Forecast model presentations during the latter part of the July 9-15 week show an evolution to a configuration that is less threatening for the U.S. Midwest heading into the last part of July than some of the stressful heat we have seen recently.
The big feature is a relocation of upper-atmosphere high pressure ridging to be more in the southwestern Plains or even the Far West. The U.S. and European forecast models differ in exactly where the ridge center is likely to be for the period ending July 23. However, there is notable agreement that the ridge will not make camp over the southern Midwest, but instead will at least be as far west as the Southern Plains (U.S. model), and perhaps even farther west, in southern California (European model). We track the location of this feature by noting where the 5,940-meter height contour for the 500-millibar constant pressure value sets up.
Heights of the 500-millibar constant pressure level can be used to estimate surface temperatures, since the height of a pressure surface is related to the temperatures of the air beneath it. When the air is warmer, the height of the constant 500-millibar pressure value rises. And, during midsummer, the surface temperature underneath a 5,940-meter (594 decameters) high location of the 500-millibar pressure value is in the 95-100 degree Fahrenheit range.
The evolution of this upper-atmosphere ridge is, of course, key to conditions for pollinating and filling corn, along with flowering and pod-setting soybeans. We have seen this summer how upper-air ridging can influence the development of stressful and debilitating heat -- look no farther than the Northern Plains drought for proof. And, earlier this week, trade fears were that the hot upper ridge was going to balloon eastward into the Mississippi-Ohio Valley area and bring about widespread stressful conditions for corn pollination. However, forecasts as of late week show this feature not being able to cross the Mississippi River on a sustained basis.
Will the ridge exercise its stressful influence yet this season? Quite likely, especially over the Plains. But, the "have" and "have not" scenario that has been a trademark of the 2017 row crop season up to this point shows no sign of changing. That is, the eastern and northeastern Midwest continues to east of the hot high pressure, with higher rainfall chances and temperatures that are not quite as high as in the western Midwest through the Plains.
Overall rain also has a more consistent focus over the eastern and the northeastern Midwest, with the potential for at least some of that activity to show up in western Midwest locations as well. We saw this during Wednesday night to Thursday morning July 12-13, when a swath of moderate to locally-heavy rain developed from south-central Nebraska eastward to southwestern Iowa.
The continued ebb and flow evolution of the upper air pattern will get additional worldwide focus through the rest of July and through August, as corn's filling timeframe and the major part of soybean pod-set and fill stages develop.
Bryce Anderson can be reached at firstname.lastname@example.org
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