What is the WSSI: Winter Storm Severity Index?

The winter storm severity index or WSSI is a product created by the Weather Prediction Center [WPC] to communicate potential impacts during significant winter weather events.

There are six components that combine to convey the potential overall impact. Each is also individually rated on a 1 to 5 scale from limited to extreme. Limited impacts typically result in minor inconveniences whereas extreme impacts cause widespread and severe property damage with life saving actions needed. The one thing WSSI does not show is the exact timing of this winter impact.

Snow amount is based on snowfall totals and rates. Here, climatology and population are considered, for example, places in the south are not as accustomed to snow resulting in a higher impact level compared to the Northeast under the same parameters.

Snow load takes into account potential infrastructure damages like down trees and power lines resulting from the weight of snow. Forested and urban areas will show larger impacts compared to grasslands.

Ice accumulation keeps in mind the impact from both ice and wind, things like widespread tree damage, transportation shutdowns and utility problems.

Places where new blowing and drifting snow could cause transportation related problems like low visibility are reflected in the blowing snow index.

Pre-existing snowfall, combined with strong winds can also lead to white out conditions. Ground blizzard index assesses this severity. Snowfall does not even need to be in the forecast to create major disruptions here.

Lastly, when temperatures rapidly drop below freezing during or after a precipitation event, it can cause icing on roadways. The flash freeze index depicts this severity level.