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COLUMN: Does warm U.S. Midwest winter mean warm Midwest summer? -Braun

CHICAGO - The summer weather forecast for the Midwestern United States has created a great deal of suspense for the agricultural market. For some time now, the prospects of a drought emerging over the next couple of months have been making headli...

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CHICAGO - The summer weather forecast for the Midwestern United States has created a great deal of suspense for the agricultural market.

For some time now, the prospects of a drought emerging over the next couple of months have been making headlines and leading to supply concerns for the leading U.S. crops.

The United States is the world's biggest exporter of corn and No. 2 exporter of soybeans. Almost two-thirds of U.S. corn and soybean production lies in the Midwest. ( http://reut.rs/1Nl8f5W ).

Many analysts have pointed to the fact that the winter of 2015/16 was one of the warmest on record in the Midwest and have assumed that this means that summer could follow suit. Would they be wrong? Statistically, no.

There are many atmospheric variables that affect the summer forecast, but interpreting the complex data requires expertise. Even then, the forecast could completely miss the mark, because sometimes that is just what the weather does.

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And although simple statistics can also fail us, what the statistics are saying right now is particularly compelling.

WARM SUMMER, SAYS WINTER

Although not a perfect relationship, a very warm winter usually results in a warm summer across the Midwestern United States.

Temperatures during winter, considered as December through February, have been warmer than this past winter only four times over the past 120 years ( http://reut.rs/1Nl8gGO ).

When considering all 11 years in which winter temperatures came within two degrees of 2016's winter, nine of those years went on to post warmer summers, on average close to 2 degrees F above the long-term normal.

Two of those years fell below normal, 1992 and 2000, but 1992 would need to be discarded as the cool summer can be attributed to the eruption of Mount Pinatubo in the Philippines a year prior, which covered the entire atmosphere in a temporary blanket of ash.

The data suggests that the summer outlook is highly variable when the preceding winter temperatures are within a couple degrees of normal. But when winter temperatures deviate significantly, as happened in 2016, summer almost always tends to follow the same direction.

Another interesting observation is how much different this year is than the previous two. The Midwestern winter of 2015 was almost 5 degrees warmer than the brutal winter of 2014, but this winter was more than 7 degrees warmer than 2015.

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When this same data is broken out by month, the relationship between winter and summer temperatures appears to be stronger in July than August, the two hottest months of the year ( http://reut.rs/1ruLH8h ).

In July, again considering years where winter temperatures fell within 2 degrees of 2016, seven years exist in which temperatures were 2 degrees or more above average. In August, there was only one year in which that happened.

This could be an important implication for U.S. agriculture because July is typically the critical month for corn yield, while August is the gatekeeper of soybean yield. This statistical approach could suggest that corn might face more weather hardships than soybeans.

GREAT LAKES AGREE?

Another way to visualize the impacts of winter weather on the U.S. Midwest is by using surface temperature data from the Great Lakes. The five lakes usually tend to control climate at a more local scale, but the overall trend in temperatures might help explain some broader details that might not otherwise be apparent.

The average surface temperature of all five lakes between December 2015 and February 2016 was the warmest of the past 22 years. It even edged the very mild winter of 2011/12 that gave way to the historic 2012 summer drought ( http://reut.rs/1ruKkXb ).

Logically, winter air and lake temperatures are fairly well-correlated, but one significant outlier exists - 1998. That year is of particular significance because similar to 2016, record El Niño during the winter turned into La Niña midway through the year. La Niña is expected to begin within the next couple of months.

As can be seen in charts two and three, the winter of 1998 was very warm, but the summer went on to be relatively average. It is one of the only El Niño-to-La Niña years not to produce a very hot summer for the Midwest.

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The fact that this past winter's air and lake temperatures most closely mimicked 2012 and 2002, another very warm year, might suggest that 1998 is not the most likely outcome for this summer, although the data set is admittedly small.

April was a bit cooler for the Great Lakes region, and average lake temperatures for last month fell to the fourth spot, placing 2016 in a rather precarious position ( http://reut.rs/1Nl8w8L ).

Warmer than April 2016 were 2006, 2010 and 2012, all warm or extremely hot summers. But just cooler than 2016 on chart 5 are a smorgasbord of outcomes, including the unusually cool summer of 2004.

If we start seeing cooler spells in May and June, then July may not be as warm as some of the analogs suggest. But statistically, a cool summer is very unlikely based on everything that has happened to this point.

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