First Month of Fall Gets a Summer Finish
The Midwest once again found another month with major temperature swings in September as the transition to meteorological fall began. It was a cool beginning to September as a trough presided in the east with a large blocking ridge in the west. Ideally, this may have been a sign of the end the heat and a move over to cooler fall weather. However, summer decided to make a comeback instead.
Harkening first to the cooler weather, though, it was certainly that to begin September - even downright chilly for the time of year for some of the major cities especially in the Ohio Valley. Dayton and Columbus both broke records for daily coolest high temperatures, both on the 1st and 2nd of the month. Dayton was only able to crawl to 59 and 60 degrees respectively, crushing old records of 65° in 1949 and 66° in 1952. It was not much better in Columbus, with 62° and 61° besting 67° and 62° back in 2006. It was the majority of the first two weeks that saw the coolest numbers, with high temperatures in Cincinnati, Columbus and Dayton falling 7 to 10 degrees below normal between the 1st and 15th, and 3 to 6 degrees short of average from Chicago to Indianapolis.
Comparing the first week of September to the month's final week (Image courtesy of the Midwestern Regional Climate Center)
One of the most anomalous warm stretches of the year may have been reserved for the latter half of September. The turnaround began between the 14th and 17th, as the aforementioned trough/ridge pattern flip-flopped. This brought pools of cooler air into the Rockies and West Coast and allowed for a surge of milder weather into the East. Chicago’s O’Hare airport observed the most extreme anomalies, with average overall temperatures 12.8 degrees above average between the 15th and the 30th (for perspective, the historical mean temperature for the period is 61.5 degrees, with 74.3 degrees being what was observed in 2017).
The highlight of the period came between the 20th and 26th, where O’Hare broke high temperature records for an unprecedented 7 straight days! Here is a bit more of an in-depth look at this record-breaking achievement:
- In an abnormal twist, this seven-day period ending on the 26th was the hottest stretch in 2017, with an average high of 93.1°.
- In fact, for the year outside of this date range, it wasn't even close. The closest was in June, with a seven-day average of 90.9 degrees ending on the 15th.
- Perhaps just as impressive - relative to the time of year - it was also enough to finish in the top-50 for warmest of this length on record (T-47th).
- For September, it is the warmest 7-day period... ever. The stretch ending on September 8th, 1960 is the next warmest at 92.6 (T-67th)
*Above Data courtesy of SC-ACIS
The most significant weather events came through Ohio during the first week of September. While not records, the not-oft achieved 1”+ of rain was recorded in Columbus on the 2nd with a total of 1.44”, and Cincinnati came in with 1.58” on the 1st of the month. Another significant cold front swept across the region on Labor Day into the 5th. With a large fetch of warm, moist air and favorable flow aloft, Ohio was placed in the crosshairs for a bout of severe weather that brought wind damage to the Dayton area and gave Columbus a close shave with an EF-2 tornado in Richland County.
Images courtesy of NOAA - edited by WeatherWorks Meterorologist Zach Murphy
One of the other significant aspects of the month though, in some ways, was what did not happen. It was especially the case from Indianapolis north and west. All of the Cornbelt finished below average in terms of rainfall. Ohio’s deficit was not quite as severe, as 2.0 – 2.5 inches fell across the state. This was generally 75-90% of normal. On the other hand, Indianapolis and Chicago were nearly bone-dry, only receiving 10-20% of its normal rainfall with totals of a mere few tenths of an inch. These areas fell into abnormally dry to moderate drought as a result according to the United States Drought Monitor.