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Note for US regions: The USDA Drought Monitor
provides higher-fidelity D0–D4 drought classification based on observational data (rain gauges, soil moisture, streamflow, and expert judgment).
ERA5 SPI/SPEI shown here is a model reanalysis — best used for spatial pattern context and cross-regional comparison.
USDM updates Thursdays; ERA5 tiles carry 5–7 day latency.
Index Reference
SPI
Standardized Precipitation Index. Negative = below-normal precipitation. Threshold: < −1 = drought, < −2 = severe.
SPEI
Adds temperature-driven evapotranspiration to SPI. Diverges from SPI when heat amplifies moisture deficit beyond rainfall alone.
EDDI
Evaporative Demand Drought Index. Positive = high atmospheric demand. Red on map = drought stress even when SPI is neutral.