The Standardised Precipitation Index (SPI) is a simple measure of drought (and also of very wet conditions) and is based solely on the accumulated precipitation for a given time period (e.g. over the last 30 or 60 days), compared with the long-term average precipitation for that period. This precipitation difference (or anomaly) is "standardised" by dividing by the long-term standard deviation of precipitation for that period.
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Last 30 days SPI
Last 60 days SPI
SPI Drought Indicators
If the 30-day SPI is showing "severely or extremely dry", then drought (if not already present) may be imminent.
If the 60-day SPI is showing "severely or extremely dry", then drought conditions are probably evident and sustained rainfall is needed over several weeks to return to normal conditions.