Climate Extremes of the United States Northern Great Plains

Global temperatures continue to increase, but summer (JJA) temperatures across much of the United States Northern Great Plains (NGP) have remained unchanged or have exhibited a slight cooling trend since the 1970s. A decrease in maximum temperatures of ca 1 °C has been observed. This trend is thought to be due in part to the impact of large-scale land management changes including the reduction of summer fallow and adoption of no-till agriculture, which impact atmospheric boundary layer processes by reducing the height of the lifted condensation level (LCL) and increasing the amount of moisture available. Recent studies have demonstrated a ca 10% increase in convective likelihood across parts of the NGP, which may also be associated with changes in extreme precipitation. Are ongoing land use changes trading extreme precipitation for extreme temperature?

Extreme precipitation has the potential to destroy crops by causing flooding in fields or delaying when crops are planted. A delayed cropping sequence pushes crops further into the late summer dry season, increasing the risk of crop failure due to drought. At the same time, planting dates have advanced across much of the NGP as producers have responded to warmer springs, increasing the chances of exposure to late season freezes. Changes in land management including the timing of important events like planting and harvesting alter the timing at which water and heat enter the atmosphere. These changes that impact atmospheric boundary layer dynamics and convective precipitation may also be associated with changes in extremes such as hail, which is also a major but understudied concern to producers.

We characterize changes in extreme precipitation and temperature events in the NGP using Global Historical Climatology Network station data from the 1950s until the present. Using daily data, the top 5% of measured precipitation days are examined to find changing trends in the frequency and intensity of precipitation. For temperature extremes, the normal for the period is calculated then days outside two standard deviations of the normal are also investigated to find the changing trends in the frequency and intensity. Using the Storm Events Database from the National Centers for Environmental Information, hail trends are also examined over the region. The results from this study will enable producers and managers to make informed decisions regarding risk given the central role of extreme events to agricultural productivity.