This experimental, station-based drought tracker uses near-real time data from several precipitation monitoring networks to estimate a drought index value at each location at several different timescales. Total precipitation for the past 30, 90, 180, and 365 days is summed up at each station with less than 10% missing observations and compared to a distribution of historical estimates at that location derived from a long-term gridded precipitation dataset (PRISM, 1895-2020). A drought index called the Standardized Precipitation Index or SPI (more info here) is calculated for the current observation in relation to the distribution of historical estimates. SPI values are simply standard deviation units so values close to 0 are near-average while negative values indicate dry conditions and positive ones wet.

30-day SPI

90-day SPI

180-day SPI

365-day SPI

How to interpret the maps

Each tab contains a map representing precipitation information summarized over different timescales ranging from the last 30 days to as long as 365 days. Each map displays precipitation observations from the network, the NOAA Global Historical Climate Network (NOAA-GHCN), and numerous other meteorological mesonets available through the SynopticData API. The NOAA-GHCN includes data from several other networks including NOAA Cooperative Observers, WBAN/ASOS at airport, SNOTEL sites, and the CoCoRAHS volunteer observer network. The SynopticData API pulls data together from numerous other networks including county-level flood control gauges, the Citizen Weather Observer Program, and Remote Automated Weather Stations. Use caution when interpreting observations from the Synoptic networks. Many of these observations come from automated stations with limited error checking. The Rainlog, NOAA-GHCN, and MesoWest sites can be toggled on or off by using the checkboxes at the upper right hand corner of the map. The latest U.S. Drought Monitor (USDM) map also is plotted on each map and is updated each Thursday. A reference grid that matches the PRISM Climate dataset which is used for the historical estimates at each location can also be toggled on using the map controls in the upper right hand corner.


Guide to station popup information

Label Description
Observed Precip Total precipitation observed at station over select time period (inches)
PRISM Avg Estimated long-term average (1895-2020) precipitation for location from gridded PRISM dataset (inches)
Diff from Avg Observed minus long-term average (1895-2020) precipitation (inches)
SPI Observed precipitation expressed as Standardized Precipitation Index value
% rank Observed precipitation expressed as percentile rank value with 100% being wettest value on record (1895-2020)
Max 1-day Precip Maximum daily precipitation amount observed during select time period (inches)
Days missing Number of missing days during select time period
Gauge Gauge name or number
Network Name of gauge’s network

Contact Mike Crimmins if you have any questions or suggestions. Site created with support from the USDA Southwest Climate Hub.

Climate Science Applications Program - University of Arizona Cooperative Extension


Last Updated: “09-28-2023”© 2022 The Arizona Board of Regents. All contents copyrighted. All rights reserved.