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Nebraska Entering A Dry Phase For Next 6-7 Years

By Steve Hu
Will summer 2018 likely be wet or dry?
While climatologists can’t predict day-to-day weather conditions in advance of the next growing season, they can use seasonal and annual average records to identify climate trends that can help growers position their operations for the expected conditions.
These climate trend projections, particularly for precipitation, are based on multi-decade climate variations that have been tested and shown to have persistent patterns. Seasonal and annual precipitation in Nebraska and the central U.S. Great Plains is one such variable with a fairly persistent quasi-20-year oscillation. This oscillation is shown in Figure 1 using data from the past 120 years.
Figure 1. Departure from normal precipitation (standard deviation) in observed summer precipitation (June-August) (gray bars) in Nebraska from 1894-2017. The 0 line represents the mean over this period. The 20-year variation component of the precipitation is shown by the red line, and the multi-decade variation is shown by the blue line. 
Summer Precipitation Trends
In addition to showing the quasi-20-year oscillation in Nebraska summer precipitation (June-August), Figure 1 shows a strong pattern of summer precipitation variations over the past 120 years. In the decades when this 20-year variation was in its “dry phase,” i.e., the red line was below 0.0 in Figure 1, most years had below-average summer precipitation. (For example, summer 2002.) On the other hand, when this variation was in its “wet phase,” or the red line was above 0.0 (Figure 1), most years in those decades had above-average summer precipitation.
Now, looking at the most recent years, we see that this 20-year variation entered its dry phase around 2015. The first year in that dry phase, year 2016, had below-average summer precipitation. As this dry phase continues to develop in the next six to seven years, we anticipate that most summers will have below-average precipitation. Although an exception to this “dry norm” may occur in one or two summers, the overall trend will include the 2018 growing season and several seasons afterward. As noted previously, individual years can fall outside the trend. (Note that 2012, one of the driest Nebraska summers in recent history, was an exception that fell outside the “wet norm” in the 20-year variation.) To predict climate conditions for individual years within the pattern, growers should consult the 30- and 90-day precipitation and temperature forecasts from the National Oceanic and Atmospheric Administration’s Climate Prediction Center.
Annual Precipitation Trends
Annual precipitation in Nebraska also has a strong quasi-20-year trend as shown by the red line in Figure 2. This annual variation has a phase similar to the 20-year variation in the summer precipitation in recent decades (Figure 1). The annual precipitation trend also indicates Nebraska may be entering a dry phase for the next few years, reinforcing indications in the summer precipitation projection (Figure 1).
Figure 2: Departure from normal precipitation in observed annual precipitation (gray bars) in Nebraska from 1894-2017. The 20-year variation component of the precipitation pattern is shown by the red line.
The Numbers Behind Figures 1-2
In Nebraska, as in other High Plains states, precipitation varies significantly from east to west and from year to year.
The mean annual precipitation from 1895-2016, averaged across Nebraska, is 22.71 inches and the standard deviation is 3.86 inches.
The mean summer precipitation from 1895-2016 averaged across Nebraska is 9.39 inches and the standard deviation is 2.15 inches.
In figures 1 and 2, the 0.0 line represents the mean precipitation level for 1895-2016: 22.71 inches for annual precipitation or 9.39 inches for summer precipitation. The numbers on the vertical axis indicate number of deviations from standard. For example, in Figure 1, the "2" indicates rainfall was two times the standard deviation of 2.15 or 4.30 inches above the normal/mean value of 9.39 inches.

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