Four of the big snowstorms that struck the Northeastern United States this winter have been ranked among the 30 highest-impact snowstorms of the last 54 years.

The assessments are made by the National Climatic Data Center. Meteorologists there use a ranking system developed to weigh not just the snow depth at any one location, but the depth, the geographic area and the population it affected over its full range.

Called the Northeast Snowfall Impact Scale (NESIS), the system was developed in 2004 by Paul Kocin and Louis Uccellini, of the National Weather Service. It's since been applied retrospectively to every major storm since 1956, and to all the big Northeast snowstorms that have occurred since the scale was developed.

The NESIS scale calculations generate an index number, which is translated into a five-level Category ranking similar to the Saffir-Simpson Hurricane Scale, including Notable, Significant, Major Crippling and Extreme. 

Only two storms have been ranked as Cat. 5 "Extreme." They were the March 12-14, 1993 storm, which was given a NESIS number of 13.20; and the Jan. 6-8, 1996 storm, rated at 11.78.

Here, for comparison, with their preliminary rankings and NESIS numbers, are this winter's four biggest snowstorms. Only the three earliest had a major impact in Baltimore. And here's a link to the full list.

17. Feb. 23-29, 2010: 5.11, a Cat. 3 "Major" storm.

21. Feb. 4-7, 2010:  4.30, a Cat. 3 "Major" storm. (Map above.)

25.  Dec. 18-21, 2009:  4.03, a Cat. 3 "Major" storm.

26:  Feb. 9-11, 2010:  3.93, a Cat. 2 "Significant" storm.