Advancing the Science of NOAA's Weeks 2-4 Prediction of Severe Weather

Severe weather – defined here as tornadoes, damaging winds, and hail associated with continental convection – accounts for nearly 50% of all billion-dollar weather disasters in the U.S. since 1980. A record number (28) of billion-dollar disasters occurred in 2023 with 18 of those disasters associated with severe weather. Through 8 August 2024, 19 billion-dollar weather disasters (15 of those associated with severe weather) have been confirmed. Clearly, severe weather and its attendant hazards have immense socioeconomic impacts and NOAA has significant interest in extending the forecast lead times for these events by advancing weeks 2–4 forecasting capabilities. A complicating aspect of this research and forecasting problem is that it involves a complex two-way interaction among different components of the climate system, from weather-to-subseasonal and subseasonal-to-seasonal (S2S) timescales. Significant efforts by the academic community, with organized extended range research and forecasting efforts such as the Extended-Range Tornado Activity Forecast (ERTAF) Project conducted from 2015–2020, have shown potential in predicting tornado activity at lead times of 2–3 weeks during spring. In response to the promising results from activities such as ERTAF and the significant interest within NOAA to extended lead times of forecasts of severe weather, as presented in NOAA’s report to Congress on S2S forecasting plans in 2020 and highlighted as a critical societal challenge area in the NOAA Office of Oceanic and Atmospheric Research’s (OAR’s) fiscal year 2023-2027 Weather Water and Climate Strategy, the Severe Weather Weeks 2–4 Tiger Team was established. The tiger team is composed of scientists from several OAR laboratories (NSSL, AOML, GFDL, PSL, GSL, and ARL), the Storm Prediction Center and Climate Prediction Center, and the Weather Program Office and Climate Program Office, and is tasked to assess the state of the science for the severe weather weeks 2–4 research and forecasting problem and recommend a path forward on how to advance NOAA’s science and forecasting capabilities. The aim of this presentation is to report on the findings of the tiger team and present recommendations on how to advance NOAA’s science and forecasting capabilities for severe weather at weeks 2–4 lead times.

Currently, no operational weeks 2–4 or S2S severe weather outlook is produced by NOAA. However, there are operational forecast products, such as the probabilistic severe weather forecasts at lead times through day-8 from the SPC. In addition, the Weather Prediction Center issues a U.S. Hazards outlook for days 3–7 that includes severe weather hazards, while the CPC product suite focuses on temperature and precipitation at week 2 through seasonal. However, extending severe weather outlooks into weeks 2–4 pose many limitations. For instance, the Storm Data database is influenced by many non-meteorological factors that result in inconsistencies in the data making it difficult to produce calibrated forecasts and conduct a meaningful verification of forecasts. Also, oftentimes the type of severe weather hazard is closely related to the convective mode and numerical models are too coarse to resolve convection. Experimental severe weather forecast products for week 2 and beyond have used an ingredients-based approach focusing on environment proxies for severe weather. These products are produced by NSSL, AOML, CPC, and academic partners, but a formal evaluation framework is needed to assess the skill of these forecasts.

The tiger team compiled a set of science goals and recommendations in short (1-3 years), medium (3-5), and long-term (5-10) horizons to overcome the current research challenges, gaps, and limitations, and advance NOAA’s science and forecasting capabilities for severe weather at weeks 2–4 lead times. In addition to the science goals, the tiger team will present an immediate objective proposing the establishment of a severe weather weeks 2–4 forecasting experiment in the NOAA Hazardous Weather Testbed. The forecasting experiment will provide a formal framework to evaluate weeks 2–4 forecasts of severe weather, facilitate collaboration among OAR laboratories and operational, academic, and private-sector partners, and spur further research and development of forecast products to transition to operations. Details on the tiger team’s recommended science goals and forecasting experiment will be presented.