Discussion: One way to generate a flash drought is presence of an amplified upper-level high-pressure ridge during summertime across a region hampered by deep layer (10-200 CM) soil moisture deficit. Precipitation deficits over a 9-month (or more) period leads to deep layer soil moisture shortages. When high-pressure areas reside over these dry regions during summer a feedback mechanism evolves whereas the dry soils lead to greater potential temperature at ground level which in-turn makes the air aloft warmer. The warming upper air strengthens the high-pressure ridge during the feedback process causing each successive day during a heat wave to turn hotter. This scenario causes a fast-emerging surface dry soil moisture pattern or flash drought.
Fig. 1: Precipitation anomalies for the past 12 months across Europe and into Western Russia.
Currently, long-term (12 months) precipitation shortages in the northern hemisphere are most pronounced across Romania, Hungary and Serbia plus Eastern Turkey into Armenia and Georgia and throughout the Russia spring wheat-growing zone (Fig. 1). Each zone is susceptible to flash drought during the warm season ahead if a high-pressure ridge anchors across or near each region.
Although short-term precipitation deficits have caused drought to emerge across the western and into the central Great Plains, a long-term precipitation shortage implying deep-layer soil moisture shortage and flash drought risk is not present (yet). In the U.S., the most pronounced region observing long-term precipitation deficit is in the Carolinas and Florida each susceptible to flash drought during the upcoming warm season given the presence of stagnant high-pressure ridging (Fig. 2).
Fig. 2: Precipitation anomalies for the past 12 months across the U.S., northern Mexico and southern Canada.