Extended U.S. Tornado Outbreak During Late May 2019: A Forecast of Opportunity

The second half of May 2019 was an unusually active period for tornadic thunderstorms across the U.S. Great Plains, Midwest, and lower Great Lakes. While this period typically coincides with the peak climatological frequency of tornadoes, preliminary reports of tornadoes were over double the expected 30-year average. Multiple-day outbreaks of tornadoes are not unprecedented in the U.S.; however, this event was perhaps the first to be forecast at subseasonal lead times (3-4 weeks) by the Extended Range Tornado Activity Forecast team. This forecast of opportunity was driven, in part, by anomalous convective forcing in portions of the tropical Pacific Ocean and subsequent changes in large-scale atmospheric angular momentum. This manuscript analyzes evolution of the hemispheric-scale circulation features leading up to to the event, examines teleconnection processes known to influence U.S. tornadoes, and provides insights into the forecast process at subseasonal lead times.