The index was developed by Barbara Mayes Boustead of the National Weather Service in partnership with the Midwestern Regional Climate Center
What makes a winter “severe”? The index evaluates winter’s harshness based on the intensity and persistence of cold weather, the frequency and amount of snow, and the amount and persistence of snow on the ground.
A score is calculated based on these characteristics and evaluated in the context of a given location’s winter weather history. Depending on how the score stacks up to years in the past (i.e. its percentile ranking), the winter is rated mild, moderate, average, severe, or extreme.
Through February 16, D.C. has received 21.9 inches of snow, more than five inches above average. Measurable snow has fallen on seven days in the past month.
Although D.C. had its warmest December on record by a landslide, the tide has since turned. January and February have both been slightly cooler than normal.
The combination of snow and cooler temperatures has elevated D.C.’s 2015-2016 winter rating from mild to severe in the last few weeks.
The index incorporates winters back to 1951 in D.C. The most extreme winter was 1960-1961 (40.3 inches of snow fell and temperatures were much colder than normal) and the tamest 1997-1998, in which just 0.1 inches of snow fell.
This year’s rating is based on observed weather to date. So, depending on whether it’s cold and snowy in the coming weeks, D.C.’s winter could be upgraded to the extreme category or drift back towards average.