Palmer Drought Severity Index: methodology
Provenance for the series on Palmer Drought Severity Index. This page has its own URL so it can be cited alongside a figure.
- Source
- NOAA NCEI nClimDiv
- Dataset identifier
- noaa_pdsi
- Update cadence
- Monthly, ~1 week lag
- Unit
- index
- Coverage
- 18 series · 1895-01-01 to 2026-09-01
- Vintage published here
- 2026-10-30
The Palmer Drought Severity Index (PDSI) from NOAA's nClimDiv dataset, monthly, beginning in 1895. PDSI is a unitless index, not a measurement.
Reading the scale. Negative is dry, positive is wet, and roughly:
| Range | Conventional label |
|---|---|
| 4.0 and above | extremely moist |
| 2.0 to 3.99 | moderately to very moist |
| -1.99 to 1.99 | near normal |
| -2.0 to -2.99 | moderate drought |
| -3.0 to -3.99 | severe drought |
| -4.0 and below | extreme drought |
The index is cumulative: it carries the preceding months' moisture balance forward, so a single wet month lifts it only partway and a long dry run drives it well past -4. Nevada's statewide value has spent much of 2025 and 2026 below -3.
Not comparable across climates as a drought severity. PDSI is calibrated to each location's own history, so -3 in Elko and -3 in Alabama both mean "severe relative to normal here", not the same amount of water missing.
Derivation and revisions are as for precipitation: area-weighted averages of 5 km grid-point estimates from GHCN-Daily, with the most recent two years updated monthly and the full record restated on a version change. Each of our runs stores a new vintage rather than overwriting.
Revisions and vintages
Every observation we store carries the date it was published, and a revision lands as a new vintage rather than overwriting the old one. The figures on this site are the most recent vintage, stamped above; a figure you cited earlier stays retrievable even after the source restates it.
The full series, including the flag and vintage of every value, is in the CSV.