The rainfall event last week certainly helped the lingering drought situation left over from the end of the 2025 growing season. Making up precipitation deficits during the driest portion of the year is somewhat difficult, but some precipitation departure metrics have some annual quirks that should be considered. Such a quirk is the subject of today's featured chart. The chart presents a heat map of trailing 30 day standardized precipitation index (SPI) values for Ames. The chart is computed by calculating SPI for each day over the period of record and then plotting a frequency at a given SPI and day of year combination. The colored lines are non-binding US Drought Monitor equivalent SPI values. Since precipitation can not be negative, SPI has minimum possible value since the difference between the average accumulation and zero is some finite number. So the red line points out the annual quirk of this index, which is that it organically improves during the cold season as accumulation averages decrease. The chart also nicely shows the spring time decrease (you can visually see the streaking of the frequencies) in SPI as climatology increases daily and individual rainfall events become less impactful over a 30 day period.

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