See Ya Next Year, 80 Degrees! - 10/15/13
The weather is changing significantly this week in West Michigan: a cool, damp pattern is moving in and if forecast models are correct, it's going to be sticking around for several days. Seeing as how we're in the middle of October, and the new, cool weather pattern will take us toward the end of the month, have we seen our last 80 degree day for the year?
If history is a good indicator, the answer is yes. I've looked at October temperatures for the last 25 years to get an idea of how often we have had an 80 degree day in the month, the earliest and latest dates for an 80 degree day, and the average "last" 80 degree day for the year. I found that over the last quarter-century, our last 80 degree day was about as likely to come in September (12 years) as in October (13 years).
The earliest, last 80 degree day was on September 8, in 2001, two weeks before the end of (astronomical) summer. The latest, last 80 degree day recorded over the last 25 years was October 20, in 2003. Considering that the cool spell which is now underway in West Michigan will keep our temps below average for at least the next seven days, which takes us to the 23rd, it seems reasonable to conclude we've likely seen our last 80 degree day this year. Of course, there's always the possibility of a record-setting warm spell coming at the end of this month or beginning of next (who would have thought we would have 80 degree weather in March, 2012?), but that's highly unlikely.
One other note, the average latest date for an 80 degree day in West Michigan over the last 25 years is September 30th.