Analyses and forecasts of 4 major East Coast Snowstorms during winter 2009-2010

A combination of factors during the winter of 2009-2010 led to the snowiest winter of record for the Mid Atlantic States, from Washington D.C. to Philadelphia,PA. This talk will document the unique aspects of 4 major snowstorms during December 2009 and February 2010 that led to the record winter, as well as a summary and discussion of the many forecasts issued primarily at the National Centers for Environmental Prediction (NCEP), including the Hydrometeorological Prediction Center (HPC). Discussions of forecasts will emphasize aspects of uncertainty and predictabilty that were at play during the days leading up to each of the 4 events.

Analyses will mirror those presented in Northeast Snowstorms (2004; Kocin and Uccellini, emphasizing surface and upper-level diagnoses, mesoscale features, such as banding, and an assessment of impact through the Northeast Snowfall Impact Scale (NESIS; Kocin and Uccellini 2004). While each storm presented unique challenges to forecasters, predictability of each storm generally ranged from 2 to 4 days for three of the storms, and up to 8 days for the snowstorm of 5-6 February 2010, representing a considerable forecast success for events that had huge societal impacts. Examples of forecasts from a variety of ensemble model forecasts, as well as human forecasts from HPC, will be presented.

NESIS values were computed by the National Climatic Data Center (NCDC) and showed that each storm was ranked as either a Category 2 or Category 3 event - on a scale of 1 to 5. However, when two back to back snowstorms on 5-6 February and 9-10 February are combined, the NESIS value jumps to a Category 5 event, in which only 2 other storms have been ranked. The impact to the lives of millions of people by these events is borne out by the high NESIS value.