The weather for April was extremely erratic and cold at times, thanks to a pattern change. A few different atmospheric mechanisms caused the shift in weather. The first was blocking (high pressure) over Alaska and Greenland which encouraged cold air to move southward. This blocking pattern developed due to the strong progression of the MJO (Madden Julian Oscillation) across the western hemisphere along with a collapsing and weakening polar vortex. This type of pattern was largely missing during the winter time. Another piece to the puzzle was an influx of atmospheric moisture due to an active sub-tropical jet stream. With these mechanisms in place, storms had sufficient cold air, moisture, and time to evolve which led to rounds of severe weather and more snow. This set-up helped to produce below average temperatures (2-4 degrees below normal) across the Midwest. In terms of precipitation, the added moisture this month inherently led to average or above rainfall with areas like Chicago almost doubling their normal amount.
The beginning of the month started off seasonable as a warm front lifted through the Midwest with a few light rain showers. In the wake of this front, temperatures warmed into the 60s and eventually 70s between 2nd and 4th with mainly dry conditions. Then, this system’s cold front pushed through on the 5th causing temperatures to fall into the lower 50s as well as producing a few instances of showers. The active pattern continued into the 7th and 8th as extremely warm temperatures in terms of April standards occurred as afternoon highs rose to near 80 degrees. This was the first time in 2020 that 80 degrees was observed in Chicago (normal is April 20th). The warm weather was coupled with moist air which helped fuel two days of severe weather. Thanks to the cold air aloft on the 7th, most of the severe weather came in the form of hail with some the size of baseballs! This led to sporadic damage especially in Chicago, Dayton and Columbus. Then, the front pushed through Indianapolis, Dayton and Cincinnati on the 8th. This round of severe weather led to numerous strong winds and 19 tornadoes! Wind gusts were observed up to 75 mph which blew down street lights, trees, power lines, and even barns. The tornadoes were mainly EF-0 and Ef-1 based on the enhanced fujita scale, which caused a plethora of damage.
Much cooler weather returned on the 9th, as overnight lows fell into the 30s. With some lingering moisture, a few rain/snow showers fell across Chicago, though only a few coatings were observed on colder and grassy surfaces. Cool and dry weather continued into the 10th before another disturbance returned warm and rainy conditions on the 11th and 12th. This disturbance was not as potent as the previous one, which led to no severe weather. The cold front then pushed through on the 12th and 13th, which produced gusts up to 50 mph as well as a rapid cooldown. Colder air ushered in on the 13th and 14th as another cold front pushed through with a round of snow showers. These were rather light, and led to scattered coatings on grassy surfaces in Chicago. A stronger system revolved through during the morning and early afternoon on the 15th as temperatures were in the upper 20s to lower 30s. This helped produce steady snow with a few heavy bursts across Chicago. While areas towards the Wisconsin border only saw a 0.5”, places southwest of I-90 towards Aurora saw up to 3.0”. With this snow falling in the morning and sticking to roadways, 50 car accidents occurred, even on major highways and thoroughfares.
Snow falls over Chicago on April 14th. Courtesy NOAA.
Unlike previous cold snaps where warmer weather returned quickly, the cold actually persisted into 16th and 17th. This allowed a potent low-pressure system to produce snow in Chicago during the morning of the 17th. Accumulations ranged from a coating to up to 4”! That being said, most areas saw 2-4” with the highest focused southwest of I-90 towards Batavia and Elgin. The system warmed up as it headed east towards Indianapolis and Columbus, allowing precipitation to fall as cold rain. The Chicago area rose to 58°F on the 18th, which made it the warmest day immediately following a day with 2+” of snow on record. The below average temperatures persisted from the 19th - 21st with mainly dry conditions. The cold pattern finally broke on the 22nd – 25th from Indianapolis to Columbus (not Chicago) as temperatures rose into the middle to upper 60s. These warmer temperatures were accompanied by periods of rain. However, the mild temperatures were short-lived as below average temperatures returned with highs falling into the 40s and lower 50s. These cold temperatures were produced by a pesky upper level low. This system spun over the Midwest from the night of the 26th and continued through the 28th producing numerous showers and periods of rain. Some areas saw up to 4” of rain particularly in Chicago and Indianapolis, which generated scattered areas of poor drainage and roadway flooding as well as stream/creek flooding. Overall, the month of April was cooler and more active than normal, thanks to a conducive pattern for multiple moisture laden systems.