Flash Drought Risk in Southeast U.S.

Discussion: Of concern is acceleration of recently emerging dryness in the Southeast U.S. (Fig. 1) rapidly toward drought (or flash drought) caused by a long duration hot and dry period (Fig. 2-3). During the hot period risk of >95F daytime temperatures is common during the next 15 days (Fig. 4-6).

Fig. 1: NOAA/USDA Southeast U.S. drought monitor indicates developing dry to moderate drought conditions.

Fig. 2-3: The GFS ensemble departure from normal temperature and percent of normal rainfall forecast through the next 15 days indicates hot/dry conditions.

Fig. 4-6: The GFS ensemble risk of maximum temperature exceeding 95F for the 1-5, 6-10 and 11-15 day periods is excessive for the Southeast U.S. (and expanding westward with time).

An emerging large region of dry soils has a tendency to increase the risk of sustained anomalous heat during the summer season. As the anomalous heat continues, additional drying of soils occurs enhancing the heat risk. As soils dry and heat risk intensifies the subtropical ridge of high pressure aloft acting as the catalyst for the hot weather strengthens. Emerging drought acts as a feedback mechanism to the atmosphere re-enforcing the hot/dry pattern. These conditions require close monitoring during heat wave forecast to affect the Southeast U.S. from late May to early June.

What is a flash-drought?

Flash drought is a more recently developed term in the weather and climate community that is used to describe a rapid onset of dry conditions that impact numerous industries inherently tied to water use. Typically drought conditions are characterized by slower onset.

Drought is mostly basically defined as a precipitation deficit in a given climate zone. However, the speed at which drought develops and the extent/severity of the drought ultimately determines the final impacts. The speed and severity of drought is influenced by factors such as lack of precipitation coupled with increased evaporative demand due to higher temperatures, lower atmospheric humidity, stronger wind speeds, incident solar radiation, and consumption of soil moisture from the surrounding ecosystem. When conditions such as this exist for for days to weeks, there is a transition from energy-limited evapotranspiration to water-limited evapotranspiration in vegetation that leads to a rapid increase in vegetation stress. These conditions are most common during the growing season when evaporative demand in the atmosphere climatologically peaks. Thus, the atmospheric conditions have the greatest impacts to agriculture and ecosystems.

Typical stages of flash drought development:

• Adequate to surplus soil moisture (vegetation and ecosystems not using much energy to obtain water)
• Onset of a period of enhanced evaporative demand (high temperatures/heat, low humidity, increased winds, coupled with little to no precipitation)
• Increase in evapotranspiration (ET) as vegetation responds to anomalous weather conditions (ET occurs to cool the plant)
• Period of rapidly decreasing soil moisture content (increased ET means vegetation pulls more water from the soil)
• Transition to water-limited conditions in soil, reduced ET (plant reduces water consumption to preserve the water available)
• Emergence of visible signs of vegetation moisture stress as soil moisture continues to decline
• Rate of intensification and final severity of flash drought varies, as these elements are influenced by the strength and persistence of the atmospheric conditions forcing the evolution of the drought, as well as the vulnerability of vegetation to lack of water and weather conditions.
• Once the period of rapid drought intensification ends, the flash drought could potentially develop into hydrological drought or simply be terminated by a heavy precipitation.

Reference: Otkin, J. A., M. Svoboda, E. D. Hunt, T. W. Ford, M. C. Anderson, C. Hain, and J. B. Basara, 2018: Flash droughts: A review and assessment of the challenges imposed by rapid-onset droughts in the United States. Bull. Amer. Meteor. Soc., 99, 911–919, https://doi.org/10.1175/BAMS-D-17-0149.1.