What You Need To Know About The Winter Storm Severity Index

You may see a winter storm warning and snowfall maps issued for your area and wonder, "How bad will this storm be?" The National Weather Service has just the map for you, and you may see us at weather.com use it on occasion as well. So, what is it?

The Winter Storm Severity Index (WSSI) is a tool created by the National Weather Service (NWS) to communicate the potential significance of impacts from a particular system.

Understanding what a winter weather forecast means for disruptions to your day-to-day activities can be difficult. This tool was created to better depict the different threats associated with each storm and to give a better understanding of how conditions could vary.

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How this tool helps winter weather decision-making: Winter watches, warnings and advisories are often issued at an individual county level - and let’s be real, winter storms don’t follow county lines.

The WSSI breaks the forecast down to give a better understanding of how conditions could vary spatially within each county.

The tool also addresses the common misconception that snow totals are the primary factor that determines the severity of a winter storm. Other threats such as ice or wind could have the most severe impact instead.

By taking into account six different winter weather factors, the WSSI gives a more comprehensive view of what impacts will be most prominent in your area.

The NWS notes that although the WSSI pulls together the impacts of different aspects of the storm in one place, it should not be used as a sole source of information for a storm.

The tool only uses forecast information, meaning it does not account for conditions that happened prior to the index being issued. This means that during an ongoing winter storm, the WSSI will not be representative of the entire event.

Those in the minor severity category will only see a few inconveniences to daily life while those who fall into the major severity category should expect considerable disruptions.

Though rare, the extreme index is reserved for severe impacts with substantial disruptions.

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What factors go into the forecast? The WSSI incorporates several different aspects of snowfall, including the expected totals, the rate at which it is falling, and the water content (light, fluffy snow versus heavy, wet snow).

The tool also includes how much ice is anticipated and whether temperatures will fall rapidly during or after precipitation and potentially lead to flash freezing. Additionally, the WSSI looks at the potential of wind to cause blowing snow or a ground blizzard.

But it doesn’t only look at meteorological factors, the tool also takes into consideration non-weather factors such as population density and land use which can exacerbate impacts.

Interpretation 101: The image below shows an example of what the WSSI looks like. The top three panels show the breakdown of individual components - snowfall amount, how heavy the snow will be, and the ice accumulation, respectively. The bottom right image shows these components combined to form the overall WSSI rating for each area.

The highest impact level for each given location is what is shown in the overall rating.

It should be noted that there are certain limitations to this tool. The WSSI is only issued for the next 72 hours, and while it can be broken down by day, there is no timing information provided within each 24-hour period.

The WSSI, though helpful to determine impacts to your area, should still be used in conjunction with winter weather watches, warnings and advisories as well as hourly forecasts to get a full understanding of each winter storm.

C​aitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.