资源环境科技发展态势分析平台

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 triple the expected 30-year average. Multiple-day outbreaks of tornadoes are not unprecedented in the United States; 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 Indian and Pacific Oceans, causing subsequent changes in Northern Hemisphere atmospheric angular momentum. This manuscript analyzes the evolution of hemispheric-scale circulation features leading up to the event, examines teleconnection processes known to influence U.S. tornadoes, and provides insights into the forecast process at subseasonal lead times.