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.
Seasonal forecasts
On October 15, 2009, the National Oceanic and Atmospheric Administration's Climate Prediction Center issued its U.S. Winter Outlook. Due to a strengthening El Niño, winter weather was expected to be affected by this. Warmer-than-average temperatures were favored across much of the western and central U.S., especially in the north-central states from Montana to Wisconsin. Below-average temperatures were expected across the Southeast and mid-Atlantic from southern and eastern Texas to southern Pennsylvania and south through Florida. Above-average precipitation is expected in the southern border states, especially Texas and Florida. Recent rainfall and the prospects of more should improve current drought conditions in central and southern Texas. The rest of the country fell into the equal-chance zone.
Seasonal summary
The winter of 2009–10 was one of the most severe winter seasons on record for North America, with significant winter storms and arctic outbreaks of bitterly cold temperatures. affecting the majority of the continent and the United States during the majority of the winter months. At one point during the winter, on February 18, all of the Lower 48 had snowfall on the ground, signifying the extremity of the amount of major winter storms that occurred in a short period of time across the U.S. The extreme activity was also indicated by the high amount of storms rated on the Regional Snowfall Index (RSI); a total of 17 storms were rated on the scale, 6 of which were rated at or above a Category 3 on the scale. The ongoing 2009–10 El Niño event was responsible for the overall winter weather pattern, leading to many blockbuster snowstorms across the eastern half of the country. The season overall opened with a snowstorm in the Rocky Mountains at the start of October, before a major storm system, partially attributed to the remnants of Typhoon Melor, affected the Western United States by mid-month starting on October 13. November did not feature much activity, aside from the remnants of Hurricane Ida becoming a nor'easter that affected the Mid-Atlantic region mid-month, causing significant damage and heavy rainfall. Additionally, snow cover across the contgiuous U.S. was well-below average for the month of November, with temperatures mostly above average.
December saw a major pattern shift as winter weather events kicked into high gear, precipitated by the negative phases of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO), which favor colder temperatures and more favorable winter storm tracks. Temperatures overall during the month of December were below-average as multiple outbreaks of cold temperatures affected the U.S.; the month ended with temperatures 3.2 °F (1.78 °C) colder than the long-term average nationally, with the coldest temperatures towards the central part of the country. A major winter storm affected regions from the Rocky Mountains to the Great Lakes from December 7–11, causing significant impacts. Only a week later on December 19, the first high-impact and major snowstorm for the eastern U.S. occurred, as a powerful blizzard buried the Northeastern United States under of snow and killing 7. Shortly afterwards, on Christmas Eve, most of the Central United States were struck by an extremely large and widespread blizzard and storm complex that produced heavy snowfall and blizzard conditions from Texas and Oklahoma all the way north towards the Canada–United States border; the storm was rated a category 5 on the RSI, the first since the January 1996 blizzard. On Christmas Day, approximately 63% of the Contiguous United States had snow on the ground and constituting a "white Christmas", the most expansive snow cover since 2003; New York City and Philadelphia would not see another white Christmas after this winter until 2024. The last few days of 2009 into the beginning of 2010 saw another nor'easter affect regions from the Great Lakes to New England.
Events October West Coast cyclone
A significant storm, partially attributed to the remnants of Typhoon Melor began to impact the West Coast of the United States, beginning very early on October 13. Around the same time, an atmospheric river opened up (the Pineapple Express), channeling large amounts of moisture into the storm, resulting in heavy rainfall across California and other parts of the Western United States. The storm caused at least $8.861 million (2009 USD) in damages across the West Coast of the United States.
Ex-Hurricane Ida ("Nor'Ida")
The mid-level circulation of Hurricane Ida's remnants led to the formation of a new low over southeastern Georgia, which eventually moved off the coast of North Carolina. This new low quickly intensified and became a powerful nor'easter that caused substantial damage throughout the Mid-Atlantic States. Convective banding features became increasingly defined throughout the day, Due to the rapid succession of these systems, United States media referred to the nor'easter as "Nor'Ida". By November 12, the system attained a minimum pressure of 992 mbar (hPa; ) along with winds of . In combination with a large area of high pressure, a long stretch of easterly, onshore winds impacted areas from Virginia to southern New England. Tracking parallel to the North Carolina coastline, the system eventually moved onshore near Cape Hatteras by November 13. Due to the high-pressure system situated over Vermont, the low turned southeastward, bringing its center back over water. Gradual weakening took place during this period, though heavy rains continued to fall across much of the Chesapeake Bay area. On November 14, a brief secondary low developed within the system, off the coast of Delaware. Continuing to weaken, the cyclone resumed a northward track after the high weakened and persisted through November 17, by which time it had moved over Atlantic Canada.
Early December winter storm
A major winter storm in early December brought record amounts of snow to the Midwest and contributed to deaths of 16 people. The storms affected a number of US states, including Arizona, Wisconsin and New York, as well as Canada. Although the initial storm had virtually subsided by December 11, further snow was expected to fall.
A weak disturbance that started overnight on December 6 stalled over the western United States and intensified rapidly. Heavy downpours began early in the morning of December 7 and continued non-stop until the evening, triggering flash-flood watches. The storm dropped as much as of rain in Cuyamaca, San Diego, causing blackouts in some locations. This storm was followed by a small semi-tropical winter storm which started overnight on December 10 and then stalled over Southern California and intensified, causing heavy downpours. In addition to of rain, this storm gave pea-sized-hail and gusty winds before weakening and moving away from Southern California on December 13. Heavy snow hit Wisconsin, New York, Washington, D.C., and parts of Maryland on the 10th and 14th. Heavy rain hit parts of Arizona on the 11th. In Vancouver, BC a strong winter storm deposited anywhere from 10 to 30 cm of heavy snow from the night of the 12th through to the morning of the 15th after a frigid arctic air that had been over the region for days with as low as -15 °C temperatures met with moist Pacific air.
Mid-December blizzard
On December 16, meteorologists identified a storm forming in the Gulf of Mexico. It produced record rainfall in regions of Texas and had the potential to strengthen as it moved through Georgia and Florida and further north. Weather models accurately predicted that this storm would meet with cold air while retaining its heavy precipitation. By the afternoon of December 19, the large, low pressure region had moved off the East Coast, intensifying and bringing heavy snow to the major Mid-Atlantic cities. Blizzard warnings were issued in Washington, D.C., Baltimore, and Long Island. As the storm moved northward along the East coast, at one point it measured across 14 states. The storm produced whiteout conditions and dumped about of snow in major cities along the Eastern seaboard.
Some regions affected by the storm experienced winds up to with gusts of . Ronald Reagan Washington National Airport, Washington Dulles International Airport, and Baltimore/Washington International Thurgood Marshall Airport saw whiteout conditions, causing flights to be delayed or canceled. Of 740 scheduled departures at Washington National and 530 at Dulles, only 14 and 12, respectively, were able to take off. President Barack Obama, arriving at Andrews Air Force Base after the UN Climate Conference, was forced to return to the White House by motorcade instead of helicopter. More than 800 flights were canceled at New York City's three major airports. Delays averaged over six hours at Philadelphia International Airport.
Christmas Eve storm complex