IEM Daily Feature
Friday, 15 April 2016

Departure Differences

Posted: 15 Apr 2016 05:34 AM

The featured chart displays accumulated to date precipitation departures for Ames over 31, 91, and 365 day periods. Rewording, for a given day on the chart the three values for that day represent the trailing 3 periods valid at that date. The chart for the past month of time tells an interesting story. While the 31 and 91 day departures have been decreasing (drier), the long term departure is actually increasing (wetter). This type of situation depicts a problem with short vs long term drought analysis. But how can this situation be happening at the same time? It all has to do with the current day periods and those that they are replacing as the window of time marches forward. The short term periods have lost the very wet December, while the long term still contains that very wet month. Our recent months have been near normal, but are replacing drier months last Feb and Mar of 2015 for the 365 day period. It is complicated!

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