NOAA's National Climatic Data Center (NCDC) began calculating NESIS operationally in 2006.
The RSI is an evolution of NESIS and, like NESIS, combines aspects of intrinsic disruption (snowfall accumulation and area) and one aspect of societal susceptibility (population) to estimate realized disruption.
For example, calibration of NESIS is based on 30 high-impact storms in the Northeast that occurred between 1950 and 2000 (Kocin and Uccellini 2004), while the RSI uses storms that occurred between 1900 and 2013.
The original NESIS algorithm uses snowfall thresholds of 4,10, 20, and 30 in.
Next, a relationship was found between these average return period values in the Northeast and the existing NESIS thresholds.
The same categorical descriptions as NESIS are used: notable (category 1), significant (category 2), major (category 3), crippling (category 4), and extreme (category 5) (Kocin and Uccellini 2004).
The RSI is a regional index that complements the NESIS (a quasi-national index) and the LWSS (a station-specific index).