RSI is a regional snowfall impact scale that uses the area of snowfall, the amount of snowfall, and the number of people living within a snowstorm. Since the index uses population information, it attempts to quantify the societal impacts of a snowstorm. RSI has been calculated for large snowstorms back to 1900 and therefore the index puts a particular event into a century scale historical perspective.

RSI is reported as both a raw index value and a categorical value from 0 through 5. The raw index value can range from 0.01 to 35.0. These values are converted to categories in the following manner:

RSI Category Definitions

Category	RSI Raw Score	Approximate Percent of Storms
5	>18	1%
4	10-18	2%
3	6-10	5%
2	3-6	13%
1	1-3	25%
0	0-1	54%

So a Category 5 snowstorm is a very rare event while Category 0 and 1 snowstorms are quite typical.

RSI is currently a new product produced by the National Centers for Environmental Information. It is envisioned that RSI will be used to help place snowstorms into a century-scale historical index. Some examples of potential include federal and state emergency managers, government and private sector financial agencies, insurance companies, and the transportation industry.

RSI is calculated for specific regions (Northeast, Southeast, …). Only the snowfall within a particular region is used to calculate the index for that region. NESIS on the other hand uses snowfall and population information from the eastern two-thirds of the U.S. in its calculation. Therefore NESIS is a quasi-national index. The "Northeast" designation is due to the fact that the algorithm used to calculate NESIS has two constants specifying the average area and population affected by 10" or more snowfall. So NESIS can be considered a quasi-national index that is calibrated to the Northeast.