"We're in better shape with regard to flooding this year than we were at this time last year," says Jim Pogue, spokesman for the US Army Corps of Engineers office in Memphis, Tenn. "That's very encouraging for us." Yet it remains to be seen what the lack of runoff may hold for river levels in the summer, he adds. If water levels fall considerably, low water can hamper barge traffic.
As consequential as La Niña is, particularly in the Western US, it isn't the only player in the country's winter weather patterns. A seesaw feature over the North Atlantic, dubbed the North Atlantic Oscillation (NAO), has played a role, as has the Arctic Oscillation, a varying wind pattern over the polar region.
As a storm track cuts across the continent, the NAO can in effect impede that flow when it's strong or stay out of the way during its weak phase. The oscillation can flip-flop in strength quickly. During the winter of 2010-11, the NAO on average remained strong, forcing the jet stream to form deep dips to the south as it crossed the continent. This allowed winter storms to track farther south than usual, bringing snow and cold to parts of the country where snowfall is rare.
The Arctic Oscillation, which can keep Arctic air bottled up near the poles, has been generally strong this winter, Dr. Weber of NCAR says. This has kept the country free of too many spells of frigid air, compared with last year, when the oscillation was weaker.
Still, these features over the Atlantic and Arctic have occasionally shifted this season to allow a few powerful storm systems to reach into the Central and Southern US, triggering fatal tornado outbreaks on Jan. 22 and again the week of Feb. 27.
The contrasts between the winters of 2010-11 and 2011-12 occur against a backdrop of a global long-term warming trend, says Martin Hoerling, a researcher at NOAA's Earth System Research Laboratory in Boulder, Colo. But efforts to explain those two differing winters in terms of global warming fall short, he adds.
Both started off with La Niñas of relatively comparable strength. Both occurred under similar conditions of summer Arctic sea ice melt-back, which some researchers have offered as a key factor for the snowy winters of 2009-10 and 2010-11.
If anything, the two contrasting winters, with their respective extremes and records, "remind us of how dynamic the weather patterns can be, unrelated to anything that is tickling them" over the long term, he says.