Ag Weather Forum

Unless you live in the American South, the turning of the leaves, lower humidity, and cooler mornings are indicative of the fall season. So far in early September, while some leaves are starting to turn, the thoughts about temperatures have been absent, as it has felt much more like the middle of summer.

Any cooler air has been fleeting at best. But the weather pattern is starting to shape up to bring in some cooler air, especially to the Canadian Prairies and Northern Plains. Eventually, that cooler air should work its way down south as well.

But in the meantime, it continues to be extremely hot across the southern tier of the country. Daytime highs well in the 90s Fahrenheit, and sometimes eclipsing 100 degrees F, have extended from August into early and eventually mid-September. The National Weather Service has had some sort of heat advisory or warning across the South-Central U.S. since the second day of August, with little exception.

Across the northern tier of the country, the heat hasn't been nearly as consistent or intense, but above-normal temperatures have been more of the norm for the last couple of months. Highs well in the 80s F and into the 90s F have been common outside of a few short stretches.

That should be changing, as September usually brings about stronger cold fronts from Canada, ushering in cooler, crisper fall airmasses. And indeed, several fronts are setting up to move through Canada and into the northern tier of the U.S. through next week. Those fronts are keeping the weather pattern quite busy, but they are also stair-stepping the U.S. down into the fall season.

While many know that September is a time when average daily temperatures are falling throughout the month, most people don't know how that happens. Summer heat lingers as long as it can, but the length of daylight is declining rapidly throughout the month, a noticeable feature. That shortening of the days means it's hard to continue building extreme heat. As cooler air builds across the North Pole and eventually into the northern reaches of Canada, it's up to cold fronts to pull that air southward. Doing so eases the summer heat, which now has a harder time building back in. So what was a high in the 90s F before a front turns into the mid-80s F behind it. Then another cold front comes through again, pushing highs down into the upper 70s F, then into the 60s, effectively chopping down temperatures in chunks rather than a gradual cooldown over the month. Some areas across the north see their average high temperatures fall from the upper 70s F and lower 80s F at the beginning of the month into the middle or upper 60s F by the end of the month.

As mentioned, there will be several fronts that move through over the next 10 days or so. Each front will bring a burst of cooler air with it, with temperatures failing to recover higher than before the previous front.

One of these fronts will finally start to make it feel like fall. A front should move through the Canadian Prairies on Thursday, Sept. 10, and into the Northern Plains on Friday, Sept. 11, continuing through the Midwest over the coming weekend. At least for the Canadian Prairies and Northern Plains, temperatures will take a dive. Some areas may fall into the 30s F for morning lows with highs in the 50s F and 60s F.

But these fronts may not make it that far south. They will have a tendency to stall from the Central Plains through the Ohio Valley or Tennessee Valley, while temperatures maintain some extreme highs south of them.

More days in the 90s F and possibly across 100 F will continue to be possible unless one of those fronts can really push southward. Models are suggesting that may be possible with a front that moves through late next week and weekend, around Sept. 18-20. If the upper-level features are positioned well, we could see a front sweeping all the way to the Gulf of America, followed by a big burst of colder, Canadian air. Models are still working that feature out, however. Southern areas of the country usually have to wait until deeper into the fall season to start to see these fronts have a significant push of cold air with them. If they move through, they usually are weak by the time they get down that far south, as the Canadian air is not yet cool enough to significantly ease temperatures. Until that happens, expect heat to be a regular occurrence from Texas and Oklahoma to the Carolinas.

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John Baranick can be reached at john.baranick@dtn.com

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