What do recent precipitation deficits tell us about next summer?
click image to zoom Those correlations, where 1 indicates perfect positive correlation in precipitation levels and -1 indicates perfect negative correlation, are presented in Table 2 and illustrated in Figure 1. On average, all correlations are very small, with some modest positive correlation between both summer and total June through November precipitation in one year and summer precipitation the following year in Nebraska. The overall lack of correlation in precipitation from year-to-year is consistent with the long-held view that weather conditions are independent over time, particularly time horizons beyond a few weeks or months. In other words, the predictability of weather year-to-year is no different than the flips of a fair coin. Regardless of what has happened recently the probability that the next weather observation will be above or below average remains 50 percent.
click image to zoom The correlations in Table 2 are computed using the full 1895-2012 sample of data. It is possible for some correlation to be present at the extremes but is masked when looking at all the data. In order to determine whether this is in fact the case, we also examined for each state both July and total summer precipitation in the years that followed the 10 years with the smallest precipitation totals in the June through November period. As shown in Table 3, on average, July precipitation in those 10 years ranged from 82 to 98 percent of normal and averaged 92 percent across the four states. Summer precipitation ranged from 96 to 108 percent of normal and averaged 103 percent. So, there is a small tendency for extremely dry June-November periods to be followed by slightly dry Julys, but the entire following summer period actually tends to be slightly wet.
We also identified the number of years that had more than a one-standard deviation precipitation deficit in either July or the full summer in the year following the 10 driest June through November periods. Those counts are as follows:
Like the previous correlation analysis, this analysis suggests little relationship between precipitation levels in dry years and summer precipitation the following year.
Careful analysis of the historical record indicates that recent precipitation levels and current soil moisture conditions provide little guidance in forming expectations for precipitation levels next summer. Historically, odds of favorable or unfavorable growing conditions have been independent of precipitation levels in the last half of the previous year. Beyond this particular analysis, we would expect to find general independence of year-to-year weather conditions. We reported on such independence, for example, in average winter and summer temperatures in a post earlier this year.
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