From late August through early October last year, drought engulfed much of the Southeast U.S. in a way that many don’t expect drought to behave -- suddenly. Only 7% of Georgia began September in drought. By the end of the month, drought covered 62% of the state. Drought throughout the Southeast region spiked from covering about 6% of the region to 44% in less than a month. While many droughts embody the frequent description of the natural disaster as a creeping phenomenon, taking months or years to develop, the Southeast drought grew widespread and severe in a matter of weeks.
It was a classic flash drought, as defined in a new research paper that seeks to clarify what flash drought is and how to better predict it.
Mark Svoboda, director of the National Drought Mitigation Center, headquartered at the University of Nebraska-Lincoln’s School of Natural Resources, is credited with coining the term in the early 2000s. Svoboda said he initially used the flash drought phrase to help explain to a USA Today reporter that a drought in the Southern Plains in the early 2000s was developing with unusually rapid intensity. “I wanted to find a term that would resonate for this quicker developing drought, and flash drought just popped in my head, as I thought people could relate it to their knowledge of flash floods,” he said. “And that took off like wildfire.”
This week, he was part of a team of 22 authors who wrote the wide-ranging study on flash droughts that was published March 2 in Nature Climate Change. The article is titled, “Flash droughts present a new challenge for subseasonal-to-seasonal (S2S) prediction.”
The paper is the end product of a September 2018 workshop, when drought experts from around the world gathered at the Aspen Global Change Institute to address S2S prediction and flash drought. The event began with the paper’s lead author, Angeline Pendergrass of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, showing images of two 2012 maps from the U.S. Drought Monitor, a service based at NDMC that utilizes multiple data sources, expert analysis and on-the-ground reporting to produce authored weekly summaries of drought conditions throughout the country and its territories.
“At the end of May, the southern Great Plains are doing OK,” Pendergrass told the gathered experts. “They’re dry, but they’re not in severe drought. By the time we’re in late July, things are much, much worse. And this wasn’t something that was predicted.”
A flash drought, as defined by the American Meteorological Society, “is an unusually rapid onset drought event characterized by a multi-week period of accelerated intensification that culminates in impacts to one or more sectors” such as agricultural or hydrological impacts.
Svoboda was part of the team that wrote the definition, and said that the Nature paper helps to more clearly define what a flash drought is (and isn’t), sets guidelines on when to detect that one has occurred and explores ways to improve monitoring and predictions.
“The growing awareness that flash droughts involve particular processes and severe impacts, and likely a climate change dimension, make them a compelling frontier for research, monitoring, and prediction,” the authors wrote.
“It’s a complex problem and we want to treat it in a complex way,” Roger Pulwarty, senior scientist in the NOAA Physical Sciences Division, said during the workshop. “It’s not easy to characterize a flash drought, and they’re not the same as saying, ‘We have a hurricane of intensity X or Y.’”
In the paper, the authors agree upon three principles that apply to flash drought -- that it involves a rapid onset, that its intensification rate is high and that the event ends in a state severe enough to qualify as a drought. Then they proposed two definitions of flash drought, one that applies internationally, as well as for prediction and research purposes, and another that applies to U.S. monitoring.
The international/prediction/research definition is based upon an experimental drought monitoring and early warning guidance tool called the Evaporative Demand Drought Index (EDDI). The tool examines differences in atmospheric evaporative demand, or essentially the atmosphere’s drying, over a time period of interest. If there is a 50% or greater increase toward drying over a two-week period that is sustained for at least two more weeks, according to the paper, a flash drought could be declared.
The U.S. definition is based on the U.S. Drought Monitor. (The USDM, which includes authors from the NDMC, U.S. Department of Agriculture and National Oceanic and Atmospheric Administration, is housed on UNL servers.) When the USDM reports a two-category change of drought over a two-week period, and it is sustained for another two-week period, a flash drought could be declared.
The authors wrote that the next steps will be to apply these definitions retrospectively to verify their accuracy in describing unusual, significant drought events that occurred well before the flash drought term was coined and interest in studying it spiked. The definitions could be further refined to reflect impacts that occur in specific regions, the authors wrote.
The Nature Climate Change paper examines specific challenges of predicting flash droughts to provide early warning of their arrival, such as forecasting precipitation deficits over sub-seasonal time periods. Svoboda said that developments in satellite technology are helping experts more quickly assess conditions like the ones that led to the 2019 Southeast flash drought.
“We have to trust this new breed of indicators that are looking at stress or demand that we can't even see with the human eye,” Svoboda said. “These tools are looking at indicators of plants that are seen by a sensor that the human eye doesn't denote until it's yellow and wilting. The satellite can see that stress in the plant before it shows visible signs to the human eye. It takes a change in mindset to evaluate, trust and integrate these new tools into the Drought Monitor process in order to be using the state of the science and being more responsive to these rapid onset flash droughts.”
-- Cory Matteson, NDMC communications specialist