The Winter Storm Severity Index (WSSI) is a National Weather Service tool, hosted by the Weather Prediction Center (WPC), that turns the official winter weather forecast into a map of potential societal impacts. Instead of showing how much snow or ice will fall, it estimates how disruptive that weather could be at each location.
How the index is built
The WSSI combines forecast weather data from the National Digital Forecast Database (NDFD) with non-meteorological layers such as population density, land cover, and tree type and density. That second set of layers is what turns a weather forecast into an impact estimate.
The current user guide lists four components: Snow Amount, Snow Load, Ice Accumulation and Blowing Snow. The overall rating at any point is the highest impact level reached by any single component. The NWS removed the earlier Flash Freeze and Ground Blizzard components in October 2025.
Why the same snow rates differently
The Snow Amount component accounts for local snow climatology. Places that see less snow on average may be less prepared for it, so the same 6 inches can rate higher in a southern city than in a snowbelt town.
Snow Load weighs the weight of accumulated snow against tree type and density, leaf cover and wind, while Ice Accumulation looks at ice on roads, sidewalks, tree limbs and power lines. Blowing Snow focuses on visibility loss and drifting. Checking the individual components tells you which hazard is driving the overall color.
Reading the maps
The 2025 user guide describes impact ratings of Minor, Moderate, Major and Extreme. Below those, a Winter Weather Area marks places where winter conditions are expected but are not expected to cause notable impacts.
The WPC page offers Day 1, Day 2 and Day 3 maps plus a combined Day 1 to 3 view.
WPC also produces a Probabilistic WSSI (WSSI-P). It runs the same algorithms and non-meteorological data across an ensemble of forecasts, 60 members in WPC's user guide, and shows the chance of reaching each impact level in rolling 24- or 48-hour windows out to 168 hours (7 days).
What it does not tell you
The WSSI shows a general level of potential impact, not conditions on a specific road at a specific hour. If the snowfall or ice forecast shifts, the impact map shifts with it.
It does not depict official warnings and is meant to be used alongside them. Local winter storm watches, warnings and advisories remain the official alerts, and the probabilistic view helps show how confident the forecast is.