One year after being pelted with widespread and abundant snowfall, much of North America stayed brown, gray, and green in the winter of 2011–12. Multiple weather patterns conspired to keep snow from forming and falling often, and warm air temperatures kept it from staying on the ground for long. This mostly snow-deprived winter should mean fewer spring floods, but also less snow melt filling reservoirs and lakes.
These maps were made with data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. The top map depicts snow cover in North America from October 1, 2011, to March 20, 2012; the lower map shows the same period in the autumn and winter of 2010 to 2011. The colors depict the percentage of days in which a parcel of land was covered by snow. The deepest blues had snow cover just 10 to 20 percent of the time, while the palest blue depicts near complete snow cover for the season. Gray areas had no measurable snow. The map does not reveal snow depth.
Turn on the image comparison tool to see the difference between the two winters. In 2010-11, the states around the Great Lakes, New England, the Rocky Mountains, the Sierra Nevada, and the Basin & Range all received snow over a wider area and for longer duration; likewise in Canada’s Alberta, Saskatchewan, and Manitoba provinces.
Those areas were less snowy in 2011-12, with some exceptions in the Pacific Northwest. (Alaska, shown in the larger downloadable map, saw significantly more snow in many locations.) Most of the eastern half of the United States and southern Ontario had far less snow this year, and very little reached the South. Snowfall reached deeper into the U.S. Southwest and Mexico, but was short-lived.
“A major reason for the snow deficits was the persistent position of the jet stream close to the U.S.–Canadian border,” said climatologist Dave Robinson of Rutgers University. “This kept the cold air at bay to the north and permitted mild conditions to persist winter-long across most states. With only infrequent buckling (troughs) in the jet to the south the number and severity of winter storms was reduced—as it is the interaction of different air masses that helps spawn storms.”
Dorothy Hall, who leads the team that creates these NASA snow maps, notes that snow cover products are often built with hydrologists and climate modelers in mind. Knowing how much water is available for irrigation and drinking depends on knowing something about the snowpack on the ground. “Water resource managers need this information so they can plan their reservoir heights,” she noted. “The more accurate the information on the amount of snow cover that will be melting, the more money they can save.”
Forecasters at the National Oceanic and Atmospheric Administration noted in early March that “for the first time in four years, no area of the United States faces a high risk of major to record spring flooding, largely due to the limited winter snowfall.” The agency noted that eight of eleven western states still had reservoir levels at or above the normal capacity—a residual effect of last year's thick snow pack.
NASA Earth Observatory image by Jesse Allen, using data provided courtesy of Jeff Miller, NASA/GSFC. Caption by Michael Carlowicz.
What a difference one year has made in the snow cover over North America.