Summer in September: Breakdown of 90s by week

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Parts of eastern Iowa will be pushing 90 degrees to finish off the workweek. Not surprisingly, it’s more common to hit 90 early in September than it is at the end of the month, and that likelihood drops off pretty quickly.

There’s also a big geographical difference in how common 90s are in September. Iowa City has had nearly five times as many 90s in September as Dubuque over the past 50 years. Why is Waterloo’s number so high? Typically, warmth builds in from the west, so even though Waterloo is farther north than Cedar Rapids, it’s more likely to get the heat.

The following statistics show the total number of highs of at least 90 in September from 1967 to 2016, then breaks the total number down by week. There’s also the average number of 90s per year by week, as well. A value of 0.5 per year indicates a 90 every other year, 0.2 would be every five years, and so on.

Cedar Rapids: 57 total
September 1-7: 27 total days/0.54 days per year
September 8-14: 18/0.36
September 15-21: 10/0.2
September 22-30: 2/0.04

Dubuque: 25 total
September 1-7: 15/0.3
September 8-14: 9/0.18
September 15-21: 1/0.02
September 22-30: 0/0.00

Iowa City: 119 total
September 1-7: 58/1.16
September 8-14: 42/0.84
September 15-21: 15/0.3
September 22-30: 4/0.08

Waterloo: 71 total
September 1-7: 39/0.78
September 8-14: 24/0.48
September 15-21: 7/0.14
September 22-30: 1/0.02