People who watch the weather have always looked for ways to predict when and where extreme weather events will happen, and how severe they will be. For some extreme events, like tornadoes, hurricanes, and floods, we’ve discovered patterns and methods that allow us to watch and even predict bad weather or hazardous conditions shortly before they happen. For example, we know that when certain atmospheric conditions happen at the same time in the same place, tornadoes are possible, so storm watchers know that they need to watch the skies in that area, and they also have radar technology to help them detect possible tornadoes. During extended or intense periods of wet weather, we can watch water levels in rivers and streams as well as weather forecasts to determine if flooding will happen.

Drought is a different kind of extreme weather event. Unlike tornadoes, hurricanes, and floods, drought doesn't have a clear beginning or ending. (To see how drought compares to some other natural hazards, see Droughts Compared with Hurricanes and Floods.) It starts and ends slowly, and for that reason it has been called a “creeping phenomenon.” It's hard to tell when a drought begins and ends, but by watching various “indicators” of drought, like water levels in streams and reservoirs, soil moisture, or the amount of rainfall or snowpack an area has received, we can keep track of droughts. Because droughts differ from one area to another, the indicators that we measure will depend on the area we're monitoring. For example, city water companies would be interested in monitoring water levels in reservoirs, while fire-fighting agencies would want to watch the dryness of vegetation.

We also need a way to make sense of the measurements we collect. For example, rainfall measurements by themselves won't tell us if we're in a drought unless we can compare them to other measurements, particularly the area's average rainfall. For these reasons, scientists have created many tools and models to help people in different parts of the world determine whether they're in a drought, and how severe it is. Many of these tools use some sort of process or formula to turn a wide variety of data, such as streamflow, soil moisture, temperature, precipitation, and snowpack, into a single number, known as an index. (For more information about the kinds of indices that scientists and drought planners use, see Drought Indices, from the NDMC's main website.) Here's a look at some of the many indices and other tools available:

The Drought Monitor (DM) and North American Drought Monitor (NADM) maps show drought conditionsin the United States and North America. The DM, created in 1999, is produced weekly by scientists from the National Drought Mitigation Center, U.S. Department of Agriculture, and National Oceanic and Atmospheric Administration. It depicts drought conditions for the United States only. The NADM, created in 2002, shows drought conditions across Canada, the United States, and Mexico. That's important, because drought can affect more than one country at any given time, andsometimes we need to cooperate with other nations to solve the problems drought creates. TheNADM is produced monthly by scientists from the National Drought Mitigation Center, U.S. Department of Agriculture,National Oceanic and Atmospheric Administration, Agriculture and Agri-Food Canada/Meteorological Service of Canada, and National Commission of Water–Mexico/National Meteorological Service of Mexico. For both maps, the scientists use many indices (such as the Standardized Precipitation Index, described above), climate outlooks, satellite information, news accounts, and "eyewitness" accounts to determine where drought is occurring, and how severe it is. The map shows "abnormally dry" conditions as well as "moderate", "severe", "extreme", and "exceptional" drought areas, in different colors.

The Standardized Precipitation Index
The Standardized Precipitation Index (SPI) was created in 1993 to measure rainfall shortages for a variety of different time periods. These time periods help show the impact of drought on different water sources. For example, it doesn't take long for drought to affect soil moisture. But drought takesmuchlonger to affect other kinds of water, like groundwater, streamflow, and reservoirs. For these reasons, the SPI was created for 3–, 6–,12–, 24–, and 48–month time periods, and it can be calculated for any number of months. The SPI is calculated using long-term precipitation records for a desired period. Both wet and dry periods can be monitored using the SPI. Positive SPI values mean that precipitation has been above normal, and negative values mean less-than-normal precipitation. The index uses different colors to indicate the severity of drought. The SPI defines a drought as occurring any time the SPI is negative and reaches an intensity of -1.0 or less. The event ends when the SPI becomes positive. The Western Regional Climate Center (WRCC) has additional information about the index. The WRCC also has an SPI mapping tool that lets you create a map of SPI values for various time periods.

Other Drought Monitoring Tools
U.S. Climate at a Glance is a tool that lets you map temperature or precipitation values for the entire United States or for cities, states, or regions of the United States for one or more months from any year from 1900 to 2003.

CLIMVIS lets you map Palmer Drought Index values for states or climate divisions, by year.

The National Weather Service's National Fire Weather map takes you to the Fire Weather Forecast for any part of the United States.

The Interactive Weather Information Network from the National Weather Service has maps with up-to-the-minute forecasts, watches, warnings, and news on weather in the United States and around the world.

LOTS of satellite images of the earth's vegetation!

Global Climate Highlights will give you a weekly look at what is going on around the world: where it's hot, cool, wet, or dry, and what major weather events have happened.

The International Research Institute for Climate Prediction keeps monthly summaries of climate events and their impacts.

You can track changes (from normal) in precipitation and temperature with the Southern Regional Climate Center's maps. They provide temperature and precipitation data for weekly, seasonal, and annual time periods.

The National Climatic Data Center’s (NCDC) Climate Monitoring Reports and Products page has lots of interactive mapping tools and reports on all kinds of climate-related topics. You can find worldwide and U.S. temperature and precipitation data as well as snow data and information on the most extreme weather and climatic events ever recorded.

The Climate Prediction Center of the National Oceanic and Atmospheric Administration has regional climate maps showing precipitation and temperature information for most of the world.

USGS Water Watch, from the U.S. Geological Survey, provides maps of real-time streamflow for the United States. The map tracks daily changes in stream and river flows; if you click on any state, you'll find more detailed information for that state.

Check out the NDMC's monitoring links for more tools and information.