Summer is on the downhill slide to autumn.
This isn’t a prediction; you don’t need fancy computer models and forecasts to make this call. You merely need climatology — a study of the history of weather (more or less).
We’re officially into the peak of the summer heat season, and the hottest month of the year ends tomorrow.
Getting to the end of the summer heat is a slow process; there is less temperature difference between July and August in a typical year than between any other two months on the calendar. (In Oneida, our average daily high in July is 85.5; the average daily high in August is 84.9.) The more pronounced cool-down doesn’t begin until September. Even then, psychology plays a role. In July, many of us take a little extra time off work, and spend it around water — the pool, the lake, the river, the sea. We enjoy the heat. By September, most of us are a slave to the grind; hot days are no longer enjoyable and we’re merely waiting for fall. Much like the watched pot that never boils, summer never seems to end when we’re anxiously awaiting its departure.
So it’s a fact that we have two hot months left before autumn truly arrives. But consider this: we’re in that time of year where every day is shorter than the day before (the sun sets at 8:45 p.m. today after a total day length of 14 hours and 2 minutes; it sets at 8:44 p.m. tomorrow after a total day length of 14 hours flat), and the lengthening nights will soon begin to trigger photoperiodism — the process through which plants shed their foliage. And while it will take us another 10 weeks or so to reach the peak of autumn colors, the less time the sun is in the sky is the less time we have each day for diurnal heating, and the more time we have for radiational cooling once the sun has set.
A few days after the 4th of July each year, we reach the peak of the summer heat here on the northern Cumberland Plateau (it’s July 7, if you want to really drill it down to specifics). On that day, our “normal” high temperature ticks up from 85 degrees to 86 degrees. And then it stays there for a while — for most of a month, actually. But guess what today (July 30) is?
It’s the last day that Oneida’s average daily high is 86 degrees. Starting tomorrow, the average daily high is 85. That might seem insignificant, and it is, but the step-down to autumn has to start somewhere, and here in East Tennessee, it starts tomorrow. On August 25, the average high drops another degree (84). Then the step-down starts to accelerate, like a snowball rolling down a hill. Within two more weeks, by September 7, our average high has dropped two more degrees, and it drops two degrees within the next week after that. By mid September (the 15th, if you’re still being specific), our average daily high has dropped under 80 degrees. Two weeks after that, we’re in the mid 70s. And so on and so forth.
Sure, it’s a long process, but it starts tomorrow. And even more important than our afternoon highs are our nighttime lows. There’s a reason those of us who don’t have heated pools stop enjoying them shortly after Labor Day. Pool water loses most of its heat at night, and our nights cool quicker than our days. By late August, our average morning low is in the upper 50s, declining at a much faster rate than our afternoon highs. It will be in the 40s by late September.
So strictly based on climatology, the window for extreme heat — like we saw part of July — is closing. Oneida has never recorded a 100-degree temperature reading later than August 10. We haven’t been above 95 degrees later than that since the horrific heat wave of 2007 (and only 5 times between 1950 and 2007).
With such a limited time for extreme heat remaining, we can start to look at the weather patterns that are currently in play. And NOAA’s Climate Prediction Center is currently forecasting excessive heat to remain confined to the Deep South and coastal areas through August 12, with a slight chance of cooler-than-average conditions here in Tennessee. Taken at face value, the CPC’s latest Week 3-4 outlook (published Friday) calls for above-normal heat to return to Tennessee for the middle and late stages of August, but the general theme continues to be that the heat is mostly confined to the South and the West, as cooler air tries to penetrate into the Plains and Midwest, so it wouldn’t be a surprise to find that the hottest weather (relative to normal) remains confined to our south.
With no signs of extreme heat showing up within the next couple of weeks, and with the time frame for such weather quickly waning, that’s why we can fairly safely say that the window for miserable weather is slamming shut.
That does not mean that summer is over. As stated above, we have 2 hot months remaining. Climatology agrees with that. Last year we saw a hot end to summer and start to fall, with 90 degrees as late as September 21 and close to that (87) as late as October 7. It wasn’t until the middle of October (October 12) that temperatures dropped to normal and stayed there. Historical records show us hitting 90 degrees as late as October 4, and in that autumn heat wave and wildfire season of 2016 we were at 88 degrees on October 20. There have been years in the past where Oneida has hit 80 degrees well into November.
But with that said, it seems fairly safe to declare that our days of excessive heat are numbered. And, if that turns out to be the case, we’ve been pretty fortunate. While parts of the world (like Alaska and Europe) have baked this summer, we’ve been pretty average here in the Mid-South. We hit 90 degrees just twice this month, and have hit that mark just 4 times all summer, after getting to 90 degrees 18 times last year. (We only hit 90 degrees 5 times in 2017, but got there a whopping 34 times in 2016.) That 2016 year could serve as a warning for excessive late-season heat, after the window has ordinarily closed. We hit 90 on 8 days out of September, including 5 days in a row, and were at 90 or above 11 days in August, including the final 6 days of the month. But there were some major differences. One, 2016 had already been an abnormally hot year; we hit 90 degrees 16 times by the end of July — 4 times as many days in the 90s as this year has featured. And we were also in the midst of a developing drought, which would later lead to all the wildfires, including the deadly blaze in Gatlinburg. Dry soil means hotter temperatures than wet soil. Not only has 2019 seen an absence of extreme heat so far, but we’re certainly nowhere close to a drought.