National Drought Mitigation Center (NDMC), the U.S. Department of Agriculture (USDA), and the National Oceanic and Atmospheric Administration (NOAA)
The U.S. Drought Monitor (USDM) is a map that is updated each Thursday to show the location and intensity of drought across the country. The USDM uses a five-category system, labeled Abnormally Dry or D0, (a precursor to drought, not actually drought), and Moderate (D1), Severe (D2), Extreme (D3) and Exceptional (D4) Drought. Drought categories show experts’ assessments of conditions related to dryness and drought including observations of how much water is available in streams, lakes, and soils compared to usual for the same time of year. The U.S. Drought Monitor began in 2000 and is a collaboration between the NDMC, NOAA and the USDA, who share the weekly author role for the product. The NDMC, based at University of Nebraska, Lincoln, leads the coordination of weekly drought monitor process and provides the maps, data and statistics to the public.
Where does this come from?
Each week, drought experts consider how recent precipitation totals across the country compare to their long-term averages. They check variables including temperatures, soil moisture, water levels in streams and lakes, snow cover, and meltwater runoff. Experts also check whether areas are showing drought impacts such as water shortages and business interruptions. Based on dozens of indicators, experts make their best judgments of regional-scale drought conditions, and then check their assessments with experts in the field before publishing weekly drought maps. Associated statistics show what proportion of various geographic areas are in each category of dryness or drought, and how many people are affected.