Category 5 snowstorms

Hurricanes have their Category 1 - 5 rankings on the Saffir-Simpson Scale. Tornadoes are ranked F1 - F5 on the Fujita scale. Now winter storms have a similar ranking system, at least if you live in the Northeastern U.S. urban corridor. The new scale, announced last week at the annual meeting of the American Meteorological Society, is called the NESIS scale, or Northeast Snowfall Impact Scale. The NESIS scale ranks snowstorms that dump at least ten inches of snow on the Northeast U.S., with a number one to five. The five categories are Extreme, Crippling, Major, Significant, and Notable. The NESIS scale differs from the hurricane and tornado ranking scales in that it uses the number of people affected to assign its ranking. Thus, a massive blizzard that whips Massachusetts, New Hampshire, and Maine with hurricane-force winds and three feet of snow--but affects no other portions of the country--would receive a Category 1 ranking on the NESIS scale, since it only affects a small number of people. The same storm tracking up the East Coast and affecting 65 million people would receive a NESIS ranking of five. There have been only two Category 5 snowstorms to hit the Northeast U.S. the past 50 years--the blizzard of March 13, 1993, and the blizzard of January 7, 1996 (Table 2). According to the scale's creators, the scale can help the emergency managers and the public make appropriate decisions to protect lives and property, assist with evaluatation of building codes, and provide a historical perspective. The scale was developed specifically for the Northeast U.S. because of the great impact winter storms there have on the U.S. economy and transportation system.



It remains to be seen if the NESIS scale will catch on and prove useful. For me, a storm ranking scale makes sense, but trying to forecast if a northeast snowstorm will be a Category 2 or 4 will be difficult, since I'll have to keep in my head a map of the Northeast population, and try and integrate in my head what total area is likely to be covered by how much snow. This is a lot harder than looking at the wind speed and assigning a category, like is done with hurricanes. Thus, I expect it will be some time before the NESIS scale catches on, but it eventually probably will as people become used to it. There are currently no plans to extend the scale to other parts of the world.

So far this winter, there haven't been many winter storms worthy of a ranking on the NESIS scale. The snowstorm of December 9, 2005, which dumped more than a foot of snow in the Boston area, was probably a NESIS Category 2 storm. The long range winter outlook doesn't show any major Northeast snowstorms in the offing for the next week, just plenty of seasonable cold air.

Jeff Masters