When it comes to describing the severity of a snowfall, inches alone don't tell the whole story. Take a 10-inch snowfall. It makes a big difference if those 10 inches fall over rural mountains or if they fall over a city. How much snow, over how many densely populated acres makes the difference between a snowstorm and a snowmaggedon.
That's why the National Oceanic and Atmospheric Administration (NOAA) uses the Northeast Snowfall Impact Scale (NESIS) to categorize the severity of Northeast snowstorms. Other weather-severity scales, such as the five-category Saffir-Simpson hurricane scale, use one main metric, such as wind speed. The NESIS factors in several inputs—snow depth, area blanketed, and population density. The result is a more multidimensional picture. It's like taking the volume of an object instead of just measuring its width.
The storm that just passed over the Northeast—while accumulating less than forecast—could still rank on the NESIS when it's all over, just for the sheer area covered by the storm.
The NESIS is sorted into five categories: "notable," "significant," "major," "crippling," and "extreme." If there were a sixth category, we imagine it would be called "100 years of darkness." Only two storms meet the "extreme" threshold: the blizzards of 1993 and 1996. During the 1993 storm, also called the "Storm of the Century," there were 10 million power outages. The storm, which caused snow in the Northeast and flooding rains in the Southeast, dropped enough H20 to flood 44 million acres with a foot of water, according to NOAA.
NOAA also employs a more generalized "Regional Snowfall Index," which applies the NESIS concept to other areas of the country. The same amount of snow in the Northeast and Southeast will wreak more havoc in the Southeast, as such events are rarer.