Extreme temperatures settled into the center section of the United States this past weekend and appear to be sticking around into the upcoming weekend. The so-called “heat dome” over the region, from Texas to North Dakota and west from Nebraska to east of the Mississippi isn’t budging.

Heat advisories have been released for 17 states as daytime temperatures exceed 90 F and high humidity levels push the heat index into the triple digits for those states. Unaccustomed to such a long stretch of extremes some of the highest heat indexes will be recorded in Minneapolis, 112 F; Des Moines, 114 F; and Chicago, 107 F.

Even the daily lows will run high, with the mid-70s F to low-80s F predicted for the week. Looking ahead, the 8-to-14-day forecast call for “above average temperatures and average (or below) precipitation.”

While the heat only adds to the extreme drought underway in Texas and Oklahoma, it raises concerns for the corn-growing regions in the Midwest.

Corn and soybean crop progress is not only variable by state, but even within states and counties as the cool, wet spring kept some farmers out of the fields for extended periods. That makes predicting the heat’s impact on the developing crop even more of a challenge this year.

According to Monday’s, USDA Crop Progress Report, corn conditions in the top 18 producing states had worsened since the previous week. Nationally, 66 percent of corn was rated in “good” to “excellent” condition, compared to 69 percent last week and 72 percent last year; 11percent ranked “very poor” to “poor” versus 9 percent last week.

Nebraska led the nation, with 82 percent of its crop rated “good” to “excellent.” Iowa is fairing well, with 80 percent of its corn with that rating.

The July 18 report also showed 35 percent of corn silking, compared to 62 percent in 2010 and the five-year average of 47 percent. Last week’ only 14 percent was reported in the silking stage.

Now, corn needs heat units to develop, but the question is will this run of high temperatures be too extreme? “Corn will rapidly accumulate heat units, as many as 30 per day,” note Roger Elmore and Elwynn Taylor, with Iowa State University’s agronomy department.

Corn needs water during tasseling, and depending on the crop’s location and the ground’s ability to hold reserves, that may or may not be a concern. Iowa, for example, is in decent shape in this regard, Elmore and Taylor note.

The concern is on high temperature during pollination. Over this week and next, it’s estimated that 85 percent of the U.S. corn crop will be pollinating. Thanks to advances in crop genetics, pollen supply and shedding is not as much of a worry today.

While a full out crop failure is not likely, weather stress during this time could produce short, ears that are less filled out, which would reduce yield.

Excessive heat also increases leaf roll, which can interrupt pollination. “By rule-of-thumb, corn yield can be cut by 1 percent for every 12 hours of leaf-roll,” Elmore and Taylor note. During silking, that shifts to a 1 percent drop for every 4 hours of leaf roll.

If soil moisture dries up, the concern sets in by the fourth consecutive day of +90 F temperatures, when a 1 percent yield decline starts to build. On the fifth day, a 2 percent yield reduction becomes a reality, increasing to 4 percent on day six. At that point, trend-line yields become a past goal.

For this year, that could present a serious challenge as the carryover is short and demand, worldwide, by many sectors is high. Users are counting on trend-line yields or better to cover new-crop demand.