Why Was September So Hot?

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For most of September, if you walked outside and didn’t know for a fact that Labor Day had passed and schools had started, you would've been forgiven for thinking it was August. September was hot and sticky across much of the United States—to the point where the seemingly endless summer heat is pushing into record territory. In particular, this past September shaped up to be one of the hottest Septembers on record for much of the southern United States.

Normally, September is a transition month in the weather world. The sun slowly begins focusing its intensity on the Southern Hemisphere, leaving behind crisper weather for those of us in the northern half of the world. Cooler temperatures and dropping humidity levels ought to make the second half of September downright refreshing, but the weather this year hasn’t followed the rules.

September 2016 wound up in the record books as the hottest September ever recorded in cities like Greensboro, North Carolina; Greenville, South Carolina; and Huntsville, Alabama. It was the second-warmest September on record for urban areas like Washington D.C., and Birmingham, Alabama. Even up in Philadelphia and down in usually sultry Baton Rouge, Louisiana, unusually warm temperatures made September 2016 the third-warmest on record.

This is no small feat. All of these cities (except for Washington) have temperature records at their major airports that stretch back generations. If your parents and grandparents lived in Greensboro or Birmingham when they were growing up, they probably never experienced a lead-up to fall as warm as what we just went through.

Even as a cold front came through to welcome the end of the month, this past September still managed to place high in the record books. The science is clear: Observational data on climate change [PDF] show that heat waves have already increased in duration and intensity. Chilly fall mornings may become less and less frequent. Sweating it out in September may be the new normal.

A large ridge of high pressure over the U.S. on September 21, 2016, indicative of the pattern that caused the month's abnormal warmth. Image credit: Tropical Tidbits

The culprit behind the delayed cooldown was a near-persistent ridge of high pressure that has made itself at home over the region. The jet stream typically shifts to the north and grows less wavy during the summer months, allowing that stale, muggy air to surge north from the Gulf of Mexico and sit over the land like a wet, miserable blanket. The jet stream gets wavier as summer fades to fall because the temperature gradients are more extreme between north and south. The wavy jet stream allows shots of refreshing air to dip southward from Canada, each burst of cool air further eroding the extent of the muggy air until winter sets in.

While daytime high temperatures weren't as brutal as you would see in July, it was still unusually warm when you take into account the daily average temperature, which is the high temperature and the low temperature averaged together. This measure gives you a good idea of the day as a whole, factoring in both how warm it was during the day and how cool it got during the night. Warm nights make warm days even more intolerable because you get little relief once the Sun goes down, and it also gives you a warmer starting point to begin daytime heating the following day.

If we look at Greensboro, North Carolina, their average daily temperature for most of September should be around 72°F, which would mean they normally see average highs in the 80s and average lows in the 60s. Their average daily temperature this September was 77°F, a full five degrees above normal, indicative of high temperatures around 90°F and low temperatures near 70°F almost every day. It’s a similar story across the rest of the region where records were either broken or came within a whisper of being topped.

The Climate Prediction Center’s temperature forecast for October 2016. Image credit: NOAA/CPC

Even though the most uncomfortable air started to break when the calendar flipped to October, NOAA's Climate Prediction Center says there are better-than-even chances of above-average temperatures across most of the country through the first half of fall. Of course, "above normal" in the late fall and early winter is all relative, so it won’t be as unbearably warm and muggy as it was for much longer than it should have been.