Snowfall accumulation from the Dec. 18-21, 2009 storm.
High resolution

To the surprise of no one affected by the Dec. 18-20, 2009 system that dumped heavy snow from the mid-Atlantic to southern New England, NOAA has rated the storm a Category 3 or “Major” winter storm on NOAA’s Northeast Snowfall Impact Scale, also known as NESIS.

NESIS characterizes and ranks Northeast snowstorms, using data calculated by NOAA’s National Climatic Data Center, based on the following factors: how much snow falls (must deposit at least 10 inches); the scale of the area impacted; and the population of the impacted area. NESIS ranks these storms on a five-tier scale ranging from Category 1 “Notable” to Category 5 “Extreme.”

 “While snowfall from the December storm ranked in the top ten for Washington, Baltimore and Philadelphia, the storm only provided a glancing blow to the New York City and Boston metropolitan areas and overall affected a relatively small area. This led to it being classified as a Category 3,” said Louis Uccellini, director of the National Centers for Environmental Prediction – a division of NOAA’s National Weather Service – and co-developer of NESIS with Paul Kocin also with NCEP in Camp Springs, Md. Both Uccellini and Kocin are published winter weather experts.

Satellite image of the snowcover enlarged here.

"Last month's storm was one of only five in the past decade that ranked Category 3 or higher," added Kocin. The others being: December, 2002 (Category 3); February, 2003 (Category 4); January, 2005 (Category 4); February, 2006 (Category 3) and February, 2007 (Category 3).

Topping the NESIS scale - and the only storms rated Category 5 - are the “Superstorm” on March, 1993

followed by the “Blizzard of ’96” in January, 1996.