Updates and Additions to the Prototype NWS Winter Storm Severity Index (WSSI) Travel

The prototype Winter Storm Severity Index (WSSI) Travel is a forecast tool in development at the Weather Prediction Center (WPC). Its algorithms take numerical weather prediction forecast data from the High-Resolution Rapid Refresh (HRRR) model and output a measure of how impactful the forecasted weather conditions could be to surface transportation. WSSI Travel exists within a suite of WSSI products available from WPC, including the deterministic and probabilistic WSSI. Whereas the deterministic and probabilistic WSSI output winter weather impact forecasts in 24 h windows, WSSI Travel produces hourly impact guidance out to 48 h every 6 h. Impacts are defined on a 4-tiered scale from “minor” to “extreme,” and displayed on an interactive map through the use of Geographic Information Systems (GIS), allowing forecasters to maintain situational awareness of surface-transportation hazards and impacts caused by impending winter storms. Such situational awareness is crucial to providing relevant and meaningful impact-based decision support services to NWS core partners and stakeholders.

WSSI Travel was first made available to WPC forecasters in the 2021-22 winter season. It included 3 components – snow accumulation, snow rate, ice accumulation – and an overall summary display, but lacked interactive elements such as zooming and panning to areas of interest. The WSSI Travel algorithms underwent an overhaul in 2022 to incorporate time of day influences to precipitation rate impacts, including those from liquid precipitation (i.e., rain and freezing precipitation) in addition to snow. The updated precipitation rate algorithms were introduced in December 2022 alongside an updated and interactive webpage that is available to all NWS forecasters. Since then, additional components have been developed and added to the webpage: snow amount, ice accumulation, blowing and drifting snow, and wind. Each component has been calibrated to reflect impacts to surface transportation, including, but not limited to: increased travel times, lane or road closures, vehicle or speed restrictions, and increased crash counts and severity. Details about WSSI Travel development, including the new components, will be presented alongside case study examples of the product output.