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Drought Indicator: Precipitation Deviation from Long-Term (30-Year) Averages


In order to provide daily updates about drought conditions, this section provides an interactive tool for examining patterns of accumulated precipitation. Use the slider bars or the dropdown lists to select a time window from 1 to 72 months in duration, for any period from 1990 to the present.

Total precipitation anomaly is calculated by dividing the total precipitation over a specified period of months by the 1991-2020 normal precipitation for the months included in that period. If the time window includes the month-in-progress, the anomaly is calculated by dividing the total month-to-date precipitation by the 1991-2020 normal precipitation for that month, pro-rated for the number of elapsed days. For example, the normal precipitation for August 1-20 would be the 30-year value for August, multiplied by 20/31. Given that daily normal precipitation may not be evenly distributed throughout a month, the month-to-date anomalies should be viewed as approximations.

The calculations use as many as possible of the station networks and data sources ingested by the PRISM Group, not just those that have existed for long periods of time. See PRISM datasets for more information; the data available from this page is "Norm" (normals) and "AN" (monthly). For information on when the grids were most recently updated, see calendar of PRISM data updates. If the time window includes the month-in-progress, "AN" (daily) data is used for the most recent month.

Results for dates >6 months old should be considered "stable" (unlikely to change until a major version change). Those for more recent dates should be considered "provisional" (still likely to change as reporting stations finalize their information).






To change the window on the slider, use the dropdown lists or drag the scale handles.







Duration (months):    Start month:    End month:


Choosing the current month as end date extends the time window to include whatever days have elapsed so far this month (e.g., a duration of "2" months is actually "2 plus the fractional month").

Questions to prism-questions@nacse.org This website is supported by the USDA Risk Management Agency © 2026, Northwest Alliance for Computational Science & Engineering (NACSE), based at Oregon State University

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