Missouri trending wetter and warmer

Missouri's seasons are getting warmer and wetter, especially winter and spring.

For farmers, this means a longer growing season, wetter fields and potential for more plant diseases and insects.

Four of the five warmest winters in Missouri on record have occurred since the early 1990s, says University of Missouri Extension climatologist Pat Guinan. The five warmest springs on record have taken place since 1977.

Guinan says Missouri has witnessed a trend of "unprecedented" annual warming over the past couple decades.

"There have been only five years since 1998 that were cooler than average," he says. "We've also seen a trend of higher nighttime temperatures in all four seasons."

Missouri's five warmest years, in descending order, are 2012, 1921, 2016, 1938 and 1931/1998 (tie).

Missouri has broken seven all-time monthly high temperature records during the past 22 years. Of these, most occurred during the cold season. In 19 of the past 22 years, the annual minimum temperature in Missouri has been above average, according to the National Oceanic and Atmospheric Administration.

However, summer days with extreme heat are less common, Guinan says. There are fewer 90-degree days, but summer nights are warmer and more uncomfortable, with more days when temperatures do not fall below 70 degrees.

These trends are due in part to water vapor content, which has been increasing in Missouri over the past several decades, he says.

One way to express atmospheric moisture is through dew point, the temperature at which the air becomes saturated. Higher dew points elevate minimum air temperatures and suppress maximum temperatures, a phenomenon that has become most pronounced during the growing season. These higher nighttime temperatures create a humid environment ripe for plant diseases.

Another change with significant consequences for agriculture: Compared to the long-term average, over the past 20 years the median date of the last spring frost is about six days earlier and the first fall frost is generally five days later. That extends the growing season by 11 days.

Missouri is also experiencing an unprecedented wet period, Guinan says. Twenty-four of the last 39 years have had above-normal precipitation. Missouri saw its seventh-wettest year on record in 2019.

While long-term (1895-2010) average annual precipitation in the state is 40.86 inches, since 1973 annual precipitation has exceeded 50 inches nine times, with fewer dry periods compared to the first seven decades of the 20th century.

Not only is there more rain, heavy rain is happening more often, leading to more flooding and wetter cropland. Missouri has seen a 35% increase in 3-inch daily rain events over the past couple decades compared to the long-term average. Missouri has also broken four all-time monthly records since 2015.

But weather can change quickly, as shown by the drought of 2012, Guinan says. Missouri has had multiyear droughts and extreme summer heat, particularly in the 1930s and 1950s. In 1936 there were more than 60 days of triple-digit temperatures in Lamar, peaking at a brutal 118 degrees on July 19, 1936. The following month saw 21 consecutive days with temperatures of 100 degrees or higher. Since 2013, Lamar has recorded no triple-digit temperatures.

Conversely, the last time an all-time monthly average low temperature record was broken in Missouri was December 1983, when a weather observer near Hamilton recorded 13 days with subzero temperatures. The coldest day was Dec. 22, when it was minus 23. A high temperature of minus 12 was reported on Christmas Day.

Through the years, Missouri farmers have learned to adapt and be resilient when weather changes quickly, Guinan says.