Prediction of Flash Droughts over the United States

We examined reforecasts of flash droughts over the United States for the spring (April-May), summer (June-July) and autumn (August-September) with lead times up to 3 pentads based on NOAA Medium range Weather model reforecasts (MRF). We examined forecasts of two types of flash droughts: heat wave flash droughts initialized by high temperature and the precipitation (P) deficit flash droughts caused by the lack of P. We find that heat wave flash droughts occur most often in spring over the North Central and Pacific Northwest. Most drought events occur in spring. Events decrease as time progresses. The MRF reforecasts capture the frequency of occurrence (FOC) pattern and are overall skillful up to 2 pentads. The skill is influenced by the skill of temperature forecasts. In contrast to heat wave flash droughts, P -deficit flash droughts occur throughout spring and summer across much of the southern United States and along the path from Texas to the Northern Plains. The MRF reforecasts capture the basic pattern of the FOC and are skillful up to lead 2 pentads. However, the reforecasts overestimate P deficit flash drought occurrence over the western part of the region where P deficit flash droughts occur especially in spring leading to large false alarm rates. For summer and autumn, the reforecasts underestimate P deficit flash drought occurrence over the western part of the domain. The MRF reforecasts are able to capture the approximately linear relationship between ET and SM, but the lack of skill in P forecasts limits the skill of the P deficit flash drought forecasts.