The Accumulated Winter Season Severity Index (AWSSI)

Indices and scales exist to aid in the ranking and classification of a number of weather and climate events, including tornadoes, hurricanes, northeastern United States winter storms, and droughts. The ranking and classification systems facilitate the communication of particular hazards to the public, allowing the collection of multiple factors into one scale that is calibrated to describe the risk or impact from that hazard. Winter seasons have significant societal impacts across all sectors, with impacts that range from direct human health and mortality to commerce, transportation, and education.

In this study, the Accumulated Winter Season Severity Index (AWSSI), a point-based index system, is calculated based on temperature and snow fall and snow depth thresholds for several sites across the United States. A parallel index also calculates AWSSI based on temperature and precipitation observations, allowing analysis of sites with missing or unreliable snow records as well as historical analysis of sites before snow records were kept. Daily scores are calculated based on the thresholds, and values are accumulated through the winter season, allowing a running total of winter severity in the midst of a season as well as a final, cumulative value representing the full season. At an individual site, the values across multiple seasons can be compared to determine the relative severity of a winter season in the context of the climatology of that site. The values are divided into five percentile-based categories, labeled from “W1 – Mild” to “W5 – Extreme”, to communicate relative severity of a winter. Additionally, the values at each site can be normalized by an averaging period to represent a scaled severity index, allowing comparison among sites. The result of the index is a scale of winter season severity that is meaningful to create a relationship between the winter and its impacts, with a climatology that can be analyzed throughout a winter season as well as historically. Applications of the index include climate variability and change analysis, as well as sector-based use.