Why the rain keeps missing your farm

This week, western Iowa continues to be dry. Nearly 8% of the state is suffereing from severe drought, according to the latest U.S. Drougth Monitor maps. Moderate drought conditions stretch from the Sioux City area down to the middle of Mills County, and as far east as Story County. Crops are showing obvious signs of moisture stress.

“We’re seeing pineapple corn. Corn leaves are rolling, soybean leaves are flipping over. You start to see the lower leaves on the corn firing,” says Iowa climatologist Justin Glisan.

Iowa drought monitor map pulled July 23, 2020
Image credit: Richard Heim, NCEI/NOAA
 

Glisan adds, “These precipitation deficits have been accumulating over the last four to six months. We’re in precipitation deficits on the order of anywhere from 8 to 12 inches in certain locations in west-central Iowa. We had ample subsoil moisture for the crop to start using as it was growing, and now, getting into pollination and maturing, that subsoil moisture is being used up. As subsoil moisture starts to diminish, and you don’t have showers and thunderstorms to replenish the subsoil profile, that’s when we start to see the moisture stress really start to show up.”

Map of Iowa precipitation departure from average in July 2020
Image credit: Iowa Environmental Mesonet

Diminished yields are possible, particularly in the regions that have been both hot and dry. Glisan reports temperatures have been above average for the last month.

Data from the Iowa Environmental Mesonet shows high temperatures have reached 90°F. or more across the state over the last six weeks. Low to mid-90s are expected this weekend, bringing triple-digit heat indices to the state. Such high temperatures can be dangerous for both people and livestock.

“When we do get into those upper 80s, low to mid-90s, overnight lows don’t cool off either," says Glisan. “Temperatures above 70°F. impact livestock stress. You start to get that stress overnight, and then temperatures get up into the 90s during the day. That doesn’t help at all.”

Map of Iowa temperature departures July 23, 2020
Image credit: Iowa Environmental Mesonet

Looking ahead

Looking at the short-term outlooks, there are signals for temperatures moderating to near normal, says Glisan. Normal Iowa temperatures for this time of year range in the low to mid-80s.

Over the next week, Glisan says there are slightly elevated probablities for drier-than-normal conditions across the northern two-thirds of the Hawkeye state. Precipitation probabilities are closer to normal across the southern third.

Why rain won’​t fall on dry farms

“That said, the forecast models recently have not been behaving properly. The rainfall they are showing that’s supoosed to fall has not fallen, and then you get showers and thunderstorms that pop up that weren’t forecasted,” Glisan says, echoing the frustrations of many farmers with stressed crops.

He’s hopeful a change in the temperature back toward normal conditions will produce more accurate forecasts.

“I look at radar as much as I can when these storms pop up because they’re so small scale that where they pop up, you can get .5 inch to 1 inch in an hour, but then 5 miles north, south, east, or west it’s dry. In July we’ve had these sporadic thunderstorms, not really storm systems that are well put together that actually make it through the entire state. That’s because of a lack of low-level humidity and soil moisture. Any storms that do pop up seem to avoid the driest parts of the state since they don’t have any fuel to feed the thunderstorm,” explains Glisan.

“A thunderstorm tires to be an efficient beast. It will look for water to feed on. If it’s not present, the storm will go somewhere else, or it’ll just dissipate. It will die out withouth the fuel that it needs,” he says.

Glisan says he hopes the last several days of sporadic thunderstorms have put enough water vapor down in the atmospmere. As that moisture cycles through its natural system, it acts to seed the low-level atmosphere, raising the probabilites of larger concentrated storm systems that could bring relief to dry areas across the state.

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