August 23, 2013

The summer heat that has been largely nonexistent in the Corn Belt the last four weeks has returned with a vengeance and appears poised to hold its grip through Labor Day. With soybeans in their critical pod-fill stage and corn moving into the dough stage, we will see how well these crops fare with a forecast devoid of appreciable rainfall for the next 10 days.

Prior to the onset of these hot temperatures, persistently cool temperatures have covered most of the Corn Belt as an upper air trough remained semi-stationary in the vicinity of the western Great Lakes. Most of the major moisture during the past 30 days fell on the backside of this upper air trough, resulting in periodic scattered thunderstorms from eastern Montana southeast through western Nebraska.

At the base of this upper air trough, heavy to excessive rainfall was observed across the eastern two-thirds of Kansas and Oklahoma, southern Missouri, and northern Arkansas. The primary fuel for this band of moisture was the interaction of the upper air trough with copious monsoon moisture flowing northeast from Arizona and New Mexico.

Although Nebraska was north of this band of heavy moisture, we still experienced an unusually cloudy period due to the frequent blow off of thunderstorm tops to our south. Solar radiation from late June through mid-August was down 20% from historical averages at Mead and Concord, 14% at North Platte, and 9% at Clay Center. Scottsbluff was only down a little over 4% during this period as it was west of the mean upper air trough. It will be interesting to see if this substantial drop in solar radiation had any significant impact on pollination or early grain fill.

Temperatures during this same period ranged from 2°F below normal in western Nebraska to 5°F below normal across northeast Nebraska. Departures were up to 8°F below normal across northern Iowa, southern Minnesota, and extreme eastern sections of the Dakotas. GDD departures were increased another 100-150 units on top of the losses incurred from planting delays in May.

It is likely that the cool conditions helped mask the dry signal that developed just prior to pollination. During the past 30 days, the western two-thirds of Iowa and most of southern Minnesota failed to receive 25% of normal moisture. There was a substantial band of moisture that developed August 21-22 from northeast Nebraska through northern Iowa and southern Minnesota. However, central and southern Iowa missed out on the moisture, just like southern Nebraska.

With the return to heat and lack of moisture in the forecast through early September, crop stress is likely to become readily apparent as ET values increase and subsoil moisture reserves are insufficient to support crop water demands for dryland farmers who missed out on our most recent precipitation event. With soybeans in the critical pod fill stage, this lack of moisture and heat are likely to substantially impact final yields.

With high temperatures forecast to consistently be in the low to upper 90s, there is an opportunity to cut into some of the accumulated GDD deficits accumulated during the past three months. Based on modeled temperature forecasts, daily GDD accumulation should average between 25 and 27 units per day through September 6. This would result in a net gain of 40-50 units. If the heat holds into mid-September, GDD accumulations would gain another 50 units and most of the deficits accumulated across the state using a May 15 emergence will be eliminated.

The eastern Dakotas, Minnesota, Wisconsin, and northern Iowa have a season GDD deficit accumulation approaching 350 units where corn planting was delayed until late May. Even if high temperatures remain consistently in the low 90s through mid-September, GDD deficits would still be over 200 units and a significant freeze risk would still be maintained.

Al Dutcher
Extension State Climatologist