January 18, 2018

NOAA’s National Climatic Data Center is now producing the Regional Snowfall Index (RSI) for significant snowstorms that impact the eastern two thirds of the U.S. The RSI ranks snowstorm impacts on a scale from 1 to 5, similar to the Fujita scale for tornadoes or the Saffir-Simpson scale for hurricanes.

Category RSI Value Description
1 1–3 Notable
2 3–6 Significant
3 6–10 Major
4 10–18 Crippling
5 18.0+ Extreme

The RSI differs from these other indices because it includes population. RSI is based on the spatial extent of the storm, the amount of snowfall, and the juxtaposition of these elements with population. Including population information ties the index to societal impacts. Currently, the index uses population based on the 2000 Census.


Six Easternmost Climate Regions

The RSI is an evolution of the  ***Northeast Snowfall Impact Scale (NESIS)*** which NCDC began producing operationally in 2005. While NESIS was developed for storms that had a major impact in the Northeast, it includes the impact of snow on other regions as well. It can be thought of as a quasi-national index that is calibrated to Northeast snowstorms. By contrast, the RSI is a regional index; a separate index is produced for each of the six NCDC climate regions in the eastern two-thirds of the nation.

The indices are calculated in a similar fashion to NESIS, but our experience has led us to propose a change in the methodology. The new indices require region-specific parameters and thresholds for the calculations. For details on how RSI is calculated, see Squires et al. 2011.

The RSI is important because of the need to place snowstorms and their societal impacts into a historical perspective on a regional scale. For example in February 1973, a major snowstorm hit the Southeast affecting areas not prone to snow. The storm stretched from the Louisiana and Mississippi Gulf coasts northeastward to the Carolinas. Over 11 people received more than 5″ of snow and three quarters of a million people in Georgia and South Carolina experienced over 15″ of snow. This is currently the 10th highest ranked storm for the Southeast region. This storm would not even be ranked in NESIS. This example illustrates why it is important to discriminate impacts between regions.

NCDC has analyzed and assigned RSI values to over 500 storms going as far back as 1900. New storms are added operationally. As such, RSI puts the regional impacts of snowstorms into a century-scale historical perspective. The index is useful for the media, emergency managers, the public and others who wish to compare regional impacts between different snowstorms. The RSI and Societal Impacts Section allows one to see the regional RSI values for particular storms as well as the area and population of snowfall for those storms. The area and population are cumulative values above regional specific thresholds. For example, the thresholds for the Southeast are 2″, 5″, 10″, and 15″ of snowfall while the thresholds for the Northeast are 4″, 10″, 20″, and 30″ of snowfall.

 

***The Northeast Snowfall Impact Scale (NESIS)

While the Fujita and Saffir-Simpson Scales characterize tornadoes and hurricanes respectively, there is no widely used scale to classify snowstorms. The Northeast Snowfall Impact Scale (NESIS) developed by Paul Kocin and Louis Uccellini of the National Weather Service (Kocin and Uccellini, 2004) characterizes and ranks high-impact Northeast snowstorms. These storms have large areas of 10 inch snowfall accumulations and greater. NESIS has five categories: Extreme, Crippling, Major, Significant, and Notable. The index differs from other meteorological indices in that it uses population information in addition to meteorological measurements. Thus NESIS gives an indication of a storm’s societal impacts. This scale was developed because of the impact Northeast snowstorms can have on the rest of the country in terms of transportation and economic impact.

NESIS scores are a function of the area affected by the snowstorm, the amount of snow, and the number of people living in the path of the storm. The diagram below illustrates how NESIS values are calculated within a geographical information system (GIS). The aerial distribution of snowfall and population information are combined in an equation that calculates a NESIS score which varies from around one for smaller storms to over ten for extreme storms. The raw score is then converted into one of the five NESIS categories. The largest NESIS values result from storms producing heavy snowfall over large areas that include major metropolitan centers. For details on how NESIS scores are calculated at the National Climatic Data Center, see Squires and Lawrimore (2006).

NESIS Calculations


Table 1: NESIS categories, their corresponding NESIS values, and a descriptive adjective:

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

 

Table 2: Ranks 47 high-impact snowstorms that affected the Northeast urban corridor.
Rank Start End NESIS Category Description Map
1 1993-03-12 1993-03-14 13.20 5 Extreme view
2 1996-01-06 1996-01-08 11.78 5 Extreme view
3 1960-03-02 1960-03-05 8.77 4 Crippling view
4 2003-02-15 2003-02-18 7.50 4 Crippling view
5 1961-02-02 1961-02-05 7.06 4 Crippling view
6 1964-01-11 1964-01-14 6.91 4 Crippling view
7 2005-01-21 2005-01-24 6.80 4 Crippling view
8 1978-01-19 1978-01-21 6.53 4 Crippling view
9 1969-12-25 1969-12-28 6.29 4 Crippling view
10 1983-02-10 1983-02-12 6.25 4 Crippling view
11 1958-02-14 1958-02-17 6.25 4 Crippling view
12 1966-01-29 1966-01-31 5.93 3 Major view
13 1978-02-05 1978-02-07 5.78 3 Major view
14 2007-02-12 2007-02-15 5.63 3 Major view
15 2010-02-23 2010-02-28 5.46 3 Major view
16 1987-01-21 1987-01-23 5.40 3 Major view
17 1994-02-08 1994-02-12 5.39 3 Major view
18* 2011-01-09 2011-01-13 5.31 3 Major view
19* 2011-02-01 2011-02-03 5.30 3 Major view
20* 2010-12-24 2010-12-28 4.92 3 Major view
21 1972-02-18 1972-02-20 4.77 3 Major view
22 1979-02-17 1979-02-19 4.77 3 Major view
23 1960-12-11 1960-12-13 4.53 3 Major view
24 2010-02-04 2010-02-07 4.38 3 Major view
25* 2013-02-07 2013-02-10 4.35 3 Major view
26 1969-02-22 1969-02-28 4.29 3 Major view
27 2010-02-09 2010-02-11 4.10 3 Major view
28 2006-02-12 2006-02-13 4.10 3 Major view
29 1961-01-18 1961-01-21 4.04 3 Major view
30 2009-12-18 2009-12-21 3.99 2 Significant view
31 1966-12-23 1966-12-25 3.81 2 Significant view
32 1958-03-18 1958-03-21 3.51 2 Significant view
33 1969-02-08 1969-02-10 3.51 2 Significant view
34 1967-02-05 1967-02-08 3.50 2 Significant view
35 1982-04-06 1982-04-07 3.35 2 Significant view
36* 2013-03-04 2013-03-09 3.05 2 Significant view
37 2007-03-15 2007-03-18 2.54 2 Significant view
38 2000-01-24 2000-01-26 2.52 2 Significant view
39 2000-12-30 2000-12-31 2.37 1 Notable view
40 1997-03-31 1997-04-01 2.29 1 Notable view
41* 2011-01-26 2011-01-27 2.17 1 Notable view
42 1956-03-18 1956-03-19 1.87 1 Notable view
43* 2011-10-29 2011-10-30 1.75 1 Notable view
44 2009-03-01 2009-03-03 1.59 1 Notable view
45 1987-02-22 1987-02-23 1.46 1 Notable view
46 1995-02-02 1995-02-04 1.43 1 Notable view
47 1987-01-25 1987-01-26 1.19 1 Notable view

*Preliminary


References:
  • Kocin, P. J. and L. W. Uccellini, 2004: A Snowfall Impact Scale Derived From Northeast Storm Snowfall Distributions. Bull. Amer. Meteor. Soc.85, 177-194
  • Squires, M. F. and J. H. Lawrimore, 2006: Development of an Operational Snowfall Impact Scale. 22nd IIPS, Atlanta, GA.

Information coutesy The National Climatic Data Center