2009–10 North American winter

The 2009–10 North American winter was one of the most active and prolific winter seasons on record in the United States and North America as a whole, and the most severe since the 1995–96 winter. Multiple winter storms and widespread arctic blasts throughout the winter led to an expansive snow cover, with the Weather Prediction Center (WPC) issuing summaries on a total of 20 significant winter weather events. None other that saw the most impacts was the Northeast and Mid-Atlantic states; from mid-December to the end of February, the region saw four blizzards affect a majority of the states, three of which occurred within the month of February alone. Those three storms pushed cities such as Washington D.C., Baltimore, Philadelphia and New York City to one of, if not the snowiest February on record. The winter also saw a prolific amount of winter storms rated on the Regional Snowfall Index (RSI) scale, with 17 such storms, and 6 of those storms further achieving a ranking of Category 3 or higher. A moderate-to-strong El Niño influenced the majority of the weather patterns during the winter, especially in the second half.

Other major winter weather events during the season also included a powerful West Coast storm in mid-October and a wide-ranging and crippling blizzard that struck the central U.S. on Christmas Eve and Christmas Day, spreading snow and blizzard conditions from Oklahoma to the Canada–United States border; it was also the first to be rated a Category 5 on the RSI scale since the January 1996 United States blizzard. In January, a a parade of storms caused significant impacts to the West Coast in addition to British Columbia in Canada. Additionally, a crippling Category 4 winter storm affected the southern half of the U.S. in mid-February, leading to record snow cover in those areas. Collectively, the winter storms and related weather caused at least $3.08 billion in damages and 108 fatalities, most of the deaths coming from the February 5–6 blizzard.

While there is no well-agreed-upon date used to indicate the start of winter in the Northern Hemisphere, there are two definitions of winter which may be used. Based on the astronomical definition, winter begins at the winter solstice, which in 2009 occurred on December 21, and ends at the March equinox, which in 2010 occurred on March 20. Based on the meteorological definition, the first day of winter is December 1 and the last day February 28. Both definitions involve a period of approximately three months, with some variability.