Subseasonal Prediction Skill of the Fall 2019 Southeast U.S. Flash Drought in SubX Models

In September 2019, excessively dry and warm conditions in the southeastern United States led to a rapid-onset drought that caused widespread agricultural disruption. Previous research by the authors shows that tropical forcing and hemispheric teleconnections played key roles in driving this event. In particular, forcing associated with the extreme positive Indian Ocean Dipole event in Fall 2019 contributed to the generation of two quasi-stationary cross-Pacific Rossby wave trains: one that intensified the southeast U.S. drought in late September and a second that ended it in late October. Given that the drought was strongly influenced by remote sources that are potentially predictable (i.e., oceanic temperatures and Rossby wave trains), a key question is whether the event was predicted at subseasonal lead times (2-4 weeks in advance) in modern dynamical forecast models. Here, we examine the prediction skill of the 2019 event in seven models participating in the Subseasonal Experiment (SubX) project. Preliminary results show that skillful predictions of the event's onset and demise over the U.S. are generally limited to within the first two forecast weeks. Nonetheless, nearly all SubX models exhibit skillful subseasonal predictions of the tropical oceanic temperature and precipitation anomalies that are linked to the southeast U.S. via atmospheric teleconnections. Indeed, some models further predict the development of the observed Rossby wave trains with some fidelity at 2-3-week lead times, particularly for the second wave train that ended the drought in mid-late October. The analysis highlights the need for an even deeper understanding of the physical drivers of the Rossby wave trains in Fall 2019 as well as the deficiencies in forecast models that limit the skillful prediction of these waves.