Towards the development of an impact-based decision support tool for surface-transportation hazards. Part II: An Hourly Winter Storm Severity Index

Abstract In line with the continued focus of the National Weather Service (NWS) to provide Impact-based Decision Support Services (IDSS) and effectively communicate potential impacts, a new IDSS forecasting tool for surface-transportation hazards is in development at the Weather Prediction Center: The Hourly Winter Storm Severity Index (WSSI-H). This second part of the series outlines the current algorithms and thresholds for the components of WSSI-H, which has been developed in line with the approach and considerations discussed in Part I of this series. These components – Snow Amount, Ice Accumulation, Snow Rate, Liquid Rate, and Blowing and Drifting Snow – each address a specific hazard for motorists. The inclusion of metrics related to driving conditions for untreated road surfaces, and time-of-day factoring for active precipitation types, help to directly tie forecasted weather conditions to transportation impacts. Impact severity level thresholds are approximately in line with thresholds used by transportation agencies when considering various mitigation strategies (e.g., imposing speed restrictions or closing roadways). Whereas the product is not meant to forecast specific impacts (e.g., a road closure or pileup), impact severity levels are designed to scale with increasingly poor travel conditions, which can prompt various mitigation efforts from motorists or transportation agencies to maintain safety. WSSI-H outputs for three winter events are discussed in depth to highlight the potential utility of the product. Overall, WSSI-H is intended to provide high-resolution situational awareness of potential surface-transportation-related impacts, and aid in enhanced collaborations between NWS forecasters and stakeholders like transportation agencies to improve motorist safety.