Northeast Snowfall Impact Scale

A scale devised in 2004 by Paul Kocin and Louis Uccellini to characterize the impact of snowstorms on the thirteen states of the northeastern United States. It was then refined by researchers at the National Climatic Data Center (NCDC), particularly with respect to automating measurement of the land area covered by the snowfall. Unlike other weather-related scales, the NESIS takes into account the population of the affected area. It indicates the relative impact of the snowstorm on people, and is not a measure of a purely physical phenomenon like wind speed. An “NESIS value” is calculated from the area covered, the depth of the snowfall, the population of the area covered, and two constants based on historical experience with large snowstorms. This value is then converted to one of five categories:

Category	Description	NESIS Value
1	Notable	1 – 2.499
2	Significant	2.5 – 3.99
3	Major	4 – 5.99
4	Crippling	6 – 9.99
5	Extreme	10.0 plus

The NESIS is not used to predict the effect of an impending snowstorm, the way Saffir-Simpson numbers are used for hurricanes. Instead, it looks back and provides a way of stating how bad the storm was, after the fact. That estimate would be valuable to emergency management agencies if it can be computed quickly enough, which is what NCDC is working on.

NESIS rating of some big snowstorms

A ranking of the 33 largest recent northeastern snowstorms, courtesy NOAA:

Rank	Date	Category	Description	NESIS
1	1993 Mar 12 – 14	5	Extreme	13.20
2	1996 Jan 6 – 8	5	Extreme	11.78
3	2003 Feb 15 – 18	4	Crippling	8.91
4	1960 Mar 2 – 5	4	Crippling	8.77
5	1961 Feb 2 –- 5	4	Crippling	7.06
6	1964 Jan 11 – 14	4	Crippling	6.91
7	2005 Jan 21 – 24	4	Crippling	6.80
8	1978 Jan 19 – 21	4	Crippling	6.53
9	1969 Dec 25 – 28	4	Crippling	6.29
10	1983 Feb 10 – 12	4	Crippling	6.25
11	1958 Feb 14 – 17	4	Crippling	6.25
12	1966 Jan 29 – 31	3	Major	5.93
13	1978 Feb 5 – 7	3	Major	5.78
14	1987 Jan 21 – 23	3	Major	5.40
15	1994 Feb 8 – 12	3	Major	5.39
16	1972 Feb 18 – 20	3	Major	4.77
17	1979 Feb 17 – 19	3	Major	4.77
18	1960 Dec 11 – 13	3	Major	4.53
19	1969 Feb 22 – 28	3	Major	4.29
20	2006 Feb 12 – 13	3	Major	4.10
21	1961 Jan 18 – 21	3	Major	4.04
22	1966 Dec 23 – 25	2	Significant	3.81
23	1958 Mar 18 – 21	2	Significant	3.51
24	1969 Feb 8 – 10	2	Significant	3.51
25	1967 Feb 5 – 7	2	Significant	3.50
26	1982 Apr 6 – 7	2	Significant	3.35
27	2000 Jan 24 – 26	2	Significant	2.52
28	2000 Dec 30 – 31	1	Notable	2.37
29	1997 Mar 31 – Apr 1	1	Notable	2.29
30	1956 Mar 18 – 19	1	Notable	1.87
31	1987 Feb 22 – 23	1	Notable	1.46
32	1995 Feb 2 – 4	1	Notable	1.43
33	1987 Jan 25 – 26	1	Notable	1.19

How the NESIS value is calculated

an equation

where

n is the snowfall category, which can take one of four values:

4 for snowfalls > 4" and <10"
10 for snowfalls >10" and <20"
20 for snowfalls >20" and <30"
30 for snowfalls > 30"

A_n is the area in square miles having snowfall greater than or equal to the category specified by n

A_mean is 73,981, which is the mean area in square miles that received more than 10" of snow, based on a NCDC study of 30 snowstorms between 1956 and 2000.

P_n is the population in the area receiving snowfall greater than or equal to the category specified by n (taken from the 2000 U.S. Census)

P_mean is 30,333,000, which is the mean population affected by snowfall >10" in the 13 northeastern states, based on the same storm sample as A_mean.

Kocin, P. J. and L. W. Uccellini.
A Snowfall Impact Scale Derived From Northeast Storm Snowfall Distributions.
Bulletin of the American Meteorological Society, vol. 85, pages 177-194 (2004).

Available on the web at www.ncdc.noaa.gov/oa/climate/research/snow-nesis/kocin-uccellini.pdf

M. F. Squires and J. H. Lawrimore.
Development of an Operational Snowfall Impact Scale.
22nd IIPS, Atlanta, GA. (2006)