Algorithm Testing of the Winter Storm Severity Index (WSSI)

The Winter Storm Severity Index (WSSI) is a tool produced by the Weather Prediction Center (WPC) to assist National Weather Service (NWS) operational forecasters maintain situational awareness and to help depict the potential for significant weather impacts based upon the latest official forecast. It is also useful to external audiences such as the media and general public to enhance clarity and communication about upcoming weather events. The WSSI uses geographic information systems (GIS) and official NWS gridded forecasts from the National Digital Forecast Database (NDFD) meteorological data with non-meteorological data to communicate the severity and spatial distribution of potential impacts. In doing this, it looks at six types of events characteristic of winter storms: snow amount, snow load, ground blizzard, blowing snow, flash freeze, and ice accumulation. The output of these components are then categorized into six levels: no impacts expected, winter weather area, minor impacts, moderate impacts, major impacts, and extreme impacts. The WSSI has evolved into a suite of tools that, in addition to the NDFD version, focus on the probability of impact (Probabilistic WSSI) and short term travel impacts (Travel WSSI). Improvement to the WSSI algorithms is continuously sought. Internal NWS and partner feedback help highlight priority areas to focus on. This presentation will showcase improvements to the ground blizzard, blowing snow and flash freeze components. Algorithm testing of the WSSI consists of determining what specific components in the tool need to be improved and developing a test plan consisting of numerous iterations and comparing the results against each other to the original formulation. This was done with the ground blizzard component, to try and better tune the algorithm for colder and winder conditions. This methodology helps identify which set up provides the most improvement towards our desired outcome for the future. Additionally, algorithm testing was conducted on a new melting snow characteristic within the flash freeze algorithm. Melting snow is now a part of the flash freeze tool, and it is treated like rainfall in the calculation for flash freeze. This presentation will also key in on the specific components of WSSI that were improved using algorithm testing and show the results of the effect the testing had on the output of WSSI. The Travel WSSI uses High-Resolution Rapid Refresh (HRRR) model weather data alongside transportation related factors and time of day factors. Weather conditions, and parameterized road surface conditions are used to generate the severity of winter storm impacts on surface transportation. A redesigned blowing snow algorithm was tested, aimed at generating impacts for reduced visibility. Snow squall and mountain pass cases were rigorously tested to ensure impacts forecast improvements for these tough scenarios. Conducting algorithm testing and analysis on these tools of WSSI, will help improve the tool as a whole and maintain the integrity of this tool for use from many sectors.