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 (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.

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

Currently, winter weather watches, warnings, and advisories are raised based primarily on “yes/no” thresholds of accumulation generally at the level of individual counties, stated 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 stated.

Therefore, WSSI seeks to combine multiple types of data.

  • 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 from none (gray) to extreme (deep red) on a temperature map. Clicking on points of interest allows the user to understand more localized conditions based on the above types of conditions.

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 there.

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 by visiting 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 update in a timely fashion.

Contact Sasha Nelson at 970-875-1794 or

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