You’ve heard of a flash flood, but what about a flash drought? Flash droughts are extreme weather events that occur rapidly, much like flash floods, and can have severe impacts on plants, animals, and people. Researchers at the National Center for Atmospheric Research in Boulder, Colorado in collaboration with Californian, Australian, and German universities have now proposed two methods to identify whether an area is suffering from a flash drought.

Currently, most drought monitoring is done on a monthly or weekly schedule. But flash droughts need monitoring methods for daily timescales. One potential variable that these researchers draw on is the Evaporative Demand Drought Index (EDDI), a measurement of how "thirsty" the atmosphere is in a certain location for a given period of time. They also propose using the US Drought Monitor (USDM). This system classifies areas by five categories that range from D0 (abnormally dry) through D4 (exceptional drought).

Using these metrics, the researchers came up with two scientific definitions for flash droughts: 

1) A 50% increase in Evaporative Demand Drought Index (EDDI) for two weeks, with the resulting high EEDI levels persisting for another two weeks

2) A two-category change in the USDM over two weeks that persists for another two weeks.

Both definitions come with their own caveats, but the researchers note that the first definition could be a good general definition for prediction and climate change research. Because the second definition uses the USDM, it is only relevant in the United States, and this definition would have to be tailored to existing drought monitoring systems for use in other countries.

These definitions are a good foundation for the next steps toward understanding flash droughts: Future research on the connectivity of soil moisture, land to atmosphere interactions, large scale atmospheric events, and the effects of a changing climate will be needed to characterize this new type of extreme weather event.