The 2011–12 North American winter by and large saw above normal average temperatures across the continent, with the Contiguous United States encountering its fourth-warmest winter on record, along with an unusually low number of significant winter precipitation events. The primary outlier was Alaska, parts of which experienced their coldest January on record.
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 2011 occurred late on December 21 (early on December 22 in EST), and ends at the March equinox, which in 2012 occurred on March 20 (March 19 in CDT and EDT). Based on the meteorological definition, the first day of winter is December 1 and the last day February 29. Both definitions involve a period of approximately three months, with some variability.
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Seasonal forecasts
On October 20, 2011, the National Oceanic and Atmospheric Administration's Climate Prediction Center issued its U.S. Winter Outlook. The outlook expected the present La Niña to persist and intensify into the winter, resulting in drier than normal conditions in the drought-stricken states of New Mexico, Oklahoma, Texas, and parts of adjacent states. The CPC noted the significance of the less-predictable Arctic Oscillation, with CPC deputy director Mike Halpert saying, "The erratic Arctic Oscillation can generate strong shifts in the climate patterns that could overwhelm or amplify La Niña's typical impacts." The outlook was broken up into ten regions. The outlook anticipated above-normal wildfire conditions across the entire southern part of the United States, cooler and wetter-than-average conditions in the Pacific Northwest and western Montana, likewise for northern California, and drier-than-average conditions for southern California. Colder and wetter-than-average conditions were favored in the Northern Plains, but the converse was expected to occur in the Southern Plains and Gulf Coast states, worsening pre-existing drought conditions. In Florida and the southern Atlantic states, equal chances existed for below, near, and above-average temperatures, with below-average precipitation favored for the region and above-normal wildfire conditions. Equal chances for below, near, and above-average temperatures existed in the Ohio and Tennessee Valleys, with wetter-than-average conditions favored, resulting in an enhanced potential for storms and flooding. Equal chances for below, near, and above-average precipitation and temperatures existed in the Northeast and Mid-Atlantic states. The outlook noted that winter weather in this region often driven by the Arctic Oscillation rather than La Niña, and as such, if enough cold air and moisture were to coalesce, areas north of the Ohio Valley and into the Northeastern U.S. could expect above-average snowfall. Colder and wetter-than-average conditions were favored in the Great Lakes region, and above-average temperatures with above-average precipitation were favored in the western islands of Hawaii, resulting in the potential for elimination of drought, particularly in Kauai and Oahu. The outlook ended by noting chances favoring below-average temperatures in the southern half of Alaska and the panhandle with below-average levels of precipitation favored in the eastern interior region of the state.
On January 4, 2012, the lowest temperature of the winter was measured in Alaska: −69.9 °F (−56.6 °C). January 2012 was Alaska's coldest January since January 1965.
Seasonal summary
The 2011–12 winter opened with a nor'easter striking parts of the Mid-Atlantic and Northeastern United States in late October. The storm brought significant snowfall totals to much of the region, with some areas experiencing gale-force winds. Some areas between northeastern Pennsylvania and southern Maine received over of snow, and parts of Massachusetts and New Hampshire received over of snow. Many trees were downed because they had not yet lost their leaves, resulting in downed power lines and more than 3,000,000 residents losing power, some of whom remained without power for over a week following the storm. The storm ultimately resulted in one direct fatality and $18.8 million in damage. Slightly over a week later, an intense extratropical cyclone—one of the strongest to strike the west coast of the state since 1974—brought high winds and flooding to Alaska. On November 8, a low initially crossed from the southern Aleutian Islands to near Gulf of Anadyr while intensifying. The low then weakened as it crossed the Chukchi Peninsula the following day. Gale-to-hurricane-force winds combined with snow to create blizzard conditions across much of northern and western Alaska, and storm-induced flooding caused at least an estimated $24 million in damage. Some locations experienced water levels several feet above the normal. One fatality was recorded before the storm's associated low entered the Chukchi Sea late on November 9, continuing to weaken as it moved off to the northwest and ceased to affect Alaska. In late November, another storm affected the United States, bringing snow to the central Mississippi Valley. The storm ultimately dropped a few inches of snow across parts of Arkansas and Missouri in the central Mississippi River Valley before advancing into the Ohio Valley where it brought rain and snow to Lower Michigan and northern Indiana. Although the storm logged no casualties, it did cause thousands of dollars in flood damage to portions of Arkansas and Kentucky. Another system materialized in mid-December; it brought heavy snow, which in some cases combined with high winds to create blizzard conditions, to northeastern New Mexico and southeastern Colorado. Along the eastern end of the snow, precipitation transitioned to a wintry mix resulting in hazardous road conditions in central and south-central Kansas. The storm did not result in any known casualties, but it did cause over $50 thousand in damage in New Mexico.
Mid-January saw the Pacific Northwest significantly affected by winter weather, with a storm bringing some of the coldest air to affect the region since winter began. Accompanying the cold air were snow showers across western parts of Washington and Oregon; heavy snows affected the coastal mountains of the region, with lesser snows affecting the Seattle area. As the precipitation receded in the Pacific Northwest, cold air associated with the system went on to pervade the Rocky Mountains and central Great Basin, with heavy snow later returning to the coastal mountains in the Cascadia region. Snow totals in the Seattle and Olympia areas as well as the Blue Mountains and parts of the Rockies reached near-record-breaking levels. Snow in western Washington transitioned into freezing rain, resulting in a historic ice storm event, with heavy snows expanding into Northern California and the Rockies, including the Tetons and the northern Wasatch Range. Cold air and snow spread into the Northern Plains, and high winds affected areas in and just east of the Rockies. Widespread rain fell along the coasts of Northern California and Oregon. The storm left three dead and four injured in its wake due to a combination of flooding and fallen trees. This storm was to be followed by another storm in early February. The major winter storm, originating from a disturbance in the subtropical jet stream, brought snow to parts of Kansas and south-central Nebraska, with cold air drawn in from the north. Areas to the east in the Middle Mississippi Valley were warmer and saw rain instead of snow. The highest snowfall totals were reported by the National Weather Service to have occurred from an area extending south from Laramie, Wyoming to the Colorado foothills as well as an area east of Denver. The foothills saw a few reports of snow totals in excess of , with Black Hawk receiving of accumulation. Lesser but still significant snowfall fell over parts of Nebraska from North Platte to near Omaha. Wind gusts of over were observed across eastern Colorado and Nebraska. Approximately 600 flights were canceled in Denver, Colorado as a result of the storm, and due to heavy snow and strong winds, Interstate 70 was closed from Denver to the Kansas border. The storm resulted in power outages affecting 15,000 people and caused an estimated $693 thousand in damage. However, this was largely due to flooding in the Southern Plains and Lower Mississippi Valley rather than the wintry impacts to the north.
Late February and early March saw another winter storm develop, impacting parts of the north-central and northeastern United States with heavy snow and blizzard conditions coupled with other forms of wintry precipitation. On February 28–29, wintry precipitation began to spread across the Upper Midwest, with winds across parts of the Central Plains through the Upper Mississippi Valley reaching . Simultaneously, light to moderate snow developed across parts of the Northeastern U.S., with snowfall rates decreasing into the night of February 29. However, snow redeveloped by the following morning, with snowfall finally coming to an end shortly thereafter. The storm's most-significant impacts were observed across the Upper Midwest, with numerous power outages occurring in tandem with the closure of roads, schools, and businesses. Interstates 29 and 90 were temporarily closed due to unsafe weather conditions, and accidents on state highways were reported to number in the hundreds. The only fatality reported to have occurred due to the storm occurred in Minnesota, with the death being attributed to exposure to extremely cold temperatures. Despite damage reports from the storm totaling $741 thousand, the storm alleviated some of the pre-existing drought conditions in the north-central U.S.
Events There were multiple events during the 2011–12 North American winter. Significant events include cold waves, snowstorms, and other notable events outside the conventional limits of winter.
Halloween nor'easter