What Happened to the Winter of 2011-12?

For much of the United States and southern Canada, the winter of 2011-12 brought unprecedented warmth and limited snowfall. What were the key players?

Although this winter, like the last, was considered to be a La Niña winter, it was like night and day in terms of snowfall for the northern and western areas. For many, it seemed like a year without a winter.

La Nina is part of the El Niño Southern Oscillation (ENSO) and represents cooler-than-average surface waters over the tropical Pacific Ocean. On average, during a La Niña, the jet stream takes a dip into the North-Central and Northeastern U.S. allowing periodic cold air invasions and opportunities for snow.

According to the National Oceanic and Atmospheric Administration (NOAA), over the lower 48 states, the winter and start to the year topped a list dating back to 1895 when records were first kept. The winter (December to February) was the fourth warmest on record. This past January was also the fourth-warmest on record. The overall period from January to April was the warmest on record.

According to Paul Pastelok, head of Long Range Forecasting at AccuWeather.com and Canada Weather Expert Brett Anderson, "By far, the overriding factor in this past winter's weather pattern was an unusually strong and persistent positive Arctic Oscillation (AO)."

The AO represents surface pressure anomalies north of 20 degrees North latitude. In a positive phase of the AO, the pressure is low in the polar region. In a negative phase, the pressure is high in the region and leads to one or more southward pushes of arctic air, known as arctic outbreaks.

On average, the AO waffles from positive to negative every several weeks allowing alternating cold air invasions, warm-ups and often major storms in between.

"The persistent, strongly possible AO pattern this winter brought month after month of a jet stream locked from west to east across Alaska and northern Canada," Anderson said.

Arctic air was locked up near the pole much of the time, meaning outbreaks over southern Canada and the lower 48 states were practically non-existent.

With the jet so far north for so long, warm air had free range over much of the continent with the only exception being along the immediate West Coast states, influenced by a flow off the Pacific.

"The strong west to east jet also whisked storms along very quickly, gave them little time to strengthen and allowed only limited time to bring front-end snow and lake-effect snow near the Great Lakes in their wake," Pastelok said.

Unlike many other weather patterns and oscillations, the AO cannot be predicted months in advance.

"From early in the winter, the persistently positive AO resulted in little snowcover over the North-Central U.S. and the southern Canada Prairies, which led to a cycle in its own," Anderson said.

The minimal snowcover impacted invading cold air masses, causing them to lose potency. Since the air was less cold by the time it reached the Great Lakes, it also contributed to warmer lake waters and a lack of ice cover.

While open Great Lakes waters can result in a bounce-back of lake-effect snowfall and did so to some extent later in the season, arctic air continued to fail to lock and load, limiting the lake effect.

This side-by-side image from NOAA compares snowcover from Feb. 1, 2011 (left) to Feb. 1, 2012 (right).

A lack of a cold, frozen and snow-covered surface then continued to cycle resulting in a lack of big, late-season snowstorms from the Midwest to the mid-Atlantic and southern New England.

In the western U.S., where warm storms can be overridden by the colder and more snowy "high country," the La Niña did exhibit some recognizable trademarks.

According to Western Weather Expert Ken Clark, "The Southwest U.S. is typically drier than average and the Northwest is typically wetter than average during a La Niña."

Up through the end of February, snowfall in the Sierra Nevada was only 15 to 20 percent of normal. The snowfall departure trended toward normal as you progressed farther north over the northern Rockies.

"Interestingly, as La Niña weakened late in the winter, more storms visited the Southwest, resulting in a recovery of snowfall over the mountains," Clark said.

The pattern in the western U.S. was of a more typical La Niña pattern. However, a strong positive AO most of the winter prevented the dip in the jet stream over the eastern U.S.

While not quite the equivalent of a walk-off home run in the 9th inning, many areas in the Sierra Nevada finished the season close to 50 percent of normal snowfall and avoided disaster. Spring snowmelt is a key player for agriculture and drinking water in the West.

The previous winter, a La Niña as well brought snowfall of 200 percent of normal in parts of California and monster snowstorms in the Northeast and Midwest.

The art of long-range forecasting incorporates looking at years of similar weather patterns (analog years). Years with similar ENSO and other sea surface and pressure oscillations are examined and a forecast is made based on anticipated similarities or differences. Because of the unpredictability of the long-term AO, an AO that balances out over the season is usually assumed.

"When the AO doesn't average out over the entire season, a seasonal forecast can really go astray," Pastelok said.

There was one area of the continent that did rack up snow this winter: Alaska.

The presence of the strong jet stream nearby brought frequent storms to the region loaded with moisture.

Slight shifts in the jet brought significant swings in temperature from one month to the next over much of the Last Frontier.

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