An example of what the Winter Storm Severity Index looked like for the Plains for a winter storm in March 2019.

(NOAA/National Weather Service)

At a Glance

A new index for winter storms from the National Weather Service is live.The Winter Stormy Severity Index is designed to better communicate the signficance of impacts from a winter storm.The index takes into account snow, ice, wind, temperatures, location and population.

Get ready to see a new forecasting tool from the National Weather Service (NWS) this winter.

The is a new operational tool developed by the NWS to better communicate the significance of any impacts from a particular system.

Determining exactly what a winter storm forecast means for disruptions to daily activities and travel can be difficult, but this newly operational index will likely make it easier to make decisions by taking the guesswork out of what the forecast for snow, ice and wind means for a given winter storm. It also pulls together all of the individual weather factors into one piece of information.

The index takes into account conditions that can change how significant a winter weather event is in a particular area and solves the common misconception that a storm's severity is determined just by snow totals. For instance, in some situations, the greatest threat can be from ice or wind.

The NWS notes that the (WSSI) is not an exact forecast for specific impacts and is not meant to be the sole source of information about a winter storm. It also only takes into account forecast information and not conditions that occurred prior to the index being issued. This means that during an ongoing winter storm the WSSI will not be representative of the entire event.

(MORE: 5 Extreme Weather Patterns to Watch For During the Winter)

Winter Storm Severity Index scale descriptions

(NOAA/National Weather Service)

There are five levels of severity for potential impacts: limited, minor, moderate, major and extreme. Limited impacts represent little inconveniences, while the extreme category is reserved for severe disruptions to daily life. Extreme storms should be rare.

The WSSI looks at several aspects of snowfall, including totals, rates and water content (heavy, wet snow versus light, fluffy snow). The index also considers winds, which can lead to blowing snow or a ground blizzard. It also looks at how much ice accumulation is anticipated and whether temperatures will rapidly fall during or just after precipitation. All of these aspects suggest where damage to trees and powerlines is a greater concern. The WSSI also takes into account population density and location (climatology), which can exacerbate impacts.

(NOAA/National Weather Service)

The image above gives an example of what the index looks like. The top three images show where different pieces of the index (snowfall totals, how heavy will the snow be, and ice) will be the most concerning. Those parts come together to form the WSSI index rating for each area, shown in the bottom image. The highest level in a given location is what the overall WSSI shows.

There are some limitations to the WSSI. It provides a 72-hour outlook and does not indicate when the winter weather will occur. This index is not intended to replace other forecast products, including winter storm warnings, winter weather advisories and specific hourly or daily forecasts.

The NWS introduced this tool on an experimental basis in 2018, and after feedback from inside and outside the NWS, it is now an official tool.

This index will continue to change and improve and may eventually .

There could also be an opportunity to adjust the index for , but the WSSI is a great step in the right direction to better communicate how significant a winter storm may be and what aspects of the system are the most concerning for a particular area.

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