Flash Drought

Unlike slow-evolving drought, which is caused by a decline in precipitation, flash drought occurs when low precipitation is accompanied by abnormally high temperatures (e.g., heat waves), high winds, and/or changes in radiation. These sometime-rapid changes can quickly raise evapotranspiration rates and remove available water from the landscape.

Geographic differences and climate patterns also impact the development of flash drought. In contrast with conventional drought—which may occur throughout the year at any location—flash drought typically occurs during warm seasons in the central United States. Widespread flash drought in the U.S. is often tied to La Niña events.

One study found that between 1979–2016, flash droughts in the western U.S. occurred more frequently in May and June, with the Northwest climate region seeing an additional peak at the end of the growing season. In contrast, flash drought frequency across the central and parts of the eastern U.S. peaked in July and August. The frequency of flash drought in the Southeast region generally peaked in May. For all climate regions, flash drought intensity tended to increase toward the beginning of the growing season and then decrease.

The prediction of flash drought is critical to successfully mitigating and preventing damages whenever possible. Scientists have identified evapotranspiration and soil moisture as early warning indicators for the development of flash drought. Changes in evapotranspiration correspond with flash drought in the contiguous United States, indicating that it may be a primary mechanism involved in the sudden onset and intensification of drought conditions. 

In one study, researchers found that all flash droughts studied were preceded by sudden increases in evapotranspiration rates caused by changes in heat, wind, and radiation. Soil moisture was plentiful prior to the onset of drought, but decreased as evapotranspiration rates rose. They concluded that changes in evapotranspiration rates are a strong indicator of flash drought, and have developed a flash drought prediction tool to advance our ability to forecast these events (Chen et al. 2020).

Despite the importance of prediction, scientists have found that flash droughts occur even when conditions just two months prior to the event appeared to be unfavorable for drought development, demonstrating that flash drought can occur even when there are no preceding signs that can be detected with known metrics (Christian et al. 2019). Better prediction of flash droughts is an active area of research.

Flash drought occurs more often than many people realize and can cause major impacts. Significant negative impacts to the agricultural sector have been better documented than impacts to other sectors and the environment. Flash drought has occurred in the southeastern United States as recently as the fall of 2019. In a widespread flash drought across the central U.S. during the summer of 2012, damages in the central Great Plains were estimated to be in excess of $30 billion.

Although most flash droughts do not persist to become prolonged drought, in one study, 5 to 10 percent of flash droughts transitioned to the highest drought category given by the U.S. Drought Monitor: Exceptional Drought, or D4 (Christian et al. 2019).