadoes touched down just west of Chicago metro along with a few wind reports. These caused a gas station to collapse along with numerous large trees uprooted. This system moved east producing quarter to golf ball sized hail as well as more reports of damaging winds. Another system was quick on its heels, allowing a cold front to push into the region. Most areas saw less than a half inch of rain while temperatures fell back into the 70s. Otherwise, the rest of the month was rain free with near normal temperatures.  Overall, the month of May was one for the history books in terms of rainfall. This is the third straight May with a new record for rainfall, with areas seeing almost 300% their normal amount for the month.