Ag Weather Forum

Heat builds in every summer. It's just a fact of nature. With the sun spending more time directly overhead during the season, that's no surprise. Some days are hotter than others and forecasting that well in advance can be tough. But meteorologists find clues by looking at the middle of the troposphere, the layer of air closest to the surface that contains our weather.

The "upper levels" that you may have heard about that most meteorologists use is sort of a misnomer. It is only about 3 miles above the surface of the earth, though that changes due to temperature and location on Earth. Meanwhile, the troposphere is about 5-10 miles thick and the exosphere, the outermost layer is somewhere between 500 and 1,800 miles above the surface. Due to gravity, the troposphere also contains about 80% of the atmosphere's mass.

But the upper levels are where the impact of friction with the Earth's surface is minimal, it is above most mountain ranges, and the air is less restricted and flows freely. That is usually an area of the atmosphere that computer models do well at simulating, and its connection to the surface can generally be assumed.

Meteorologists look at upper-level ridges and troughs on maps that can help to give the general weather conditions at the surface, and can get the sense of the overall conditions over large areas. Fine tuning and local features will help to get the specifics for small locations or points. Ridges are areas of higher pressure and typically mean hotter and drier conditions at the surface. Troughs are therefore the opposite as areas of lower pressure that mean cooler and wetter conditions at the surface. In truth, that is only partially true, but sparing the meteorological specifics, that can generally be assumed.

Meteorologists search out these ridges and troughs as usually the first step in forecasting to get an overall sense of the weather conditions before diving into the specifics. And what we usually note are the location of ridges of high pressure, the strength of which may lead to heatwaves.

Currently, a large ridge has anchored itself in the eastern U.S. and Canada, pumping up temperatures not only in this region, but the general area just off to the west. High temperatures well into the 90s Fahrenheit have taken hold of the Central and Southern Plains, Midwest, Southeast, and Northeast regions in the U.S. In contrast, a pair of troughs are found in the West and Canadian Prairies, which have much colder air underneath them in those regions.

But the upper-level pattern will change over the course of this week. The portion of the trough in the Canadian Prairies will lift northeast into Hudson Bay, allowing the ridge to expand across more of the U.S. and increasing temperatures further. The western trough will make a move eastward this weekend, pushing a system through the northern tier of the U.S. and southern Canada that will put a temporary stop to the heat, at least across the north.

Another trough looks to move through the northern U.S. and Canada a couple of days later, as the ridge shifts temporarily to the West mid-late next week. That ridge will then move eastward again late next week and weekend.

Therefore, if you're looking at your 10-15 day forecast and you live in the Corn Belt, you will probably see your high temperatures at hot levels the next few days, take a small dip during the weekend or early next week, take another dip in the middle of next week, then rise again going into next weekend. Of course that is only the general pattern, and models do not fully agree with the shifting upper-level pattern or the specifics at the surface.

The upper levels may be the starting point, but they do not fully determine your forecast. It is much more complicated than that. You could be hotter in a trough due to higher south or westerly winds and clear skies. Or you could be a few degrees cooler under the ridge thanks to a nearby front, northerly winds, cloud cover, or rain. But in general, that's what meteorologists are looking at when they speak about the upper-level pattern and the general conditions in your area.

The movement of the ridges in particular, will be important for the summer temperature forecast. A stagnant, stubborn ridge could create a lasting heatwave. But as long as ridges are moving, conditions will be more variable, allowing for shorter bursts of heat and potential for increased rainfall.

To find more weather conditions and your local forecast from DTN, head over to https://www.dtnpf.com/…

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

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