The WestWide Drought Tracker: Drought Monitoring at Fine Spatial Scales

The expression of drought differs depending on the geographic and climatic setting. These variations are related to the phasing between the annual cycles of temperature and precipitation. The form of precipitation (rain or snow) causes drought sequences to play out differently through the introduction of temporal lags in the hydrologic system. In the western United States there are strong relationships between climate and elevation that are realized through temperature lapse rates and orographic precipitation enhancement. In addition, the seasonality of precipitation can vary considerably from lower to higher elevations over short distances. Collectively, this results in fine scale spatial variation in both climate and climate variability that necessitates more detail than regional averages provide. Since most streamflow in the West originates as mountain snow, the behavior of climate in relatively small elevated areas has wide implications for regional rivers, reservoirs, soil moisture recharge, and the economic sectors that depend on them, and thus on the status of hydrological drought. The WestWide Drought Tracker (WWDT) at the Western Regional Climate Center provides a more localized perspective on drought by providing access to drought indices at spatial scales that represent most of the important drought-related spatial variability in climate seen in the western US. The WWDT provides user-friendly access to a suite of drought indices including: a) Palmer Drought Severity Index, Palmer Z-Index, and self-calibrated Palmer Drought Severity Index, b) Standardized Precipitation Index and Standardized Precipitation Evaporation Index for time scales from 1 to 72 months, and c) temperature and precipitation departures and percentiles for 1-12 month intervals. These products are derived using monthly climate data from PRISM on a 4 km grid spanning the contiguous 48 states, and from 1200 stations in the Historical Climatology Network (HCN). The datasets are available at each individual HCN station and PRISM grid point, in addition to areal aggregations of PRISM data for states, climate divisions, hydrologic basins, counties, and fire management areas. WWDT products can be accessed as maps or time series from 1895 through present via the web pages of the Western Regional Climate Center, current to the end of the last month and updated twice a month. A first-look of the drought conditions at the beginning of the month is performed interpolating temperature and precipitation anomalies from the North America Land Data Assimilation System-2 to the PRISM grid. Datasets are updated a second time each month following the update of PRISM output.