National Weather Service developing new tool to track snow, ice severity |

National Weather Service developing new tool to track snow, ice severity

The Winter Storm Severity Index, or WSSI, a new experimental product being tested by the National Weather Service, suggests "limited" impact of winter storms moving into Northwestern Colorado through 1 p.m. Jan. 30.
National Weather Service/courtesy

As winter storms continue to roll across the United States, the National Weather Service is rolling out a new tool — the Winter Storm Severity Index, or WSSI — which aims to provide an indication of the level of winter precipitation severity and societal impacts.

Currently, winter weather watches, warnings, and advisories are based primarily on “yes/no” thresholds of accumulation, generally at the level of individual counties, according to Project Lead Andy Nash, of NWS in Burlington, Vermont, in his Guide for Users.

“The reality is that severity and impacts from winter weather are due to more than just amounts  — one five-inch snowstorm is not like the next five-inch snowstorm — additionally, there can be great variation in weather conditions frequently within individual counties,” he wrote.

With that in mind, WSSI seeks to combine multiple types of data, including the following:

• Snow amount: Severity due to the total amount of snow or rate of snowfall accumulation, with adjustments made based on climatology and urban areas. For example, four inches of snow in Atlanta is more severe than four inches in Minneapolis.

• Snow load: Severity due to the total weight of snow on trees and power lines.

• Blowing snow: Severity of impacts, mainly to transportation, due to blowing and drifting snow.

• Ice accumulation: Severity of impacts to transportation and downed trees and/or power lines due to the accumulated ice in combination with wind.

• Flash freeze: Severity of impacts, primarily to transportation, in situations where temperatures rapidly fall below freezing during precipitation.

• Ground blizzard: Severity of impacts, mainly to transportation, of ground blizzards that develop due to a pre-existing snowpack and strong winds.

Conditions, when mapped, are given a severity rating and color, ranging from none (gray) to extreme (deep red) on a temperature map. Clicking on points of interest allows users to understand more localized conditions based on the above types of conditions.

Winter Storm Severity Index (WSSI) ratings from none to extreme.

In one example of how to interpret the data, the WSSI maps a storm impacting Wyoming and Colorado showing “major” to “extreme” severity in southeast Wyoming and “limited” around Denver.

The forecast data shows “snow amount” around Denver, so snow is the primary aspect of the winter weather, whereas for Wyoming, “ground blizzard” is the primary aspect.

The final interpretation is to expect quite severe ground blizzard conditions in southeast Wyoming, while Denver will experience snow that won’t disrupt daily life.

The new system is available at the WSSI Project website

The WSSI does not depict official warnings and should be used in context with official NWS forecasts and warnings.

As an experimental product, it may not updated in a timely fashion.

Contact Sasha Nelson at 970-875-1794 or